The Worlds that Wait

The Voyager 2 Saturn encounter in August, 1981, marks the end of an era in planetary exploration. All of the planets known to the ancient astronomers have been visited by spacecraft; the reconnaissance of the inner solar system will be finished. There will be no new planetfall until Voyager 2 succeeds in getting a close view of Uranus in 1986.

Reconnaissance is not enough, however. Many tantalizing questions cannot be answered by spacecraft flashing quickly past these worlds, and new and unexpected questions have been raised by the very data sent back to us.

We are now ready to begin a different phase in the exploration of the solar system, a period of careful, systematic study. For this we will need heavier, more sophisticated spacecraft, longer observation times, and much larger data returns. Such missions and their new generations of instruments will be based on the immense knowledge that we have already obtained.

The next generation of planetary exploration missions which will be launched in the mid-1980s and beyond will begin with the Galileo mission to Jupiter, which is intended to begin the detailed exploration of the outer gas giant planets.

The concept for Galileo is a two spacecraft mission. One is a probe that would plunge into the Jovian atmosphere, measuring the chemical composition as it descends, until, more than 100 kilometers (62 miles) down, it succumbs to the intense pressure. The second spacecraft would go into orbit around Jupiter, continuously photographing the planet's clouds and weather, measuring the magnetic field, and taking close-up pictures of its moons. Galileo would study Jupiter in detail, not on a quick flyby, but over a period of a year or two, and it will bring us a long way toward understanding this giant planet, its huge magnetic field, and its mysterious moons.

Although we have explored the inner terrestrial planets in some detail, one remains hidden. The surface of Venus, beneath its thick, opaque atmosphere, is still largely unknown. We have some crude radar maps made by Pioneer Venus, but their resolution is only a few kilometers, not enough to let us compare the details of Venus with those of the other worlds whose surfaces are in full view.

A new spacecraft mission has been proposed to fill this gap. Called VOIR (for Venus Orbiting Imagingt Radar), it would carry a large radar into orbit around Venus. The radar waves can penetrate the clouds and make accurate images of the surface, detecting features as small as a football field. The resulting maps would cover almost the entire surface of Venus, an area four times as large as the total land area of the Earth, and would be adequate to show such features as volcanoes, meteorite craters, crustal fractures, and river channels.

We will then be able to compare the geology and history of Venus in detail with what we have learned from other worlds, to discover how Venus fits in with the rest of the terrestrial planets.

The year 1985 will be a special occasion for the study of small bodies in the solar system. It will mark the return of Halley's Comet to the neighborhood of the Sun. This bright comet, which returns about once every 76 years, has been sighted on over two dozen visits to the inner solar system since its first recorded appearance in the year 240 B.C. Now, however, for the first time we may go out to meet it. Several countries, including Japan and the USSR, are planning to send spacecraft out to investigate the comet as it swings around the Sun. NASA is considering a coordinated program of Earth- and satellite-based observations when the comet appears in oursky.

Other missions are under study for the more distant future. After Galileo explores Jupiter, Saturn should be the next step in our detailed investigation of the gas-giant planets. Plans are under discussion for a Saturn Orbiter, resembling Galileo, that would make a long survey of the ringed planet and its family of at least 15 moons. The Orbiter might also carry two probes, one to plunge into Saturn's atmosphere and a second to analyze the atmosphere of the large moon, Titan.

Closer to Earth, among the terrestrial planets, Mars still demands our attention. It is better understood and more hospitable than Venus, but it is still a puzzle in many ways. We need to find out the nature of its rocks and soil and to search more thoroughly for possible life. A more sophisticated lander, a sort of super Viking, is one possibility. A more exciting and advanced, yet wholly feasible, idea is to send a robot spacecraft to land, collect a load of Martian rocks and soil, and return to Earth, just as the USSR Luna spacecraft collected samples from the Moon. It may be that only on Earth, with the full resources of our laboratories focused on returned Martian samples, can we finally settle the ancient questions about Mars: its composition, its his tory, and its life or lack thereof.

concept drawing of a Martian resaerch and mining expedition

Prospecting the plains or mars.
A Mars Sample Return mission, shown here according to one design concept, would carry on where the Viking mission left off. This mission would continue the study of the chemical, geological, and physical properties of Mars and would search further for evidence of past or present life by revisiting the red planet. Upon landing, the robot spacecraft would install instruments and operate a small Rover vehicle (left fore ground). Most important, it would collect rock and soil specimens that would be brought back to Earth for laboratory studies by means of a sample return capsule in the spacecraft's ascent stage (streamlined device atop lander spacecraft at right).

The small bodies of the solar system - thousands of asteroids and billions of comets - are also important targets for future space exploration. None of them has ever been visited by a spacecraft, and their compositions and detailed natures are largely unknown. They are primitive objects that date back to the earliest days of the solar system. Comets, preserved in the cold regions beyond Pluto, may give us an unchanged sample of what the original solar nebula was like. The asteroids retain the history of how all the tiny bodies formed and grew in the original solar system, before they were collected into larger worlds like the Earth.

Even after the quick flybys past Halley's Comet in 1986, we will still need to send out more spacecraft to scan these objects at close range, to rendezvous with other comets and study them for long periods of time, to determine the exact nature and composition of asteroidal surfaces, and even to collect and return samples of these bodies to Earth.

To orbit distant worlds like Saturn and Uranus, or to go out to rendezvous with comets or asteroids, we need a boost, a more powerful means of propulsion than we now have. One possible solution is a propulsion system that would operate only in space, the Solar Electric Propulsion System or SEPS. SEPS could be used to carry spacecraft launched from the Space Shuttle even further and faster into space.

The energy source for SEPS will be not fuel but sunlight. Large solar panels on the engine convert sunlight into electricity. The electricity is applied to a supply of atoms, perhaps of the element mercury. The atoms become ionized (electrically charged) and are hurled out of the engine. The reaction from this jet of departing atoms pushes the rocket infinitesimally forward. A SEPS engine can produce only a weak thrust, but the thrust can be applied continuously for weeks or months, until the supply of mercury runs out. Even a very small force will accelerate the spacecraft to a surprisingly high speed when it acts over a long time. SEPS, or something like it, is essential if exploration of the solar system is to continue. Without it, we can do little more than we have done, and many aspects of the solar system will remain a closed book. Our access to distant worlds, and our ability to make more thorough studies of nearby worlds, depend on the thrust that SEPS, or a comparable system, can provide.

Some of the future of space exploration may lie closer to home, on the Moon. We have learned much about the Moon, but it remains mysterious in many ways. We actually have sampled only nine lunar locations. The nature and composition of the rest of the Moon are unknown and much debated. We have no samples at all from the far side of the Moon, that never faces Earth. We do not know what causes the mysterious glows and clouds that have appeared and disappeared on the Moon on various occasions over the past 200 years. We have not explained the fossil magnetism preserved in lunar rocks, nor unraveled the mystery of whether the Moon has an iron core. We have virtually no information about the Moon's polar regions, where water and other gases may still remain as frozen de posits in the permanent shadows.

Apollo was a glorious beginning to lunar exploration, but we cannot answer the above questions until we study the Moon on a systematic global basis. The Russians have already demonstrated, with their automatic Luna spacecraft, how samples can be brought back, at relatively low cost, from additional regions of the Moon.

Another method of global lunar exploration involves a spacecraft that would be placed in an orbit that passes over the Moon from pole to pole. In such an orbit, the spacecraft would eventually scan the entire lunar surface. A battery of instruments would map the chemistry, gravity, magnetism, and thermal properties of the whole Moon. Other instruments would search the polar regions for frozen water in the permanently shadowed areas. This project would complete the mapping begun by Apollo. It would give us the first thorough scientific data base for a whole new world, with which we can better understand the global properties not only of the Moon but of other planets as well.

NASA's consideration of such a mission has not proceeded beyond the study stage. Other countries, including the European Space Agency (ESA), are also considering the polar orbiter project.

The Moon may some day be more than just a scientific treasure. A major concern of our long-term future in space is the question of resources. If we decide to build large structures - power satellites, research stations, or habitats - in space, where will we get the materials? Will we lift them up out of the Earth's strong gravity field at a great cost in fuel, launch facilities, and possible environmental impacts? Or can we use materials that are already present "up there" in space?

Studies of moon rocks and meteorites tell us that the Moon and the asteroids do contain critical and necessary elements: aluminum, silicon, iron, titanium, and oxygen, and even hydrogen and carbon in some asteroids. There is a continuing controversy over whether, how, and when these materials can be mined and used in space. So far, no fundamental barriers have been identified, but there are a host of technical, economic, and social problems to be resolved before a new generation of "forty-niners" goes into space. These discussions and studies should continue, so that we will understand better what we can now do in space if we want to. Only a generation ago, the idea of going to the Moon was science fiction. A generation from now, the mining of the Moon might be routine.