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Mission to Jupiter
Editor's Note: This is the 26th in a series of essays
on exploration by Steven J. Dick.
The recent
publication in the NASA History series of Michael Melzer's Mission to
Jupiter: A History of the Galileo Project details another extraordinary
voyage of exploration, and highlights the trials, tribulations and triumphs in
the long process required for success in planetary science. The Galileo
spacecraft was launched Oct. 18, 1989, arrived at Jupiter on Dec. 7, 1995, and
undertook its tour of Jupiter over the next two years.
This Prime
Mission would have been quite enough to declare success, but it was followed by
the Galileo Europa Mission, extending for two more years until December 1999.
During this time the spacecraft studied not only studied Europa but also other
Jovian satellites, especially (and appropriately) the other "Galilean
satellites" discovered by Galileo almost 400 years before. Aside from
satellites, the Galileo spacecraft would also study the magnetic fields,
particles and atmosphere of Jupiter itself, notably with its releasable
Atmospheric Probe. Even then, Galileo's explorations were not over; its lifetime
was further extended by the Galileo Millennium Mission, which ran from January,
2000 through the end of mission operations in January 2003. Galileo's death
occurred by design when it was sent on a collision course with Jupiter on Sept.
21, 2003 and burned up in Jupiter's dense atmosphere.
Galileo
represented a new phase in the study of the outer planets. The Pioneer probes
and Voyagers 1 and 2 together completed the preliminary reconnaissance of those
gas giants. But Galileo undertook a much more systematic, in-depth, and holistic
analysis of the entire Jupiter system, featuring close flybys of its satellites,
and intense study of its particles and fields. And even before the spacecraft
reached Jupiter it visited Venus, two asteroids, and twice returned to Earth,
along the way observing a spectacular celestial crash.
 The Galileo spacecraft and
its Inertial Upper Stage booster rocket were deployed from the space shuttle
Atlantis October 18, 1989. Shortly thereafter, the booster rocket fired and
separated, sending Galileo on its six-year journey to the planet Jupiter. Upon
its arrival at Jupiter in December 1995, Galileo released a probe into the
atmosphere to survey the composition of the planet's clouds. The orbiter has
relayed probe information, surveyed its surroundings, and photographed Jupiter
and the Galilean satellites, among others. Image credit:
NASA.
Like many spacecraft, Galileo had an extended and painful
birth. The total mission from launch to destruction lasted 14 years, but the
"Jupiter Orbiter Probe" project from which it emerged had begun another 14 years
before launch. Funding was won from Congress in 1977 and the project was renamed
Galileo the following year. The spacecraft was originally scheduled to be
deployed in 1984 aboard the Space Shuttle, which would not fly its first mission
until April 1981. Once deployed in Earth orbit, the plan was for it to be
launched by a Centaur upper stage toward Jupiter. Meanwhile, in the early 1980s
the ever-watchful Office of Management and Budget proposed not only that Galileo
should be cancelled, but that planetary exploration should be halted and JPL
shut down. All three made it through that crisis, and in December 1985 the
spacecraft was transported to the Kennedy Space Center for launch on the Shuttle
Atlantis.
The following month, in one of NASA's tragic and memorable
moments, the Space Shuttle Challenger was lost with all its crew at launch.
Return to flight for the Shuttle program would take almost three years. During
that time it was decided the liquid-hydrogen-fueled Centaur was too dangerous to
take aboard the Shuttle, and that a solid-fueled Inertial Upper Stage would be
used instead. Because of the lower thrust of the IUS, and in order to minimize
fuel consumption, Venus and two Earth gravity assists (affectionately termed
VEEGA) replaced the direct trajectory, greatly lengthening the travel time to
Jupiter.
Like its birth, Galileo's journey was full of drama -- as full
as any of the ocean voyages in the Age of Discovery (minus the natives). Four
months after launch it reached Venus for the first gravity assist. By December
1990 the spacecraft had returned to Earth for another gravity assist. Several
months later, the mission encountered a major problem. Now far enough from the
Sun, attempts were made to deploy its high gain antenna, but to no avail. That
meant data return would be severely limited. Engineers were forced to use the
low gain antenna, analogous to using a dim light bulb compared to a powerful
searchlight. Nevertheless, in due course and with a great deal of patience and
ingenuity, 70 percent of the mission's scientific goals were completed. In the
midst of this problem the spacecraft proceeded to the asteroid belt, where
Galileo encountered Gaspra on October 29, 1991, capturing spectacular images. A
year later it was back to Earth for its second Earth gravity assist.
On
Aug. 28, 1993 Galileo was in the asteroid belt again, encountering Ida at a
distance of only 1,500 miles. Among other surprises, images examined months
later revealed a moon of Ida, later named Dactyl. Finally Jupiter was in
Galileo's sights, but meanwhile unprecedented events were unfolding at Galileo's
final destination. Comet Shoemaker-Levy 9 had broken up, and from July 16-29,
1994 21 pieces of the comet collided with Jupiter, giving it a spectacular
series of "black eyes."
Still 140 million miles from Jupiter, Galileo
captured images nonetheless, even as most telescopes on (and orbiting) Earth
were trained on the event from their much more distant vantage point. At the U.
S. Naval Observatory where I was working at the time, I recall our astonishment
that we could see the dark splotches on Jupiter, even with small telescopes and
from the middle of Washington, D.C. A year later, at 39 million miles from
Jupiter, Galileo survived the most intense interplanetary dust storm ever
recorded. At about this time, with 147 days to go to Jupiter, the Jupiter
Atmospheric Probe separated from Galileo, in freefall toward its
destination.
 This "family portrait," a composite of the
Jovian system, includes the edge of Jupiter with its Great Red Spot, and
Jupiter's four largest moons, known as the Galilean satellites. From top to
bottom, the moons shown are Io, Europa, Ganymede and Callisto. Europa, the
smallest of the four moons, is about the size of Earth's moon, while Ganymede is
the largest moon in the solar system. North is at the top of this composite
picture in which the massive planet and its largest satellites have all been
scaled to a common factor of 15 kilometers (9 miles) per picture element. The
Solid State Imaging (CCD) system aboard NASA's Galileo spacecraft obtained the
Jupiter, Io and Ganymede images in June 1996, while the Europa images were
obtained in September 1996. Because Galileo focused on high resolution imaging
of regional areas on Callisto rather than global coverage, the portrait of
Callisto is from the 1979 flyby of NASA's Voyager spacecraft. Image credit:
NASA.
By the time Galileo finally arrived at the Jupiter system in
December 1995, the comet impact sites had dissipated. But the spacecraft had
plenty to observe. On the day of arrival the Atmospheric Probe plunged into
Jupiter, slowed by drogue parachutes, while Galileo flew by Europa and Io. The
spacecraft then settled down to its Prime Mission Orbiter tour, which consisted
of 11 orbits of Jupiter. They were not normal orbits, but orbits that unfolded
in a flower petal pattern while maximizing scientific visits to the Jovian
moons. Thus, the first orbit lasted 7 months, culminating in a close encounter
with Ganymede. Ganymede, Callisto and Europa were visited multiple times over
the next two years as part of the Prime Mission. During the extended Galileo
Europa Mission, Io was also visited, mapping its cloud of charged particles and
the density of sulfur streaming from Io's erupting volcanoes.
Further
observations of Io and Jovian system dynamics were made during the Millennium
Mission extension, including the eruptions of Io's Loki volcano, the largest and
most powerful in the solar system. Europa proved no less fascinating, with its
water-ice surface and large ocean lurking beneath. And the Atmospheric Probe
results proved revolutionary. As Melzer puts it, Probe data "made it necessary
for scientists to revisit many of their beliefs about the formation and
evolution of our solar system's giant gaseous planets. Measurements of
atmospheric composition, wind velocities, temperatures, cloud characteristics,
electrical storms and elemental and molecular abundances painted a very
different picture of Jupiter from what was expected." Galileo found a turbulent
Jovian atmosphere, complete with lightning and thunderstorms a thousand times
the size of those on Earth. Distinct from the Pioneer and Voyager planetary
flyby missions, Galileo was the first to orbit the giant planet, and completed a
total of 34 orbits by the end of its mission in 2003.
Melzer also makes
clear how complex spacecraft development has become, both in terms of
institutions, technology and highly trained individuals interacting at many
levels. The lead NASA field center for the project was the Jet Propulsion
Laboratory in Pasadena, California, with oversight from the Solar System
Exploration Division at NASA Headquarters in Washington. NASA's Ames Research
Center developed the Jupiter probe that entered the planet's atmosphere. NASA's
Lewis Research Center (now Glenn) in Cleveland helped design the spacecraft
propulsion system and integrate the payload with the shuttle. The shuttle
itself, as well as the astronauts, were under the jurisdiction of Johnson Space
Center in Texas, while Kennedy Space Center provided the launch facilities. The
Department of Energy provided the controversial plutonium-powered electrical
generators for the spacecraft. Finally, the Galileo project involved
international cooperation with the Federal Republic of Germany, which provided
instrumentation, propulsion for the Orbiter and additional telemetry from the
German Space Operations Center.
Galileo the man would have marveled at
Galileo the machine. And we should marvel too at what we can do, and see, in the
new Age of Discovery.
Further Reading
Melzer, Michael.
Mission to Jupiter: A History of the Galileo Project (NASA SP-2007-4231,
NASA History Division, Washington, 2003). Available from the NASA Center for
Aerospace Information, 301-621-0390, or online. This book is also available
online at http://history.nasa.gov/sp4231.pdf.
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