Many of the 200-plus satellites the Goddard Space Flight Center has built, managed, or operated over the past 40 years have been dedicated to exploring the heavens - the almost limitless space that extends billions of light years beyond our planet. And at the time Goddard was founded, this territory was truly a new frontier. We could look through telescopes at stars and distant galaxies, and we could theorize about the forces and dynamics at work in the universe. But a mere 50 years ago we had as little concrete knowledge about what surrounded our planet as Lewis and Clark had of what lay between them and the Pacific Ocean.
The first steps in charting the frontier beyond our planet came with the early V-2 and sounding rocket flights in the late 1940s...
....and early 1950s. But satellites were the first ships that could explore the heavens in real detail and depth. The satellites Goddard launched into space gave physicists and astronomers the opportunity to turn theory into science, opening the door to whole new areas of experimentation and revitalizing the field of science itself.
In the process, their research has challenged many of our beliefs and sparked our collective imagination. For the scientific exploration of space is really a search for answers about where we came from, how our planet and life forms evolved, and what other life, planets, and phenomena might exist beyond our own world. In the last 40 years, we have found some answers and stumbled on even more complex mysteries and questions. Theorists now contemplate the possibility of a ten-dimensional universe, while experimental researchers are finding evidence of black holes, energy bursts hundreds of times greater than a supernova star explosion, and even evidence to support many aspects of Albert Einstein's most mind-bending theories. Recently, for example, scientists discovered possible evidence for Einstein's theory that the space-time "fabric" can be dragged, or warped, by the gravitational fields around rotating stars and planets.1
From a time when most of us thought that Earth must be the only inhabited planet in the universe, we have learned enough about how elements and galaxies evolve that numerous scientists now believe there has to be intelligent life somewhere else. We have found evidence that indicates the universe really may have started with a single, explosive "bang" of energy and has been expanding ever since. We have learned a tremendous amount about the birth, evolution and death of stars, the formation of planets, and other perplexing cosmic events, but we are still only probing the surface of these mysteries.
For centuries, we have been trying to put together the puzzle of how the universe works. Indeed, our curiosity to know more about our world and our universe, without any immediate practical application to our daily lives, is one of the unique characteristics of the human race. The puzzle is far from complete, but gaining the ability to explore space from space - a capability in which the Goddard Space Flight Center has played a pivotal role in developing - has uncovered many important pieces.
For the most part, the exploration of space has been pursued in two different fields of science, although the lines between  the different types of research are not always clear or distinct. One of the lasting legacies of the space age, in fact, is that it has prompted the integration of previously independent fields of scientific research.
One of the main focuses, and strengths, of space science at Goddard has been what might be generally termed space physics. This is the realm of physicists who investigate gravitational, magnetic and electrical fields in space and a variety of particle radiations, such as electrons, protons and the nuclei of many elements, that are emitted from the Sun, the galaxy and the cosmos beyond. In addition to giving us a better understanding of the near-space environment and the substances, forces and dynamics that affect our planet, this research can provide clues as to how other planets and galaxies are formed.
Complementing this research at Goddard is the work of astronomers, who use the electromagnetic spectrum (from radio waves to visual and ultra-violet light to gamma rays) to study the physical and chemical properties of more distant objects and phenomena in the universe. In contrast to space physicists who tend to use satellites to make "in situ," or on site, measurements, astronomers have to rely on remote sensing techniques, because the objects they are investigating are still long distances away even from a satellite in space. While a space physics satellite might measure the number of particles hitting a detector in a given period of time, an astronomy satellite looks at the radiated light or energy coming from objects at different wavelengths in the electromagnetic spectrum.
The early satellites were dominated by the space physicists, for several reasons. For one thing, many of the Goddard's satellite scientists came out of the sounding rocket community, which had focused more on space physics than astronomy. In addition, astronomy experiments generally required more sophisticated satellites that could point to particular objects for a length of time.
Space physicists also needed satellites more than their astronomy colleagues, because experiments in space physics could not be done on the ground. They required in situ measurements, which made physicists more willing to undertake the risks and rigors of satellite research than astronomers, who could still conduct a fair amount of research from ground-based observatories.
Yet space still offered astronomers something valuable. Satellites gave them not only the opportunity to observe objects...
...in greater clarity, but the chance to look at objects in portions of the spectrum that were blocked by the Earth's atmosphere. The results of these efforts have been beyond expectation. Over the past 40 years, Goddard's scientific satellites have discovered possible answers to some of our oldest questions and opened doors to worlds we didn't even know existed.
Goddard's early work in space science focused on simply figuring out what existed in the upper reaches of the atmosphere and beyond. Scientists had already determined that the atmosphere consisted of several different regions. Closest to the Earth was an area called the troposphere, where our weather occurs. Above that was the more stable stratosphere. Directly above the stratosphere was a highly charged region called the ionosphere. The ionosphere is what allows radio signals to travel beyond the horizon, because its electrically conductive properties reflect radio waves back down to Earth. In the mid-1950s, however, there was little information about what lay above that region.
Dr. James Van Allen, a researcher at the University of Iowa, made one of the first fundamental discoveries in this unexplored space above the atmosphere when his instruments on sounding rockets and the Explorer I and Explorer III satellites detected a mysterious "belt" of radiation above the ionosphere. Scientists had deduced that there must be energetic particles from the Sun that flowed toward the Earth. The polarized magnetic field of the Earth would split and deflect those particles at the equator, sending the positive particles travelling toward one pole and the negative particles toward the other. Among other things, these particles were thought to be the cause of the polar auroras, commonly known as the Northern and Southern lights.
What Van Allen discovered was that the dynamics of the Earth's magnetic field were more complex than scientists had thought. In an area near the equator, the Earth's magnetic field not only deflected particles but also trapped some of the lower energy particles, creating high-altitude "belts" of radiation around the Earth at lower latitudes.
The existence of the "Van Allen Radiation Belts," as they were called, forced scientists to revise their theories of how the Sun's particle radiations affected the Earth's atmosphere.2 In the process, they made another fundamental discovery that...
....occupied much of space physicists' satellite research in the 1960s - the existence of a previously unpredicted region above the ionosphere called the magnetosphere.
The discovery of the Van Allen Belts focused attention on the interaction between the Sun and the Earth's magnetic field. Many of Goddard's early satellites focused on this area and, as more data emerged, a new picture began to emerge. The Sun, it appeared, radiated particles in a steady stream into space. Dubbed the solar wind, these particles were deflected around the Earth by the Earth's magnetic field in much the same way as water is deflected around the bow of a boat. In 1963, a Goddard satellite called Interplanetary Monitoring Platform (IMP) 1, detected a distinct, turbulent "bow shock" area where the solar wind hit the magnetic field of the Earth. Additional Goddard satellites determined that this flow also created a magnetospheric "tail" behind the Earth, similar to the wake created by a boat. Inside that wake, confined by the solar wind, was the region known as the magnetosphere.
Understanding the dynamics of the magnetosphere - why particles were trapped at certain latitudes and not at others, how it was affected by events on the Sun such as solar flares, and how it affected other processes in the atmosphere and in interplanetary space - was the subject of many of Goddard's satellite research projects in the 1960s. In fact, understanding the larger-scale dynamics of the interactions between the Sun and the Earth has continued to be a goal of numerous satellite research projects even up through today.
In many ways, the Explorers class of satellites epitomized the Goddard Space Flight Center. The first Explorer was developed and launched before Goddard was formed, but Goddard's scientists, engineers, technicians and support staff were responsible for making the Explorer series.the amazing success story that it is. To date, there have been more than 75 successful Explorer missions, and the new Small Explorer series promises to carry that tradition on into the 21st century. The Explorer satellites were innovative, most of them were relatively small and simple, and many of them were built in-house at Goddard. They conducted research in almost all the space science disciplines, from particle and field research to high energy astronomy. In the early days, however, the Explorers were perhaps best known for their particle and field research in the magnetosphere and interplanetary space.
One of the early concerns of NASA's managers, for example, was micrometeorites and the threat they might pose to both unmanned and manned space missions. Several early Explorers3 included micrometeorite detectors, which discovered that the threat of damage from these particles was much lower than some scientists had feared.
Another concern was the possible radiation hazard solar cosmic ray events might pose for astronauts, especially on the Apollo Moon missions. Cosmic rays are actually energetic particles that travel through space. Scientists now have identified three distinct types of cosmic rays, each of which brings us information on a different aspect of the universe. One type, known as "anomalous cosmic rays," offer a sample of the interstellar gas in the nearby region of our Galaxy. So-called "galactic cosmic rays," travel at much higher speeds, reaching us from distant regions of the Galaxy and perhaps from beyond. "Solar cosmic rays" emanate from solar flares and other events on the Sun.4
During the Apollo program, however, NASA was concerned primarily with solar cosmic rays. When events such as solar flares occur on the Sun, a greater number of solar cosmic rays are released into space. One of the goals of Goddard's Interplanetary Monitoring Platform5 (IMP) series of satellites was to determine how great a hazard these events might create for humans travelling in space.
The IMP series of Explorers, which project scientist Frank McDonald named in honor of his children, consisted of 10 separate satellites that investigated galactic and solar cosmic rays, the interplanetary medium, and the distant magnetosphere. They also provided real-time monitoring for possibly hazardous cosmic radiation events during the Apollo missions.
Another Explorer satellite played an important role in measuring the results of the Starfish high-altitude nuclear bomb test in 1962. The test had been an effort to analyze the effects of an explosion in the upper atmosphere, but the blast cloud had travelled much higher than the test designers had intended, and both the military  and NASA were noticing difficulties with a couple of their satellites. At the request of President John F. Kennedy, Goddard initiated an intense effort to modify a satellite from another mission to go up and measure what the results of the test actually had been. Designated Explorer 15, the satellite discovered that the explosion had created an artificial radiation belt around the Earth. Goddard also discovered that the radiation from the blast was also responsible for damaging several satellites that were in orbit at the time, including the Alouette spacecraft and a Telstar communication satellite.6
Other early Explorers made important discoveries about the composition and boundaries of the Earth's magnetosphere, ionosphere and upper atmosphere and how these regions were affected by the solar "weather" and other cosmic events.7 By the end of the 1960s, these and other satellites had given scientists a fairly good general description of the magnetosphere, the features of the Sun-Earth relationship, and the interplanetary medium. The task since then has been to refine and expand that picture with more detail and depth.8
The Orbiting Geophysical Observatory Projects
The Explorers were not the only satellites geared toward helping scientists understand the near-Earth environment and how it is affected by the Sun. The Orbiting Geophysical Observatory (OGOs) projects that Goddard managed were an attempt to get a more integrated picture of the dynamics at work by co-locating many different experiments on a single satellite. The OGO I satellite, launched in September 1964, for instance, included 20 different instruments.
The OGO series actually incorporated two unique concepts. One was the inclusion of so many different experiments on a single spacecraft, which was a radical change from the smaller, more focused, Explorers. The second was that the OGO spacecraft and interfaces with experiments were designed to be standardized. In addition to cost savings, this "streetcar" design would allow experiments that weren't ready by the deadline for one launch to "catch" the next spacecraft in the series, like catching the next streetcar on a commuter schedule.
The OGO concept was a learning experience for the scientists and engineers involved, and it never worked as well as its designers would have liked. Having so many experiments on one satellite created numerous interference problems, and the success of the instruments varied widely.  The performance improved over time, and eventually six different OGO satellites were launched, the final one in 1969. But while the OGO concept was abandoned, the OGO work laid the groundwork for future standardized spacecraft designs. This standardized spacecraft approach has remained a recurring theme at NASA, first with the Multi-mission Modular Spacecraft (MMS) of the late 1970s, and even more recently with the Rapid Spacecraft Procurement Initiative.9
International Sun-Earth Explorer
Another approach to getting a more integrated picture of Sun-Earth dynamics was the International Sun-Earth Explorer...
....(ISEE) satellite series, a joint project between Goddard and the European Space Agency (ESA). The ISEE A satellite, built at Goddard, and ISEE B satellite, built by the ESA, were launched together on a single Delta rocket in October 1977. By placing the satellites in similar orbits at a variable distance from each other, scientists could pinpoint fluctuations in the boundaries of the Earth's magnetosphere. Ten months later, a third ISEE satellite built by Goddard was launched into a complementary orbit and gathered additional data on the same phenomena being measured by the first two satellites.10
In 1982, a Goddard project engineer named Robert Farquhar realized that a near pass of the Giacobini-Zinner comet predicted for 1985 created a golden opportunity for scientists to learn more about the composition and evolution of a comet nucleus. So the orbit of ISEE 3 was altered to fly by the comet. In a complex three-year maneuver, the ISEE 3 satellite was moved into an orbit that swung through the Earth's magnetospheric tail, sling-shotted around the Moon to put it into an orbit around the Sun, and came in close enough range to gather data from the comet in 1985. Renamed the International Cometary Explorer (ICE), the satellite was also able to observe the solar wind upstream of Halley's comet on that comet's visit in 1986.
International Solar-Terrestrial Physics
As satellite technology improved, it allowed us to add more detail to  our understanding of the dynamics between the Sun and Earth. We know that there are particles from the Sun that flow continuously toward and around the Earth, creating the boundaries of our magnetosphere.
The number of particles flowing toward Earth is not constant, however. Active solar events such as flares and coronal mass ejections (CMEs) release high quantities of energetic particles that shoot toward Earth at speeds up to two million miles an hour. Influxes of these particles result in geomagnetic storms around the Earth that cause a number of strange effects. These storms are believed to be the cause of the spectacular Northern and Southern lights, but they can cause numerous problems. They can interfere with telephone, television and radio signals, damage the electronics in spacecraft, disturb compasses and marine navigation instruments, and create power outages. As a result, we have more than just a scientific interest in learning more about these events, especially as our society has become more reliant on electronic technology. If we can figure out the dynamics that lead to these storms, we can take measures to mitigate their effect.11
In 1988, Goddard was given responsibility for a comprehensive international effort to explore these "solar-terrestrial" dynamics with much more precision and depth. The collaborative program was called the International Solar-Terrestrial Physics (ISTP) program, and its goal was to observe the impact and behavior of the...
....solar wind and its interaction with the Earth simultaneously from different perspectives. The program's initial goal was to look at these interactions from three different spacecraft called Wind, Polar, and Geotail, simultaneously for at least six months in space. In 1992, the program was expanded to include plans for two additional spacecraft called SOHO and Cluster.
The first of the ISTP spacecraft was "Geotail," a joint project between the Japanese ISAS and NASA. Launched in 1992, it looked at the dynamics and effect of the solar wind in the magnetospheric "tail" on the night side of the Earth.
NASA's Wind spacecraft was launched initially into an unusual orbit around the Earth, with the furthest point in the orbit almost a million miles away. Researchers then moved the satellite's orbit to a spot known as the Earth-Sun Libration point, or Lagrangian point L1. L1 is a point approximately 1/100th of the distance from the Earth to the Sun, where the centrifugal and gravitational forces of the two balance each other. The goal of the Wind satellite was to gather data on the solar wind before it reaches the Earth's magnetosphere - first at the point where it encounters the "bow shock" of the Earth's magnetosphere and then, at point L1, before the solar wind reaches the influence of the Earth at all. In 1998, the satellite's orbit was changed once again to a "petal" orbit of successive elliptical loops around the Earth to explore the solar wind in additional locations.
The third spacecraft in the ISTP series is the NASA-ESA Solar and Heliospheric Observatory (SOHO). In 1995 it was launched into an orbit around the L1 point to study the physical processes in the Sun that affect the release of solar cosmic rays and the solar wind itself.
A fourth project, developed by the ESA, was named "Cluster" because it was actually a set of four spacecraft designed to gather three-dimensional information on the shape and dynamics of magnetic structures. Unfortunately, the Ariane-5 booster rocket carrying the Cluster spacecraft exploded during its launch in 1996. But in 1997, the ESA announced that it would launch a replacement, Cluster II, on two Soyuz rockets in mid-2000.
The last ISTP spacecraft, called "Polar," was launched in 1996. It was placed into into a polar orbit of the Earth to observe the activity of solar particles once they entered the Earth's magnetosphere, ionosphere, and atmosphere.
The ISTP program was an ambitious one, and coordinating the efforts of numerous research institutions and countries on several different satellites was challenging from both a management and technical perspective. For example, Goddard had development responsibility for both the Polar and Wind satellites. The idea was to get all five satellites into orbit at the same time, but both Polar and Wind were behind schedule and encountering technical difficulties. Goddard's management decided to solve the problem by finishing the Wind satellite first and then tackling the Polar spacecraft. This caused some difficulties with Goddard's partners, because, most of the instruments built by Goddard  scientists just happened to be on the Wind spacecraft. But eventually, both Wind and Polar were successfully launched and are still returning useful data.
Yet despite the challenges of a multi-spacecraft, international effort, the ISTP program has achieved major breakthroughs in scientific observations of the Sun and its effect on the Earth, including the processes by which solar plasmas and particles are transported to the Earth. These findings are adding valuable pieces to the puzzle of the sun's relationship with our home planet.12
Although they are not part of the official ISTP program, Goddard has also developed and launched three additional satellites to enhance our understanding of cosmic rays. The first of these missions was a Small Explorer (SMEX) satellite called the Solar, Anomalous and Magnetospheric Particle Explorer (SAMPEX).
It was put into orbit with an air-launched Pegasus rocket in 1992 to study the composition of particles arriving from the solar atmosphere and interstellar space and how they are transported into our atmosphere. A second SMEX satellite, called the Fast Auroral Snapshot Explorer (FAST) is taking a closer look at the plasma physics of the polar auroras. The auroras are created by accelerated electrons from particles in the magnetosphere hitting the upper atmosphere, just as a television picture is created by a beam of electrons hitting the inside of the front screen. The goal of the FAST satellite, which was launched in 1996, is to better understand exactly how those particles are accelerated.13
A third satellite, the Advanced Composition Explorer (ACE),was launched in 1997 and is in orbit around L1, almost a million miles away from Earth. Its nine instruments are measuring the type, charge, mass, energy, direction of travel and time of arrival of ...
....anomalous, galactic and solar cosmic rays in an attempt to find clues as to their source and the processes that brought them here.
Because the different kinds of cosmic rays contain matter from vastly different times and places, researchers hope the satellite can help us better understand the formation and evolution of the solar system, among other things. The effort is not that different from the earlier satellites such as the Interplanetary Monitoring Platforms (IMPs); the difference is that technological advances have given the ACE instruments collecting power 10 to 1,000 times greater than previous satellite instruments.
The goal of space physics satellites has not changed dramatically over the past forty years. Scientists are still looking to satellites to gather data on the particles, magnetic fields and dynamics in both the near-Earth environment and interstellar space. And their goal is still to understand what exists beyond our atmosphere, how those particles and fields affect us here on Earth, and what all of this can tell us about the formation of our own solar system and other planetary systems that lie beyond our reach. What has changed is the technology. The early Explorer satellites drew the rough outlines of the picture. As we have developed better and more capable satellites, we have continued fill in the colors, the details, and the depth of what is turning out to be a very complex picture, indeed.
Understanding the Sun
One of the critical steps in filling in the details of the Sun's relationship with the Earth has been to learn more about the Sun itself, and several Goddard satellites have been devoted to this goal. Actually, the Sun offers a unique opportunity for several fields of study. In addition to its direct impact on Earth, the Sun is also a star. In the universal scheme of things it may only be a very average-sized star, but it is a star, and the only one close enough for us to observe in any great detail. So in learning more about our Sun's internal processes and its impact on our own planet  and solar system, we can infer a great deal about the dynamics of stars in remote parts of our Galaxy and beyond. And because the Sun is relatively close to Earth, scientists can obtain in situ measurements of the particles it releases as well as remote images of its processes in various wavelengths of the electromagnetic spectrum.
The Sun consists of a complex nuclear furnace surrounded by several different layers of gaseous plasma. Most of the elements in the universe are created within stars when nuclei of "lighter" elements fuse together to make heavier and more complex elements. Hydrogen, for example, is the simplest element known, with only a single proton in its nucleus. Helium is next, with two protons. Hydrogen can be turned into helium in high-speed collisions between nuclei that then release tremendous amounts of energy, but those collisions require temperatures of at least five million degrees. The formation of more complex elements require even higher temperatures, which is why scientists think most elements heavier than iron are formed in supernova explosions of dying stars.
The energy of our own Sun is a result of hydrogen nuclei fusing into helium at its core. That energy travels outward through the Sun, slowly degrading from gamma rays to X-rays and then ultra-violet and finally to visible light as it nears the visible surface, or photosphere, of the Sun. The process is a slow one. Light travels from the photosphere to Earth in approximately eight minutes, but that energy was created by fusion reactions that took place thousands of years ago in the Sun's core.
Near the surface of the Sun, energy begins to be transported by convection as well as radiation. Scientists have long surmised that dark "Sun spots" on the surface of the Sun were masses of cooler gas where strong magnetic fields limit the typical convective currents that would otherwise keep bringing heat up from lower regions.
At times, eruptions called solar flares explode outward from the photosphere, sometimes associated with huge ejections of up to 100 million tons of mass from the Sun. The particles released in those eruptions and Coronal Mass Ejections (CMEs) travel outward into space at speeds up to two million miles an hour, contributing significantly to the solar wind and sometimes causing numerous problems on Earth.
The region above the photosphere is called the chromosphere, an area of decreasing density analogous to the Earth's upper atmosphere. Above that is the Sun's corona, a region whose density is so low that we generally cannot see it from Earth except when a solar eclipse blocks out the...
 ...scattered sunlight in the Earth's atmosphere. During an eclipse, the corona appears as a white halo around the edges of the Sun. High-speed, feathery jets of plasma, called polar plumes, can also be observed shooting out from the sun's corona at its poles, where its magnetic field lines are more open.
Scientists have been observing the processes of the Sun since at least the days of Galileo. But many of the sun's internal processes can only be studied by looking at the radiation they create, and ground observatories are limited in the type of electromagnetic wavelengths they can see. Rocket and satellite solar astronomy allowed scientists to view the sun's processes in ultraviolet, X-ray and gamma ray wavelengths that would have been blocked by the Earth's atmosphere. This opened up an invaluable window to understanding the Sun, because many of the critical solar processes involve these short wavelengths in essential ways.14
Orbiting Solar Observatory
The first Goddard satellites that were designed specifically to look at the Sun were the Orbiting Solar Observatories (OSOs). The...
....first OSO spacecraft was launched in 1962 and successfully measured electromagnetic radiation from the Sun over time in ultraviolet, X-ray and gamma ray regions of the spectrum. The next two OSO satellites, however, both ran into trouble. The second OSO was destroyed in a pre-launch accident and the third stage of the Delta launch vehicle carrying the next OSO satellite fired prematurely. The result, according to Goddard's matter-of-fact mission notes, was that "the satellite thus entered the Atlantic Ocean rather than planned 350-mile circular orbit at 33-degree inclination."15
Eventually, eight OSO satellites were launched into orbit, all with the same basic goal of observing the Sun's processes in wavelengths not visible on Earth. OSO V, VI and VI had an additional focus on studying solar flares because of the potential threat these events might pose to astronauts in space. These OSO satellites provided the first close, extended look at the sun in important regions of the spectrum that could not be observed by any ground-based methods, and they provided the first steps toward understanding the complex processes of the star that has a critical impact on life here on Earth.
Activity on the Sun increases and decreases over the course of a cycle that typically lasts about 11 years, although there are exceptions to that time frame. In 1980, Goddard launched a satellite to try to get a more comprehensive  understanding of solar events, particularly solar flares. The satellite was called the Solar Maximum Mission (SMM) because its 1980 launch was timed to coincide with the peak activity of the Sun's 11-year cycle. The Solar Max satellite incorporated gamma ray, X-ray and ultraviolet spectrometers to look at solar flares across all the higher energy portions of the spectrum. These results were coordinated with ground observations in the visible and radio wavelengths in an effort to compile a comprehensive picture of the dynamics involved in these powerful solar events across the entire electromagnetic spectrum. The picture was filled out even further by the ISEE 3 satellite, which took in situ measurements of the particles released by the solar flares the SMM satellite and ground stations were observing.
The SMM marked the first time a satellite had been designed and launched specifically to look at solar flares. It also demonstrated one of the advantages satellites offered scientists. In addition to allowing observations of particles and radiations which cannot be seen or measured from the ground, satellites make it possible for scientists from different disciplines to work together, on the ground and in space, to get a much bigger and more comprehensive picture of cosmic phenomena.
The Solar Max satellite also marked the first time a satellite was successfully repaired in space. The SMM was the first of Goddard's Multi-mission Modular Spacecraft (MMS) designed to be serviceable in orbit by Space Shuttle astronauts.16 The concept got the opportunity to be tested eight months after SMM's launch, when three fuses in the satellite's attitude control system failed. The failures made it impossible for the spacecraft to point precisely at observation targets on the Sun, severely compromising the potential success of the mission.
So in 1984, astronauts aboard the Shuttle Challenger retrieved the satellite into the Shuttle's cargo bay, replaced the damaged components, and released the spacecraft back into orbit. Although the repair mission was ultimately successful, it underscored the difficulties of such an endeavor. The SMM spacecraft had been designed with a special attach point that could be mated with a docking device carried by an astronaut to retrieve the satellite and bring it back into the Shuttle cargo bay. But a screw head near the satellite's attach point was apparently sticking up out of place, and the astronaut was unable to get a lock on the spacecraft. To make matters worse, the astronaut grabbed onto one of the satellite's solar arrays in an attempt to get a better hold on the satellite, sending the spacecraft tumbling out of control.
Down at the control center at Goddard, engineers began a frantic effort to restabilize the spacecraft. Engineers estimated that the batteries on the tumbling satellite would be dead within eight hours, after which it would have to be brought back to Earth for repair or abandoned in space. To conserve power, the engineers decided to turn off even the satellite's transmitters. This meant that they would have no information on the health of  the crippled satellite, but they were desperate to buy more time to gain control of the spacecraft. After 19 tense hours, controllers managed to use the Earth's magnetic field and the magnetic torquers on the satellite to stop the spacecraft's tumbling, point it toward the Sun to re-power its batteries, and restabilize its movement. The Shuttle astronauts were then able to use the orbiter's robotic arm to capture the satellite the next day. The Solar Max repair mission underscored the difficulty of conducting science in the unforgiving realm of outer space.But the resourcefulness of the ground controllers at Goddard and the Shuttle astronauts made the mission a success. The effort also gave Goddard's engineers valuable experience in space repair missions - experience that would prove critical to the success of the first Hubble servicing mission a decade later. 17
The next maximum activity time in the sun's 11-year cycle is expected around...
....2001. So in the last couple of years, several new satellites have been launched to look at the Sun in even more detail as it transitions from a low-activity time to the height of its active cycle. The biggest of these is the Solar and Heliospheric Observatory (SOHO) satellite, a joint ESA-NASA project that was launched in 1995 as part of NASA's International Solar-Terrestrial Physics (ISTP) program. The SOHO satellite is orbiting the L1 Lagrangian point, which gives it a view of the Sun unobstructed by the Earth. The 12 SOHO instruments each gathered a particular type of information to help give scientists a better picture of the inner workings and dynamics of the Sun and its complex magnetic field, as well as what causes its plumes, flares, and coronal mass ejections.
Complementing the SOHO satellite is the Transition Region And Coronal Explorer (TRACE). TRACE is a Small Explorer satellite built by Goddard and launched in April 1998. The TRACE instruments don't have as wide a range as SOHO's instruments, but they can take images of the photosphere, transition region and corona of the Sun in much finer detail. Between the two satellites, scientists hope to obtain simultaneous measurements of all the temperature ranges of the solar atmosphere as it moves toward the height of its activity cycle.
Complementing this research is a NASA-ESA satellite called "Ulysses," which is in a unique polar orbit around the Sun. Most satellites have orbits that are still  within what is called "the plane of the ecliptic," which means on the same horizontal plane as the rest of our solar system. Our solar system, looked at from a distance, would appear as a flat disk. All the planets rotate around the Sun in pretty much the same horizontal plane, and most satellites stay within that same horizontal path as they orbit the Earth. But to orbit the poles of the Sun, the Ulysses satellite had to be flung out of that plane, into an orbit that operated perpendicular to the rest of the solar system. It was a difficult maneuver that required sling-shotting the satellite around Jupiter before turning it back to the Sun in order to give it the energy to break away from the flat disk of the solar system. But its orbit has allowed the Ulysses satellite to obtain valuable information about the dynamics of the Sun at its poles, which appear to play a major role in the creation of the solar wind.
Goddard also developed the Spartan 201 satellite, which is a short-duration spacecraft released overboard by Shuttle astronauts at the beginning of a mission and then retrieved and brought back to Earth when they return. The Spartan 201 satellites have carried various instruments to look at specific aspects of the sun's perplexing corona.
The combined efforts of these satellites have discovered some amazing things about how our Sun works. One of the mysteries of the Sun, for example, is the temperature difference between its outermost layer, the corona, and the chromosphere layer beneath it. Because the majority of the...
....Sun's energy is generated at the core, one might assume that its layers should be progressively cooler as its energy travels outward and dissipates. But that's not what happens. Earlier satellites had discovered that while the sun's photosphere and chromosphere are about 6,000 and 10,000 degrees Kelvin, the corona beyond them is a blistering two million degrees Kelvin. How the corona can be so much hotter than the regions below it has mystified solar physicists for many years.
Based on observations from these latest satellites, scientists have concluded that at least one source of the corona's high temperature is a process called "magnetic reconnection." In essence, the surface of the Sun has a very complex magnetic field structure that is constantly changing. As these magnetic structures continually "snap" and break down, they appear to release energy up into the corona, in the form of heat.
But the picture satellites are bringing back to us is even more complicated than that. Indeed, one of the main discoveries these solar satellites have made is just how amazingly complex the sun's processes are. Recent observations from the SOHO satellite, for example, have detected streams of plasma that move across the sun's surface and dive deep into its interior, almost as the jet stream, trade winds, and ocean currents circulate around the Earth. Those streams don't all move at the same speed or direction, however, and the Sun itself appears to rotate at different speeds in different places. In its interior, the Sun rotates as a unified object, but on its visible surface the Sun rotates slower at its poles than it does at its equator. Scientists now think this churning interaction between different rotation patterns and charged plasma currents may be what creates the Sun's complex magnetic field and causes the turbulent eruptions of solar flares and coronal mass ejections.
Instruments on SOHO and Ulysses have also looked at the polar plumes and coronal holes near the sun's poles, which scientists now believe play a significant role in the creation of the solar wind. Once, scientists assumed that the solar wind streamed out from the Sun in all directions. Satellite data has now shown that the picture is much more complex. The Sun's surface, chromosphere and corona are highly structured with very complex magnetic fields. At places, the fields cause plasma to "loop" out and back, trapping it on the Sun. In other places, "holes" in the fields allow plasma to escape a high speeds, becoming the solar wind. And the turbulent interactions of the various parts of these magnetic fields may be the cause of more explosive ejections of material, such as solar flares or coronal mass ejections, which release tremendous amounts of plasma in a short period of time. One such solar storm in January 1997, for example, released a magnetized cloud that stretched 30 million miles across, pounding the Earth's magnetosphere with particles for 30 hours, interrupting communications and causing spectacular polar auroras. With the new generation of solar satellites, scientists are beginning to put together enough pieces to start to make sense of this complex solar puzzle. Before long, we may actually learn enough to not only understand these storms, but to predict them.18
 Exploring the Universe
The Sun offers a unique opportunity to study the processes of a star at close range, allowing scientists to examine both the particles and the electromagnetic radiation it emits. Goddard scientists also designed instruments for space probes and planetary missions to examine the magnetic fields and particles of other planets and regions within our solar system. To figure out the composition and behavior of material, planets or stars beyond our immediate surroundings, however, scientists rely on analyzing the light and energy that reaches us in various wavelengths of the electromagnetic spectrum. Another main focus of Goddard's scientific satellites over the years has been building spacecraft to explore the universe in different regions of this spectrum.
The electromagnetic spectrum progresses from radio waves, with the lowest energy and lowest temperatures, to microwave radiation, to infrared light, to visible light, to ultraviolet light, to X-rays, and finally to gamma rays, which have the highest energy and occur at the highest temperature of any waves in the spectrum. The upper end of the scale contains very short wavelengths with a higher frequency, while the lower end consists of much longer wavelengths with a lower frequency. In a sense, the spectrum behaves much like a rope held between two people. To get the rope to undulate in big motions doesn't require much energy. But to get it to move up and down in frequent oscillations requires quite a lot of energy.
Energy is also linked to temperature. High energy events have high temperatures, while lower energy events are much cooler. So each particular point on the spectrum correlates to a very particular temperature as well as energy or light level. Scientists spend a lot of effort looking at the Sun in X-rays and gamma ray regions, for example, because some of the processes they want to study take place at very high temperatures - which means they can be observed only at the high frequency end of the electromagnetic spectrum.
Each element of the periodic table also has a particular "fingerprint" in the electromagnetic spectrum. If nitrogen is present somewhere, for example, emissions from that location will show a particular spike at a very precise wavelength in the spectrum. Other elements will spike at different unique wavelengths. This is how scientists can determine the chemical composition of gas clouds, comets, or other matter in the galaxy.
Yet in exploring the universe, one more factor comes into play. When we look at a distant star, galaxy, or nebula, the light or energy we are receiving has travelled up to...
...13 billion light years to reach us. So when we look at any of these objects or phenomena, we are really looking back in time. What we see is the light that star generated many years ago, and the further away the source is, the dimmer it will appear. The universe is also expanding. So the stars in distant galaxies are not only old, they are actually moving away from us. And the more distant they are, the faster they are receding. This movement affects the light we see.
Light waves, like sound waves, change as they travel toward or away from us. Just as a train whistle appears to get higher in pitch as it approaches and lower in pitch as it recedes into the distance, the light from objects in the universe will get longer in wavelength if those objects are moving away from us. If astronomers detect an infrared source in our own galaxy, it probably began as an infrared source. But that infrared light is coming from an object in a distant galaxy, it may have begun as visible or even ultraviolet light. Scientists call this phenomenon "red shift" because the starlight from distant galaxies "shifts" down toward the redder, longer wavelength end of the spectrum as the galaxies move away from us. The more distant they are, the faster they're moving, and the greater the redshift.
So when scientists look for light from the very early days of the universe, for example, they are looking for light that is very old and has come from very far away. It may have been extremely bright at one time, but now we are more likely to detect it as a dim light and in the lower, infrared and microwave portions of the spectrum. Radiation from closer, more highly energetic events in the universe, by contrast, will be detected in the higher ends of the spectrum. Unfortunately for scientists, the Earth's atmosphere blocks many of these portions of the spectrum. So it has only been since the advent of satellite technology that we have been able to fully explore many of the universe's greatest mysteries.
The real benefits of these efforts, however, have come from the coordination of many people, projects, and data. Individual objects or phenomena may be especially bright in one particular region of the spectrum, and observations in new regions, such as X-ray wavelengths, may tell us things about objects we never knew before. But one of the biggest advantages of conducting satellite astronomy across the entire electromagnetic spectrum is that it gives scientists the ability to look at the same objects and phenomena in many different wavelengths.
 Many so-called X-ray stars, for example, are also visible to the eye. What X-ray satellites told scientists was that these stars were also emitting peculiar, high-temperature energy, which meant there was something different about these stars than visible stars that did not show up in X-ray wavelengths. Radio astronomers detected pulsars several years before X-ray satellites discovered that these phenomena also had high-energy emissions. The Crab Nebula, the remnant of a star that exploded almost one thousand years ago, can be observed in almost every wavelength. Yet each different wavelength provides a slightly different piece of information that helps us put together a more accurate and complete picture of any particular object or phenomenon in the universe.
So while some findings may be touted as "X-ray," "gamma ray" or "ultraviolet" discoveries, all of these depend heavily on observations in other wavelengths. One of the greatest difficulties in figuring out the source of the exotic "gamma ray bursts" that satellites have detected for...
...three decades, for example, was that scientists were unable to link these high-energy bursts with any visual objects or phenomena until 1997. And even then, only three or four out of thousands of gamma ray bursts have been identified.
Clearly, there is much left to learn. But Goddard's achievements in space-based astronomy, along with ground-based astronomy efforts over the last 40 years, has already shown us a universe that is more dazzling, complex, mysterious and powerful than we ever even imagined.
The lowest energy portion of the spectrum consists of radio waves. It is also one of the few regions of the spectrum other than visible light where the waves can penetrate the dust clouds in our galaxy and the water vapor, dust, ozone and other elements of our atmosphere. As a result, radio astronomy has played an important role in the exploration of the universe, uncovering signs of many phenomena and objects that are invisible to the eye. Astronomers were studying intergalactic radio wave signals even before there were satellites. But from the ground, radio signals may be affected by the ionosphere and extraneous man-made radio noise. Radio astronomy satellites offered a chance to monitor signals from the solar system, galaxy and the cosmos without any of those interferences.
Goddard's first radio astronomy satellite, Explorer 38, was launched in July 1968 to look at radio signals from the Earth's magnetosphere, Jupiter, and other  cosmic sources. In order to receive the weak radio wave signals, the satellite had twin antenna booms that were extended from the main body of the spacecraft once it was in orbit. Tip to tip, the antenna were taller than the Empire State Building. Seven years later, Goddard launched another radio astronomy satellite, Explorer 49, to look for radio signals at slightly different wavelengths.
Among other things, radio astronomy can identify certain elements whose "fingerprints" fall in the radio wave portion of the spectrum. Goddard's Submillimeter Wave Astronomy Satellite (SWAS), for example, is being designed to examine the chemical composition of interstellar galactic clouds to help determine the process of star formation. The SWAS satellite is the third mission in Goddard's Small Explorer (SMEX) program, and is scheduled for launch in early 1999. 19
Some of Goddard's most significant astronomical research has been conducted in the microwave/infrared region of the spectrum, right above radio signal wavelengths. It's a very difficult region to explore, because it involves such low temperatures. The phenomena scientists are trying to detect have temperatures as low as three degrees Kelvin, or almost absolute zero. The heat most instruments and detectors generate is higher than that and would obscure any signals at these wavelengths. So an infrared or microwave satellite has to have cryogenically cooled instruments, which is a very cumbersome and difficult proposition. Yet this portion of the spectrum offered our best window back to the dawn of time, which prompted scientists to try to solve the difficulties inherent in exploring it.
Over a 15-year period, a Goddard team succeeded in designing, building and launching a satellite capable of exploring portions of the universe in this challenging region of the spectrum. The Goddard-built satellite, called the Cosmic Background Explorer (COBE), went in search of data to test the "Big Bang" theory of the origin of the universe - and found it.
The origins of the COBE satellite actually date back to at least 1965, when two researchers at Bell Telephone Labs in Holmberg, New Jersey detected a background microwave "noise" coming in equally from all directions of the universe, not related to any particular object or event.
 Scientists realized that this low-energy background noise might hold the secret to how our universe was formed.
Several different theories had been put forth as to how the universe began and how it was evolving. One of the most popular was the "Big Bang" theory, which held that all the matter and energy in the universe was created in one initial explosion and had been expanding ever since. According to the theory, matter and energy had been changing and evolving in various combinations since then, but no additional major inputs of energy or matter had occurred in the 14 billion years since that initial explosion.
If the Big Bang theory was correct, the very beginning moments of the universe would have consisted of a cosmic oven of tiny particles of matter and anti-matter colliding into each other at tremendous rates. Each collision would have annihilated the matter, creating energy, but the temperature of this primordial soup would have been so hot that that energy, in turn, would be constantly creating new matter and anti-matter. As the energy decreased and dissipated from the initial explosion, a point would have been reached when new matter would no longer be created. At that point, there was evidently a little more matter than anti-matter. The matter clumped together to form galaxies, and the remaining energy became the cosmic background radiation.
If no additional energy inputs had occurred in the billions of years since then, that background radiation should...
....behave like a "blackbody," which emits heat and in a uniform and specific manner at all points. If there had been some other source of heat or energy during the evolution of the universe, that smooth blackbody curve of radiation would have distinctive bumps in it where the energy increased.
To find out the behavior of this cosmic background radiation, scientists had to develop instruments that could detect wavelengths typical of a body with a temperature of three degrees Kelvin - the wavelength at which that very old cosmic background radiation would be at its brightest. A young Goddard scientist named John Mather had done his doctoral thesis on cosmic background radiation, and in 1974 he proposed a follow-on experiment that might work on such a satellite. The result was COBE - a project that eventually...
....involved the efforts of almost 1500 people, cost between $300-400 million, and took 15 years to complete - but which unlocked the door to the dawn of time.
The COBE satellite consisted of three main instruments, all of which had to be cryogenically cooled to almost absolute zero. The Far Infrared Absolute Spectrophotometer (FIRAS) would examine the cosmic background radiation to see if it behaved like a blackbody in the three-degree Kelvin temperature range. If it did, the radiation should increase in a smooth curve approaching the wavelength equivalent to three degrees Kelvin, peak at the 2.78 degrees Kelvin wavelength, and then decrease in a similarly smooth manner as the wavelength and frequency increased beyond that.
A second instrument, called the Differential Microwave Radiometer (DMR) was designed to make an all-sky map of the brightness of the background radiation to look for evidence of how galaxies developed from the homogeneous soup of the universe's earliest stages. Miniscule temperature fluctuations in the cosmic background radiation would reveal places where matter was denser than others, pointing out where and how galaxies began to form.
The third instrument, called the Diffuse Infrared Background Experiment (DIRBE), was designed to search for residual light from the earliest galaxies. If there had been galaxies in the early universe, it was so long ago that we probably couldn't detect individual light sources. But there might be a distinct residual "afterglow" from those galaxies at very long wavelengths. Scientists hoped the DIRBE instrument could detect this faint evidence of celestial bodies in the primordial universe.20
As with numerous other Goddard satellite projects, COBE was a tremendous challenge, requiring great technological leaps and innovation. Engineers and scientists could not write a specification for the instruments or spacecraft to operate them because nobody knew how to build them. It had never been done before. So the COBE team simply set out to design its own systems and instruments. It was not an easy process. Instruments would work on the bench and then fail when they were put into the cryogenic containers. Problems in weight, power, and cryogenic systems plagued the effort. If they didn't know before, researchers soon found out that there was a good reason no one had done this before. As Dr. Mather later put it, "The team's naivete was a blessing and a strength. We didn't know how hard it was going to be, so we went ahead and did it."
 The satellite was originally designed to be launched into its 560-mile high orbit from the Space Shuttle. But the 1986 Space Shuttle Challenger explosion, just as the COBE team was nearing launch readiness, changed that. The Shuttle fleet was grounded and would probably never launch from the the west coast site COBE needed. Scrambling to save the project, the COBE team decided to redesign the satellite to fit on board a smaller Delta rocket that could be launched from the west coast. Among other things, that meant the satellite had to be much lighter and smaller.
In an intense two-and-a-half year effort, COBE was successfully redesigned to fit the alternate launch vehicle. Ironically, the emergency effort and limitations actually made the effort easier in some ways. There was no question of expanding weight or size, and there was no time to discuss multiple options at length. Choices had to be made quickly and decisively. The team was also helped by the fact that, in the wake of Challenger, NASA saw COBE as something that could offer the agency a much-needed success story. Consequently, the effort suddenly became a high-priority project. Team members who had had to split their time with other projects were put on COBE full-time, and the team was relocated so the engineers and scientists could all work together in the same place.
In January 1989, COBE was successfully launched into orbit. The effort to get it there had been tremendous, and some Goddard employees jokingly referred to COBE as "the project that ate the Center," because so many people were involved in it by the time the satellite was finally launched. But when the results started coming in, everyone realized the effort had been well worth it.
As the data from the FIRAS instrument began to come in, the project scientists were rendered almost speechless. The data points and the predicted curve were not just close ... they were identical. The results were so astounding that when they were presented to the American Astronomical Society in January 1990, they drew a standing ovation from the....
 ....normally reserved society scientists. There could be no doubt about it. COBE's data pointed clearly to a universe whose energy had been generated in one initial explosion and had been radiated out in a uniform manner at all points ever since.
It took a little longer to get firm results from the DMR instrument but eventually it indicated that there might be enough of a temperature differential in the cosmic background radiation to account for the clumping of matter into galaxies. The DIRBE results took even longer to analyze but, in early January 1998, the team scientists finally announced that they had, in fact, found evidence of an infrared background glow from the earliest stars and galaxies.
Do these results mean that we have conclusively settled the question of the universe's origins? No. If anything, the lesson of the past 40 years has been that the more we learn about space, the more we realize how much we have left to learn. Every step we take into the cosmos seems to reveal an even more complex universe than the one we previously thought we inhabited. The task of COBE was a staggeringly difficult one. Its scientists were like archaeologists trying to peer back more than 13 billion years in time and detect the faint whisper of clues that still lingered, ghost-like, in a world at the dawn of time. And the project uncovered an astounding piece of evidence no one had ever been able to see before. But there are still unanswered questions and anomalies in the Big Bang theory.
Because we know there is much left to learn, Goddard is already planning a follow-on mission to the COBE satellite. Scheduled for a launch date around the year 2000, the Microwave Anisotropy Probe (MAP) satellite will explore the tiny temperature differences discovered by COBE in even greater detail. Among other things, scientists hope that by learning more about the density and arrangement of matter in the early years of the universe, we can begin to predict whether or not the universe will keep expanding.21
Orbiting Astronomical Observatories
The very first astronomy satellites concentrated on the visual and ultraviolet regions of the spectrum. In part, this was because scientists had yet to realize the potential of some of the other wavelengths, but it was also because the very high and low frequency wavelengths required more complex technology to explore.
But even these first astronomy satellites came after the initial round of simple, particle and field-detecting spacecraft,....
....because astronomy satellites in general were harder to develop. Goddard had inherited some very good optical technicians from the Naval Gun Factory that had been located next to the Naval Research Lab, but developing a spacecraft for a telescope was harder. An astronomy satellite had to be stabilized and have the ability to point at one object for a relatively long period of time.
Goddard's first astronomy satellite was also its first attempt at a larger, observatory-class spacecraft. Called the Orbiting Astronomical Observatory (OAO), it was designed to explore the sky in both the visual and ultraviolet regions of the spectrum. Ultraviolet light is one of the wavelengths blocked by the Earth's atmosphere, and it was one of the regions astronomers were most interested in pursuing in the early 1960s. The astounding high energy world of neutron stars and black holes had yet to be discovered. The OAO satellite, however, was a tremendous technological leap that incorporated new sensor technology, new experiment technology and complicated ground system software that required constant updating. Experimenters on the ground who were commanding the OAOs were often only a couple of orbits ahead of the spacecraft, which made it a high-stress project.
The first OAO was launched in April 1965 but developed problems soon after reaching orbit. The high-voltage system on the star trackers that it needed to stabilize itself began arcing, and its battery overheated. After a mere 20 orbits, the satellite failed. It was not until December 1968 that the first successful astronomical satellite, OAO-2, reached orbit. OAO-2 allowed the Smithsonian Astrophysical Observatory to compile the first complete ultraviolet map of the sky, creating a catalog for use by astronomers. The satellite also provided new information on the composition of interstellar dust and hot stars.
Four years later Goddard launched OAO-3, also known as the "Copernicus" observatory, which lasted eight years and was an extremely successful astronomy satellite. Copernicus reached much farther into the ultraviolet region of the spectrum than the earlier OAOs had and gave scientists information much more detailed information about the chemical composition of certain stars and interstellar gases.
The OAO satellites were complex to operate and somewhat limited in their abilities, because they were pioneers in the space astronomy effort. But they provided valuable information and laid the groundwork for further discoveries by the Hubble Space Telescope and Goddard's International Ultraviolet Explorer.22
The Hubble Space Telescope
The origins of the Hubble Space Telescope date back to the late 1960s, when NASA managers began thinking about follow-on projects to the OAO satellites. Originally called the "Large Space Telescope," the facility was renamed before launch in honor of Edwin P. Hubble, an astronomer who determined in 1929 that the speeds with which galaxies are moving...
....away from us is proportional to their distance. This "Hubble's Law," is a crucial tool still used by astronomers in trying to determine not only the location of distant galaxies, but also the size and shape of the universe itself.
NASA Headquarters decided to award management of the Hubble project to the Marshall Space Flight Center, although the scientific instruments and ground system would be Goddard's responsibility. Goddard would also be responsible for operating the telescope once it was launched. To reassure the external scientific community, which feared NASA scientists might have too much of an inside edge on this powerful astronomical tool, selection and processing of individual experiments using the telescope were delegated to an independent Space Telescope Science Institute set up specifically for that purpose in Baltimore, Maryland.23
Goddard scientists and engineers had fought hard to get full responsibility for developing the telescope, and there were hard feelings over the decision that lingered between the two NASA Centers for years. As one scientist described it, the fight over Hubble wasn't just a disagreement, "It was a war." But with the decision made, Goddard's team shrugged their collective shoulders and, as usual, got down to the work at hand. But the decision to split the responsibility between the two centers would come back to haunt the agency.
The Space Shuttle Challenger explosion delayed the launch of the Hubble, which was designed to be launched from a Shuttle. But in April 1990, the telescope was finally put into orbit, amidst great hope and expectation. This was the largest telescope ever put into space, and it was expected to return images clearer and more detailed than anyone had ever witnessed before. But as operation of the telescope was turned over to Goddard and the first images began to appear, it became obvious that something was wrong. The images were blurry.
Coming so soon after the Challenger disaster, the flaw in the Hubble telescope was a devastating blow to NASA's credibility in the scientific community and the public at large. Goddard engineers knew what was at stake. But they were at a severe disadvantage in terms of trouble-shooting the problem, because they had only been nominally involved in building of the actual telescope.
Goddard quickly put together a team to start working on a fix. In addition to Goddard personnel, the team included  representatives from the Marshall SpaceFlight Center and the Space Telescope Science Institute. Lockheed, which had built the spacecraft, and the Perkin-Elmer company24, which had an excellent reputation in optics and had built the telescope's primary mirror, also sent experts to Goddard to join the trouble-shooting team.
The team members agreed they had three top priorities. First, they had to figure out what was wrong with the telescope. Second, they had to figure out what it would take to fix it. And third, they had to figure out how scientists could use the telescope until they could get it fixed.
The team soon discovered that the error was caused by a tiny flaw in the telescope's primary mirror. A piece of tooling used to test the mirror's accuracy had been installed backwards, causing the two-and-a-half meter diameter mirror to be 2.34 microns flatter at its edges than it should have been. The discrepancy was microscopic, equivalent to 1/50th the width of a human hair. But it was enough.
The good news was that the Hubble telescope had been designed to be serviced in space, so it had modular instruments and components that could be pulled out and replaced. The bad news was that because the error was in the telescope's primary mirror, it affected the operation of all five of the telescope's instruments.
One of the instruments, the Wide Field/Planetary Camera, was scheduled to be replaced in a servicing mission, anyway. So a corrective optical lens could be built into the upgraded replacement camera. But replacing all the instruments would be far too costly.
A group headed by researchers at the Space Telescope Science Institute that also included engineers from Goddard, Marshall, and the European Space Agency (ESA) began studying other possible solutions. In the meantime, scientists and engineers began looking at what science could be done with the telescope until it was repaired.
Even the blurry images were better than anything that had been available before, which was encouraging. And scientists soon found that the telescope's performance was acceptable for bright objects. It was faint objects that caused the biggest problem. Some of the images were also degraded because the telescope's flexible solar arrays, which had been designed and built by the Europeans, had developed a "jitter"...
 ....as the spacecraft transitioned from night to day. Researchers found ways to work around the jitter, developed software to help correct the fuzzy focus on the images, and concentrated the initial work of the telescope on brighter objects in the universe. But while these adjustments were remarkably innovative and allowed some very good science data to be drawn out of the telescope, they couldn't completely compensate for the flaw. In order to get the promised science out of the telescope and restore NASA's credibility in the eyes of the scientific community and the American public, the team needed to actually fix the source of the problem.
In just a few months, the Hubble team came up with a radical idea for a solution. The scientists and engineers proposed...
....replacing the least-used instrument on the Hubble with a module that would contain a corrective optics system for the remaining three instruments. The Corrective Optics Space Telescope Axial Replacement (COSTAR), as they called it, would contain ten separate mirrors, ranging in size from a dime to a quarter, that would refocus the light reflected from the primary mirror before it entered the three "axial" instruments.25 In order to work, these tiny mirrors would have to be accurately polished to 1/50th of a wavelength of white light.
It was a risky proposition. Even if the complicated COSTAR instrument could be built with that degree of accuracy, it would have to be installed in space, and it had to work perfectly. NASA couldn't afford another embarrassing failure. Some NASA managers suggested limiting the effort to the less challenging task of replacing the Wide Field/Planetary Camera, which was the instrument that took all the visible light photographs. But team leaders argued that Goddard and NASA owed the scientific community a fully working telescope. The COSTAR project was approved.
With only 80 employees and more experience in large-screen television systems than space instruments, Tinsley Optical Laboratories in Richmond, California was an unlikely candidate to make the COSTAR mirrors. But it turned out that the processes for building both products were surprisingly similar, and every employee of the small company took on an almost personal responsibility for making sure the mirrors were made correctly.
 The Hubble repair mission became the number one priority at Goddard, as well. The team had a target date of June 1993 for the repair mission to take place, with a outside deadline of June 1994. Eventually, the team committed to having the mission ready to launch by 1 December 1993.
To make that date, there was a lot that had to be done. The problem with the solar arrays had to be found, and new arrays had to be built. Meanwhile, the telescope had developed problems with its gyros, which would have to be replaced, as well. To make all these repairs in orbit also would require more Extra-Vehicular Activity (EVA) time on the part of the Shuttle astronauts than any mission to date, and the work would be challenging.
The Goddard-managed recovery team convinced managers at the Johnson Space Center to assign the astronauts for the repair mission a full year earlier than usual, giving them almost two years to prepare for the mission. Johnson also agreed to put only veteran Shuttle astronauts on the mission, so nobody on the mission would be adjusting to space for the first time while the team tried to work on the telescope.
The astronauts shuttled back and forth between Goddard and Johnson, learning the spacecraft's systems and rehearsing the servicing effort as well as every contingency and emergency managers could envision. They even spent a record 400 hours in Johnson's neutral-buoyancy water tank practicing the five EVA missions the repair would require.
All of this activity, of course, had to be done in the fishbowl of scrutiny following the discovery of the Hubble flaw. By the time the servicing mission was launched, no fewer than 18 external review committees were overseeing the team's efforts.
But on 1 December 1993 the team was ready, as promised, and the Space Shuttle Endeavor thundered off its launch pad on an 11-day mission to retrieve and fix the Hubble telescope. As the astronauts prepared to...
 ....head home a week and a half later, the mission appeared to have been a success. The proof, however, would lie in the images transmitted back to Earth once the telescope was back in orbit.
Goddard scientists and engineers watching at the Space Telescope Science Institute held their breaths as they waited for the first images, knowing that the consequences of another error would be terrible. As one team manager put it, "We felt like the future of the Agency was riding on this effort. We really did." They needn't have worried. A crystal-clear image of a star appeared on the main computer screen, sending cheers and applause through the room. An effort that some experts had given no more than a 50% chance of success had succeeded beyond everyone's wildest expectations.
In recognition of the challenge, the effort, and the accomplishment, the Hubble team was given numerous awards. The team was even awarded the 1993 Collier Trophy - recognizing the servicing mission as the greatest aeronautical achievement in the nation that year.
With the flaw repaired, the Hubble Space Telescope finally began fulfilling its long-awaited promise. The images its high-resolution camera and instruments have brought us since 1994 have been nothing short of awe-inspiring. From the towering pillars of starbirth in the Eagle Nebula to the artistically spectacular shock waves from exploding and dying stars, the Hubble has brought the distant universe to our doorstep in brilliant, breath-taking technicolor.
The Hubble provides extraordinarily clear images, illuminating far more detail about a wide variety of phenomena in the solar system, the galaxy and the universe than scientists ever had before. In 1994, the Hubble captured the collision of a comet into the planet Jupiter. It has been able to distinguish the shape of galaxies at distances so far away, and therefore so far back in time, that we can start to see a pattern in how galaxies evolve. It has also let us see at least the visible and ultraviolet results from the collisions of galaxies and the death of giant stars.
In fact, the visible and ultraviolet images26 from the Hubble have been able to give scientists a lot more detail about the life cycle of stars. It has generated clear images of dusty stellar nurseries, newborn stars with potential solar system material massing around them, bright, Sun-like stars in nearby galaxies, swollen giant stars approaching destruction, and the remnants of supernovae at different stages following these stars' explosions.
The gas clouds surrounding smaller dying stars are cool enough that they lend themselves well to analysis in the ultraviolet and visual ranges, and Hubble has produced a virtual catalog of the different shapes these "planetary" nebulae can take. Some nebulae are round, but others are shaped more like hourglasses, butterflies, goblets, rectangles, or streaming jets. Scientists are still puzzling over this phenomenon because, up until their death, all these stars have the same basic round shape.
 The Hubble Space Telescope's capabilities were improved even further by another planned servicing mission in 1997. These upgrades included the addition of a Near-Infrared Camera and Multi-Object Spectrometer (NICMOS), which will offer much greater detail in the cooler infrared region. Another upgrade is called the Space Telescope Imaging Spectrograph (STIS). Among other things, this instrument is useful in finding possible galactic black holes, because one of the signatures of this phenomenon is a swirling motion of the galaxy surrounding it. As the galaxy swirls, part of it is moving away from us, shifting its light lower in the spectrum, while another part moves toward us, shifting its light higher. The STIS spectrometer can create an extremely detailed cross-section image of a galaxy, measuring shifts in wavelength at 500 different points across the galaxy simultaneously. By comparison, previous spectrometers looking at these celestial bodies could sample only a dozen or so points. Scientists hope the more detailed images provided by the STIS will help identify swirling and other types of galactic movement.
Even before the STIS instrument was put on the Hubble, however, the technology developed for it was put to work here on Earth. The digital imaging technology used in the instrument has provided doctors with a new technique that facilitates non-surgical biopsies on women who may have breast cancer.
Additional servicing missions are planned for the Hubble in 2000 and 2002.
Scientists hope that these improvements will allow the telescope to extend its useful life until at least 2005. A study is currently underway for a "Next Generation Space Telescope" (NGST), which scientists hope will succeed the Hubble. If it is approved, the NGST will have much greater light collecting power than even the Hubble possesses and will be optimized to look in infrared light at the earliest galaxies of the ancient universe.
Of course, the more "invisible" wavelengths are every bit as important in piecing together the puzzle of our universe as those we can see. But the Hubble is particularly popular because we are, in the end, a visual species. The data from the Hubble's instruments have given scientists many valuable new insights about the universe. But the photographs and enhanced infrared and ultraviolet images from the telescope have reached far beyond the scientific community. They have sparked the imagination of millions of people who now gaze up at the night sky with a reawakened sense of curiosity and wonder. For without even a single word of...
....explanation, the Hubble images have made us realize how powerful, beautiful and mysterious the universe can be.27
International Ultraviolet Explorer
While the Hubble Space Telescope was still in its embryonic planning stages, another satellite project devoted exclusively to exploring the ultraviolet realm was taking shape. The ultraviolet discoveries of the OAO satellites in the 1960s had made scientists eager to explore further into this previously invisible realm of the spectrum with a telescope specifically designed for that purpose.
The satellite was called the International Ultraviolet Explorer (IUE), which was put into orbit in 1978. Although some components came from other places, the spacecraft was constructed in-house at Goddard. Having learned some difficult operating lessons from the larger, more cumbersome OAOs, the scientists decided to design the IUE to have a geosynchronous orbit. While this would dramatically simplify ground operations for the satellite, because it would be in sight of a ground station all the time, it meant that the spacecraft had to be small and light enough to reach an orbit 23,000 miles above the Earth. It would also have to have very high-resolution cameras and an extremely precise pointing system, accurate to within a few arc-seconds.
Another unique aspect of the IUE was that it was developed as an international project between NASA, the European Space Agency (ESA), and the British Science Research Council. The detectors and solar panels were supplied by the Europeans, and the telescope and spacecraft were designed and built by Goddard. The OAO's star tracker problems taught Goddard's engineers that a better way was needed to stabilize an astronomy satellite. So they worked with the Bendix Corporation to develop an inertial gyro system for the IUE that turned out to work so well that the spare was used in the Hubble Space Telescope. In fact, the IUE continued to work flawlessly for 11 years with only two of its initial six gyros, and the team even managed to operate the satellite for the last six months of its 19-year life with only one remaining gyro.
Technology also had advanced far enough by the mid-1970s, when the IUE was being built, that it could incorporate a vastly simpler ground computer system. The interface between an experimenter's software system and the satellite's system was designed to be "transparent," so that outside astronomers could use the IUE telescope  as easily as they used their own ground-based observatories, making adjustments in real time. This change meant that observers no longer had to be experts in satellite instrumentation. As a result, it opened up a whole new era in space science and generated a lot more support for satellite research in the astronomy community.
The IUE was scheduled to launch in mid-January 1978. Only a couple of weeks before that, however, a short circuit was discovered somewhere in its internal wiring system. On New Year's Eve, the satellite was sitting in many pieces on the floor of Goddard's clean room as engineers and technicians searched for the trouble spot. Even if the problem was solved quickly, there wouldn't be time to complete another full test on the satellite if it was going to make its launch date. But the technician in charge of the repair solemnly promised the engineers and scientists in charge that the satellite would be fixed and reassembled not only perfectly, but on time. It was, and on 26 January 1978 the IUE satellite was launched into orbit.
Observing time on the IUE was shared between the United States and European partners. Goddard controlled the satellite 16 hours a day, and the Europeans controlled it the remaining eight hours.
The IUE was originally designed for a five-year mission. To everyone's surprise, it...
....kept returning useful data for almost 19 years, adding a tremendous amount to our understanding of our solar system, our galaxy, and the universe.
One very basic and important contribution of the IUE was that it allowed scientists to fill in observations of thousands of celestial objects in a previously unobtainable portion of the spectrum. But there were also some objects and phenomena that could be studied particularly well in ultraviolet wavelengths. The temperature of many of the gas clouds surrounding stars, in between stars, and in between galaxies, for example, means that they create a signature in the ultraviolet range. As a result, the IUE was able to teach scientists a lot about the temperature, density, and behavior of this circum-stellar, inter-stellar, and inter-galactic matter. Among other things, IUE data indicated that just outside of our galaxy was a "halo" of hot gas that scientists had not known existed.
By analyzing the gas clouds surrounding the nuclei of active galaxies, the IUE was able to help measure the size, temperature and behavior of these high-energy objects. In addition, the satellite played a key role in observing the Supernova 1987A that occurred in the nearby Large Magellanic Cloud galaxy. Scientists were not sure, at first, which of two closely located stars in the galaxy had exploded. The IUE accurately detected which star had exploded, which was significant because Supernova 1987A was the closest supernova to occur since the invention of the telescope. Scientists knew that the star which had exploded had been one they had observed before, but the two possible candidates were different types of stars. IUE's results told scientists that the star that had exploded was the "blue supergiant" star - a fact which surprised researchers but provided significant new information about stars and supernova explosions. IUE also continued to help study the Supernova's debris as it expanded from the initial explosion.
The satellite also confirmed that many other stars in the galaxy had chromospheres, just like our own Sun. In addition, it provided important information about how gas flows behave in binary star systems, disproving one previous theory about how matter flowed to and from companion stars.
The IUE satellite taught us many things about events in our own solar system, as well. Although radio astronomers had previously found radiation belts around the planet Jupiter, IUE made the first  extensive study of auroral activity on the planet.
IUE was also extremely helpful in allowing us to better understand comets. Once, scientists thought comets were little more than dirty ice balls. And the IUE did determine that Halley's comet had tremendous water reserves, spewing off up to ten tons of water per second during its last flyby of our Sun. But the IUE also detected sulfur emissions from the IRAS-Araki-Alcock comet's nucleus, providing solid evidence that comets were much more complex than scientists had once thought. In fact, quantitative evidence of numerous elements has now been detected in comets, lending support to the idea that comets might be cosmic "Johnny Appleseeds," seeding planets in the galaxy with water and other elements necessary for life. Whether or not this is true may depend on whether comets originate inside or outside our solar system - a question that will take further study of comets in order to answer.
By the time the IUE was finally shut down in September 1996, it had accommodated almost four thousand guest observers. Those scientists, in turn, had generated more than 3500 scientific papers from IUE data, making it the most prolific satellite project in NASA's history.28
Extreme Ultraviolet Explorer
As technology progressed even further, it allowed scientists to contemplate telescopes that could observe the universe in more difficult wavelengths, such as the extreme ultraviolet range. Some scientists believed that there would be no use in investigating this short range between ultraviolet and X-ray wavelengths, because hydrogen and helium atoms in their normal state block these emissions. Because hydrogen and helium atoms are the most plentiful elements in the universe, it might stand to reason that a satellite looking for this type of emission from distant sources might not find very much.
But some scientists argued that from a scientific standpoint, we should observe the universe in all available wavelengths, and that we wouldn't know for sure what could be seen in the extreme ultraviolet world unless we at least made the attempt to look. NASA consequently approved another Explorer project, called the Extreme Ultraviolet Explorer (EUVE). The EUVE satellite was built in-house at Goddard, using instruments developed at the University of California, Berkeley, and launched in 1992.
While the number of extreme ultraviolet sources observable in the universe has, indeed, proven to be small, the satellite has discovered many more than most scientists expected. So far, the EUVE has identified approximately 900 stars and 11 galaxies with radiation in the extreme ultraviolet range. What that means is that there are at least portions of interstellar and intergalactic matter that are hotter and/or less dense than expected, allowing extreme ultraviolet radiation to penetrate them and reach the EUVE telescopes. The EUVE observations have enabled us to study very hot gases around stars and  galaxies, helping us in our ongoing effort to piece together an accurate puzzle of what our universe is and how it operates.29
The High Energy Universe
In the early 1960s, most scientists thought that most exciting promise of satellite-based astronomy lay in the ultraviolet wavelengths. But by the end of the decade, scientists realized that the higher regions of the spectrum had even more potential for significant and paradigm-changing discoveries. Some of Goddard's most significant contributions to astronomy, in fact, have been in this high-energy astronomy field.
A group of scientists from the Naval Research Lab (NRL) began exploring X-rays coming from the Sun in 1948, using short-duration sounding rocket...
....flights to get above the Earth's X-ray-absorbing atmosphere. The first X-ray source from outside the solar system was discovered in 1962 by a sounding rocket research group headed by Riccardo Giacconi, from American Science and Engineering, and Bruno Rossi, from the Massachusetts Institute of Technology. The researchers thought they were going to look at solar-induced X-rays from the Moon, but they detected another object in the sky emitting X-rays in far greater quantities than the Sun. Because the source was a star in the Scorpio constellation, it was named Sco X-1. Their results, confirmed a month later by the NRL group, cracked open the door to a universe that until then had been completely hidden from view. They also sparked interest in developing follow-on satellite research projects to probe further into this "invisible" universe.
Throughout the 1960s, bizarre new sources of high-energy radiation were found. One particularly puzzling find was an X-ray source in the Crab Nebula, a gas cloud that was the remnant of a supernova explosion that occured almost a thousand years ago.
A supernova is one possible result when a star uses up the nuclear fuel at its core and "dies." The energy from nuclear reactions at the core of a star are what keep it from collapsing under the weight of its own gravitational field. When those reactions cease, the star will collapse on itself. If it's a small to medium-sized star, like our own Sun, it will become what's known as a "white dwarf." A white dwarf  is about one-one-hundredth the size of the original star, which means that our Sun would end up about the size of Earth. That much compression generates a tremendous amount of heat, turning the star "white-hot" - hence the object's name.
A star five times the size of our Sun will collapse with even more force, generating a tremendous "supernova" explosion that propels most of its material out into intergalactic space. This explosion is so powerful that it can generate energy as intense as the light from 10 billion suns. The shock waves from the explosion can spread as far as 100 light years, seeding the galaxy with heavier elements and heating the interstellar gas enough to trigger the formation of new stars. The material that is left is compressed so far that the atoms themselves are crushed. The positively-charged protons in the nuclei capture the negatively-charged electrons circling them, becoming neutrons. The resulting object is known as a "neutron" star. The collapsed matter in this kind of star is great that a particle 1/10th of an inch in diameter would weigh as much as an aircraft carrier.
When an even larger star explodes, the gravitational weight of the remaining material can be so dense that it collapses in on itself indefinitely, creating a strange phenomenon known as a black hole. The gravitational pull of a black hole is so great that neither matter nor light can escape from it. Black holes are still not well understood, but rocket and satellite research over the last 30 years has taught scientists a great deal more about their existence and behavior.
Astronomers actually had observed white dwarfs as far back as 1862, and had come up for an explanation for the objects by 1933. But until the late 1960s, neutron stars and black holes existed in theory only. The first real evidence of a neutron star was finally found in the Crab Nebula, which had perplexed astronomers ever since they had detected X-ray emissions coming from the cloud in 1962. The X-rays were puzzling because the Crab Nebula was the remnant of a supernova that occurred in 1054 A.D. High energy emissions from an explosion that long ago should have dissipated by now. So the continuing presence of X-ray emissions was a mystery.
The mystery was solved in 1968 when radio astronomers discovered a very strong, regular and rapid pulsing signal coming from deep within the Nebula. At first, the odd signals were nicknamed "LGMs," because some people thought they might be "little green men" signalling us from a distant galaxy. But scientists soon realized that the pulsing signals were from a new class of object that was apparently rotating at a rate that would tear most objects apart - thirty times a second in the case of the Crab Nebula. In order for something to hold together at that rate, it would have to have a staggeringly high density level. This was the clue that led scientists to the discovery of neutron stars.
Scientists deduced that, in the process of being compressed from large star into an ultra-dense remnant core in a supernova...
....explosion, the normal rotation of the star must be accelerated greatly - in some cases as much as 100 million times. The magnetic field of the star also must be amplified by the compression, creating a stream of strong radio waves that emanated from each pole. Because the star was rotating at such high speed, these signals appeared to us as pulses, leading astronomers to name these objects "pulsars."
The discovery of the pulsar-type of neutron star explained the mysterious high-energy emissions from the Crab Nebula and gave scientists another hint about the complexities and amazing phenomena that lay undiscovered in the "invisible" realm of the high-energy universe. It also made astronomers even more interested in exploring this realm from the unencumbered perspective of space.30
The first three satellite projects dedicated to high-energy astronomy were Explorers developed and managed by Goddard. Explorer 42 was the first of the Small Astronomy Satellite (SAS) series, as the three were called. It was launched by a joint team from NASA and the University of Rome, Italy on 12 December 1970, from a platform off the coast of Kenya. Since the satellite was also launched on Kenya's day of independence, it was named "Uhuru," which is the Swahili word for "freedom."
The Uhuru satellite provided the first detailed and accurate view of the X-ray sky, cataloging 161 X-ray sources with luminosities as great as one thousand times the strength of our Sun. That catalog was a significant contribution to the astronomical community. In fact, it was cited more than any other scientific paper in the year it was published.31
The Uhuru satellite also discovered that many, if not all, X-ray stars were part of binary systems in which a very strong, dense collapsed star was actually pulling matter off of a nearby larger star, producing X-ray emissions in the process. Some of these collapsed stars were neutron stars, but Uhuru made an even more significant breakthrough by studying a particularly strong binary X-ray system called Cygnus X-1. In looking at the satellite data, scientists realized that they had found what would prove to be the first tangible evidence of a black hole. In a black hole, matter is pulled off a companion star and pulled into the hole's immense gravity field. As the matter swirls down into the hole it is compressed, heating it to temperatures of tens of millions or even a billion degrees, creating flashes of X-ray and  gamma ray energy before it finally disappears into the black hole.32
Other X-Ray Satellites
Several satellites have pursued X-ray astronomy since Uhuru. In 1975, Goddard launched SAS-3, which looked more closely at some of the X-ray sources mapped by the Uhuru satellite. That work was followed in 1977 by the first of three High Energy Astronomy Observatory (HEAO) satellites. The HEAO projects were managed by the Marshall Space Flight Center, but Goddard provided the project scientist on the first two HEAO satellites, which focused on X-ray astronomy.
HEAO A, like Uhuru, was a scanning mission that mapped the sky in numerous X-ray frequencies. The second observatory, HEAO B, carried the largest X-ray telescope ever built. Two X-ray telescopes had flown successfully aboard Skylab,...
 ....giving scientists the confidence that a focusing X-ray telescope could work on a satellite like HEAO. Since HEAO B's November 1978 launch date was the centennial of physicist Albert Einstein's birth, the satellite was named the "Einstein Observatory." This observatory became an extremely successful project. One of the secondary objectives of project managers was to bring X-ray astronomy into the mainstream of the astronomical community, so the Einstein Observatory was designed to incorporate guest observers. Eventually over 400 guest astronomers used the satellite's telescope.33
The results of these satellites have changed our entire view of the cosmos. The HEAO satellites indicated to scientists that all stars are X-ray sources at some level. Because supernovae, neutron stars and even the matter surrounding black holes emit most of their energy in the X-ray region, these satellites gave astronomers the opportunity to study these incredibly powerful events and objects in the universe for extended periods of time.
X-ray satellites have also found important clues to the structure of both our own Milky Way galaxy and other galaxies in the universe. Before the Einstein Observatory was launched, only three other normal galaxies outside our own had been detected in the X-ray region of the spectrum. By the end of the HEAO project, well over 100 galaxies that gave off X-ray radiation had been identified.
The satellites also found evidence that explosive activity was present in almost all galaxy nuclei, including our own. Some galaxies are more energetic than others, however. Some of the most energetic of these are called "quasars," or "quasi-stellar objects," because they are so distant that they initially appeared to be single stars. These quasars can be detected as far away as ten billion light years and are so powerful that one the size of our solar system would put out more energy than ten trillion of our Suns. The source of this incredible energy is still something of a mystery, but scientists are beginning to find evidence that indicates these galaxies may have tremendous black holes at their centers.
In fact, it's possible that all galaxies begin life as quasars. Early star clusters spiralling toward the center may form a giant star that eventually collapses and forms a black hole. After the black hole at its core devours all the star material near it, the galaxy may settle down, becoming a "normal" galaxy like our own. These quasar galaxies may provide clues to the formation of galaxies in the early universe, because the energy we detect from them originated so far back in time.34
Investigations with X-ray satellites have also given us a better understanding of how galaxies interact with one another. Many of the universe's galaxies, it turns out, are clumped together in clusters. Especially in dense clusters, the galaxies can collide with each other. The friction from these collisions may slow a galaxy down, causing it to spiral in toward the center of the cluster where its stars may be  torn apart by a larger galaxy there that "feeds" on inward-spiralling galaxies. Over time, this would result in galaxy clusters with a giant galaxy at the center surrounded by much dimmer galaxies. This theory was given a boost when HEAO A found evidence of just this type of galaxy cluster.
This process is occurring in our own galaxy, as well. X-ray satellites have shown us that the Milky Way has an active galactic nucleus sending out bright X-ray signals that may be a from a black hole. Our galaxy has also experienced collisions with other, smaller galaxies. Scientists recently discovered a small galaxy that had collided with ours in the Sagittarius constellation, for example. The odd thing about this galaxy was that even though scientists estimated it had orbited around that location about ten times, it was still intact. For a small galaxy not to be ripped apart by the gravitational forces of a larger galaxy with which it collides, it must contain very dense material. It may be that this small galaxy contains a large amount of "dark matter" - one of the great remaining mysteries of the universe.
Dark matter, which may make up as much as 90% of the matter in the universe, cannot be observed. But something beyond the matter we can see must exist in space, or even our own spiraling galaxy cluster should fly apart. Dark matter also may provide the answer to whether our universe will continue to expand or will eventually start collapsing back in on itself. Based on the observable matter we know, there is not enough density in our universe for it to stop expanding. But if there is enough dark matter, with enough mass, the universe may actually have enough density to stop expanding at some point in the distant future.35
Recent X-ray Satellites
Scientists at Goddard and elsewhere are continuing to work together to learn more about how stars and galaxies evolve and behave. Goddard has provided part of the scientific payload on two Japanese satellites designed to further explore the X-ray region of the spectrum. And in 1996, Goddard launched the Rossi X-Ray Timing Explorer (XTE) satellite to take a more precise look at collapsed stars and massive black holes in quasars and galaxies. The XTE satellite is the first U.S-sponsored X-ray mission since HEAO B, and it was designed as an observatory satellite, in that 100% of its observation time is open to guest observers.
The XTE set out to find answers to some of the many remaining questions about the X-ray universe, such as the cause of a mysterious X-ray background radiation, similar to the infrared and microwave background radiation explored by the COBE satellite. And although previous missions have identified active galactic nuclei and neutron stars, scientists still are trying to understand the dynamics of these objects.
Yet in the process of trying to better understand these known objects, the XTE satellite has discovered some even more amazing phenomena. It found a young pulsar spinning twice as fast as any pulsar ever discovered - a remnant of a supernova  four thousand years ago in a nearby galaxy called the Large Magellanic Cloud. This new pulsar is turning at 60 times per second, or twice the rate of the Crab Nebula pulsar, which had been the most energetic pulsar found up until the XTE's discovery. Scientists estimate that this new pulsar may have been turning as rapidly as 150 times per second when it was created. The magnetic field of the new pulsar, on the other hand, is even weaker than that of the Crab Nebula, leading scientists to wonder if neutron stars might progress through a predictable evolutionary process. When they are created, pulsars appear to rotate at very high speeds and have relatively weak magnetic fields. Over time they slow down, and their magnetic fields appear to increase. Indeed, the magnetic fields of these stars may be the reason they slow down over time.
In exploring this phenomenon, the XTE satellite also uncovered a neutron star with what appears to be the most intense magnetic field ever found in the universe. This discovery may confirm the existence of a special class of neutron stars called "magnetars," with magnetic fields estimated to be one thousand trillion times the strength of the Earth's magnetic field. The neutron star associated with this magnetic field is only spinning once every 7.5 seconds, in contrast to faster-rotating neutron stars with weaker magnetic fields. A neutron star born with that great a magnetic field might slow down so quickly that it would be undetectable at the X-ray and radio wave frequencies where most neutron stars are found. So this discovery might account for the large number of supernovae remnants in the galaxy which do not appear to have neutron stars at their centers. It may also help us understand the rate at which stars die and seed the galaxy with the heavier elements necessary for life as we know it.
There is still much we don't understand, however. So the exploration continues. The Advanced X-Ray Astrophysics Facility (AXAF), a large X-ray telescope observatory with high-resolution instruments at least 100 times more sensitive than those on the first HEAO X-ray telescope, is currently scheduled for launch in late 1998. AXAF was originally designed as one of four "great observatories," to explore the universe in all parts of the electromagnetic spectrum. The other three are the planned Space Infrared Telescope Facility (SIRTF), the Hubble Space Telescope, and the Compton Gamma Ray Observatory (GRO). AXAF and SIRTF have been downsized considerably from their original designs, but both should still bring back important clues that will help us better understand our galaxy and our universe.36
Gamma Ray Satellites
Some of the most perplexing and exotic phenomena in the universe are those that emit radiation at the highest frequencies of the electromagnetic spectrum. Gamma rays, which are produced by reactions within an atom's nucleus, are produced deep within the core of stars, but they rarely reach the surface. In most cases, we have only found observable  gamma rays associated with phenomena such as in the birth or death of a star, or around black holes. But phenomena that create emissions at these extraordinarily high frequencies are very unusual in the universe. Out of 500 pulsars detected in the radio and X-ray regions of the spectrum since 1968, for example, only six emit gamma rays.37
Yet gamma rays are evidence of some of the most powerful energy events known to scientists, so they are as fascinating as they are mystifying. As we have improved our satellites and instruments, we have begun to understand a little more about what creates these rare, high-energy sources in the universe. But it is still a strange, perplexing and exciting frontier.
The gamma ray universe has been explored by scientists with Goddard satellites as far back as 1961, when a gamma ray telescope was put into orbit aboard the Explorer 11 satellite. Explorer 11 detected the first gamma ray sources outside our own solar system and helped rule out one of the major competing theories for the evolution of the universe. According to the "steady state" theory of the universe, the universe was neither expanding nor contracting, but stretched infinitely into space and time. Celestial bodies moving away from us would be replaced by the...
 ....formation of new matter, which would create new stars and galaxies. Explorer 11, however, found that the intensity of gamma rays in the universe was a thousand times weaker than what should have existed if the steady-state theory were true.38
Explorer 11 was an important first step into this high-energy field, even though it located only one or two gamma ray sources during its time in orbit. The next step was taken in 1972, when Goddard launched Explorer 48, which was the second spacecraft in the Small Astronomy Satellite (SAS) series. SAS 2 carried a gamma ray telescope and detected numerous additional gamma ray sources. Even more importantly, it laid the groundwork for a larger gamma ray observatory, HEAO 3. This third High Energy Astronomical Observatory, launched in 1979, conducted the first extensive sky survey of gamma ray sources.
HEAO 3 also provided some important clues about what exists at the center of our galaxy. In addition to a continuous spectrum of gamma rays coming out of our galaxy's nucleus, HEAO 3 detected a spike of gamma ray energy at one particular and precise wavelength. An energy surge at that precise frequency happens when electrons meet anti-electrons, or positrons, and annihilate each other. The spike of energy detected by HEAO 3 told scientists that there must be a strong source of positrons, or "anti-matter" at that location, and the most likely source of these extremely high-energy particles is a black hole. In other words, these results told scientists that there might well be a black hole at the center of our galaxy.39
HEAO 3 and SAS 2 were both relatively small satellites with specialized instruments. The next step, then, was to explore the high energy universe across a broad range of the gamma ray portion of the spectrum. This was the mission of Goddard's Compton Gamma Ray Observatory (CGRO), a 17-ton satellite that was launched from the Atlantis Space Shuttle in April 1991. The Compton was named in honor of Dr. Arthur Holly Compton, an astrophysicist whose Nobel prize-winning discoveries about high energy processes are central to the techniques used by the observatory's instruments.
The Compton Observatory's instruments were ten times more sensitive than those on HEAO 3, allowing it to make numerous important contributions to our understanding of gamma ray sources in the universe. One of the most perplexing sources of gamma rays, for example, is something called a "gamma ray burst." These short-lived bursts of high energy...
 ....radiation were first detected by Air Force satellites in the 1960s but have remained a mystery ever since. In a few seconds, one of these bursts can put out more gamma ray energy than our Sun could produce in one thousand years.
Since its launch, the Compton has detected more than a thousand of these bursts. It has also discovered that the bursts are not limited to our galaxy, but are spread evenly across the sky. This has tremendous significance, because it means the bursts are coming from very far distances. For something to appear that energetic after travelling that far a distance, its initial energy has to be staggeringly high. Scientists now believe that gamma ray bursts may originate from locations as far away as 12 billion light years, which means they date back almost as far as the universe itself. Perhaps the most mystifying thing about gamma ray bursts, however, is that no known phenomenon could explain a burst of energy that high.
Several theories have been proposed about the source of gamma ray bursts, including the collision of two neutron stars, or the collapse of a massive giant star into black hole. This second theory was given additional weight recently by a group of researchers from the University of Cambridge in the U.K. Massive stars have very short life spans, so they die pretty much where and when they are born. If gamma ray bursts are linked to the death of these stars, they should correlate with the location and near-time of these stars' birth.
Gamma ray sources are typically very hard to pinpoint, but an Italian-Dutch satellite called BeppoSAX, launched in 1996, gave scientists the ability to nail down the location of these bursts with much more accuracy. The Cambridge scientists plotted the location of three gamma ray bursts detected so far and found that they were located in star-forming regions. The results are far from conclusive, but this evidence certainly gives some additional credence to the theory that gamma ray bursts may be caused by massive stars falling into black holes. In any event, the Cambridge research indicated that gamma ray bursts are extremely rare events, occuring in any galaxy perhaps as seldom as once every 40 million years.40
The gamma ray burst picture got even more complicated when scientists realized that a gamma ray burst detected by the Compton Observatory on 14 December 1997 had put out more energy in a few seconds than any event since the beginning of the universe. For a few seconds, the energy of the burst was as bright as the rest of the universe combined and, in a core region about 100 miles across, it would  have created conditions similar to those that existed one millisecond after the Big Bang. Scientists estimate that the burst originated about 12 billion light years from Earth, and released hundreds of times more energy than a Supernova explosion. Scientists are baffled as to the possible source for such a tremendous burst. All they know is that something obviously exists in the universe that has the ability to produce more energy than any current theory can explain. But this kind of event and mystery is what propels science forward.
Goddard's Compton Observatory also discovered traces of Cobalt 57 that scientists believe was produced by the 1987a Supernova explosion in the Large Magellanic Cloud. This evidence helped confirm that heavier elements in the universe are, indeed, formed in the process of supernovae explosions. In addition, the CGRO satellite has uncovered new objects and phenomena, such as gamma-ray quasars (as opposed to quasars that emit their energy in X-ray wavelengths), identifying a new class of active galactic nuclei.41
Astronomers have been exploring the heavens for centuries. But the ability to go into the heavens themselves has taken us into a whole new dimension. In the past 40 years, rocket and satellite technology has changed dreams and theory into science and knowledge. Because of the efforts of the people at Goddard and its many industry, university, NASA, and international partners, we now can see into the Sun to explore how it works and how it affects life here on Earth. We can map the fields and forces surrounding our own planet and probe the atmospheres of planets further out in our solar system. We have uncovered stars, galaxies and mysterious high-energy phenomena that would have boggled the minds of astronomers even 100 years ago.
In the process, we have found evidence that has supported some important scientific theories and disproven others. We have peered back to the dawn of time and discovered worlds and events that have forced us to change many of our ideas about the galaxy and universe in which we live. In fact, we have sometimes ended up almost falling over ourselves trying to keep up with new discoveries and information that keep changing the picture of our universe just as we thought we understood what was going on.
The truth is, the universe is an astoundingly complex and almost infinitely large territory that we may never fully understand. Columbus, Magellan, and Lewis and Clark may have faced the same element of the unknown as Goddard's space scientists, but a continent or an ocean is a much more limited area than a galaxy or universe. The amazing thing is the amount of headway we've actually been able to make into this mysterious and unforgiving territory in only 40 short years.
In some cases, Goddard's efforts have helped scientists find answers about objects or phenomena, such as the existence and boundaries of Earth's magnetosphere or...
...the origins of pulsars. In others, the Center's work has uncovered pieces of information that we only know must fit into the puzzle somewhere - we just don't know yet how or where, or even how significant these pieces may turn out to be.
But as Goddard scientists point out, astronomer Tycho Brahe made observations in the late 16th century about the orbits of the planets without being able to make any bigger conclusions about them. A few years later, however, mathematician Johannes Kepler used that information to come up with his laws of planetary motion. Those laws later provided Issac Newton with the basis for his universal law of gravity, which transformed science and made modern cosmology possible. In other words, progress and learning are ongoing, incremental processes that require many building blocks to be in place before a building takes shape. And while we are putting together the pieces, we can't always see ahead to see what the building will look like.
Even when a building appears or an "Aha" moment occurs, it's still not the end of the cycle. We continue to build, because we continue to be curious, and there is always something new to learn. It's one of the most wonderful things about life as we know it. The universe we live in is so large and so intricate that as long as our curiosity and spirit of adventure remain alive, there will always be new territory to explore and new wonders to uncover. One of the marvels of space, we have discovered, is that it's not simply a new or final frontier. It's an endless frontier, with the ability to to inspire both our minds and our souls with the power of its elements, the depth of its mysteries, and the beauty of its music.