The Unmanned Exploration of the Solar System: Mariner, Viking, Pioneer, and Voyager
The Unmanned Exploration of the Solar System: Mariner, Viking, Pioneer, and Voyager
On January 2, 1959, the Soviet Union launched Luna 1, the first manmade object designed to explore another celestial body. Luna 1 passed about 3,600 miles above the moon's surface, performing some basic scientific observations before entering a solar orbit. Since then, space probes—primarily American and Soviet/Russian—have explored every major body in the solar system except Pluto. They've landed on the Moon, Mars, and Venus; flown through the erupting dust and gases of Halley's comet; mapped asteroids; and dropped probes into the atmospheres of Jupiter, Venus, and (soon) one of Saturn's moons, Titan. In the latter half of the twentieth century, our vision of the solar system changed from that of small, blurred dots in a telescope to a collection of real, unique planets. As William Burrows said in Exploring Space, "It was the interplanetary exploring machines . . . simultaneously drawing in the edges of the world and expanding them."
Throughout history, mankind has looked toward the skies. Even before recorded history, people noticed that while most stars seemed fixed in the sky, some of the brightest ones moved. These were called planets (meaning wanderers) by the ancient Greeks. Powers were attributed to them, and elaborate mythologies arose to explain their origins and characters. Until just a few centuries ago, our understanding of the planets stayed at about that level of sophistication. Then Galileo's telescope began to put a face to the planets, while Copernicus, Kepler, and Newton explained their motions. But even in 1960 man's knowledge of the planets was limited to blurry telescopic images seen through a thick and dirty atmosphere from millions to billions of miles away. Although our theorizing grew ever-more elaborate, we had yet to see what lay outside—or inside, for that matter—Earth's orbit.
Our close-up exploration of the solar system began with the Soviet probe Luna 1, but that gave only fleeting views of our nearest neighbor. The first probe designed to explore beyond Earth's orbit was launched in July 1962. Unfortunately, instead of traveling to Venus, Mariner 1 ended up at the bottom of the Atlantic Ocean. In December 1962 Mariner 2 became the first probe to send back information from another planet, Venus. Throughout the decade the United States and Soviet Union sent probes to Mercury, Venus, and Mars, sending back photos and data from each of these planets, largely on fly-by missions that scooted past, never to return. But, by the end of the 1960s, the inner solar system was becoming known, if only in passing. This culminated with the landing of the Viking landers on the surface of Mars in 1976.
In 1972 the first missions were launched to explore the outer solar system. Sent as pathfinders to prove contemporary technology and navigation, Pioneers 10 and 11 were launched a year apart to explore Jupiter and Saturn, and to pave the way for the follow-up Voyager missions. The Pioneer spacecraft performed superbly, returning a tremendous amount of data. They were followed by the more sophisticated Voyager probes, which were able to visit all four major planets in a single "Grand Tour" of the solar system, a once-in-175-years opportunity. The Galileo, Cassini, and Magellan probes followed, each dedicated to studying a single planetary system for several years. At the same time, orbital observatories launched by NASA, Europe, and Japan helped expand our knowledge of the rest of the universe even more dramatically; other missions visited comets, asteroids, and observed the Sun.
Key to this entire process was the development of technology that could send large space probes to any point in the solar system and return scientifically valuable data. These missions required powerful rockets, compact instruments, navigational software, onboard computers, electrical generators, and communications equipment that could function for decades over billions of miles. Even missions closer to home required care, as evidenced by the loss of the billion-dollar Observer.
Unmanned exploration of the solar system has been largely successful, returning enormous amounts of information and tens of thousands of images. The success was hard-won, however. The unmanned space program continually fought for funding against the space shuttle and space station programs, and was nearly abandoned on a number of occasions. Despite the public acclaim that accompanied each new set of spectacular photos, the memory of failure lingered.
If scientific and technological advances made exploration of the solar system possible, space programs helped drive earthbound technology, too. The lack of abundant sunlight in the outer solar system led to the development of radioisotopic thermal generators for electrical power, new imaging techniques for photography, and ways to stabilize cameras while a spacecraft sped by a planet or moon at tens of thousands of miles per hour. Problems such as Galileo's jammed high-gain antenna or Voyager's flighty tape recorder forced engineers and scientists to devise clever diagnostic and repair procedures for a spacecraft several light-hours away. Weight and power limitations forced them to discover ways of transmitting data using no more power than is needed by a refrigerator lightbulb. Finally, although more prosaically, this data had to be plucked from background noise, stored, analyzed, and cataloged so that scientists decades later could locate, retrieve, and read it.
In addition to overcoming these technological obstacles, NASA had to surmount political and economic challenges as well. In the closing days of the Apollo program, the American public lost interest in space exploration as their attention turned toward the Vietnam War and domestic problems. NASA's funds were cut, forcing the cancellation of at least two planned lunar landings. To maintain public interest and government funding, NASA promoted their man-in-space program, the centerpiece of which was the space shuttle. It worked—both the public and legislature were intrigued. Congress may well have been attracted by the number of jobs the program would create; the fact that many of those jobs would be in vote-rich California appealed to both presidents and presidential aspirants alike.
Unfortunately, time and cost overruns began to tarnish the space shuttle's image and, by association, all of NASA. Unreliable schedules, cancelled launches, and the Challenger explosion all diminished NASA in the eyes of both Congress and the public. The subsequent loss of the Observer, early problems with the Hubble Space Telescope, loss of the Mars Climate Orbiter, and the Galileo antenna problems only added to the public's skepticism about NASA. Fortunately, the agency had successes as well: the Mars Pathfinder, the repaired Hubble Space Telescope and other orbital observatories, the Magellan mission to map Venus, and Galileo all performed extraordinarily well.
The public and Congress alike, however, seemed to take success for granted while castigating NASA for failures. At the same time, increased public and political pressure to balance the U.S. budget placed even more financial stress on NASA. As of this writing, NASA continues to operate under financial pressure, although the future seems a little brighter. The heady Apollo days of virtually unlimited budgets are unlikely to return, forcing NASA to continue to budget and consider carefully all space exploration for the foreseeable future. Their new motto: "faster, better, cheaper."
Other nations are also launching their own spacecraft. Despite efforts by the Japanese, Europeans, and Russians, however, the United States remains the only nation to send spacecraft beyond Mars—and the only one to have both the capability and plans to continue solar system exploration for the near future.
In spite of its problems, space exploration has captured public imagination and changed the perception of man's place in the universe more than any other enterprise in history. As we learn about our solar system and the universe, we develop a better appreciation for Earth's value and fragility. We've seen that the universe is awe inspiring, beautiful, vast—and largely empty. A photo of Earth rising beyond the desolate lunar surface taken by the crew of Apollo 8 is a dramatic emphasis of this fact.
The discovery of other planetary systems suggests we might not be alone in the universe, although no planet hospitable to life has yet been found. Given this situation, we can continue to hope and dream, but we must still consider the possibility that Earth and its inhabitants represent the only outpost of life we will know. This realization may prove to be the most lasting legacy of space exploration.
P. ANDREW KARAM
Burrows, William. Exploring Space. New York: Random House, 1990.
Chaikin, Andrew. A Man in the Moon. New York: Penguin Books, 1994.
Kippenhahn, Rudolf. Bound to the Sun. New York: WH Freeman and Company, 1990.
Miner, Ellis. Uranus: The Planet, Rings, and Satellites. New York: Ellis Horwood, Ltd., 1990.
Morrison, David. Exploring Planetary Worlds. New York: Scientific American Library, 1993.
NASA'S WORK ON ADVANCED SPACECRAFT PROPULSION SYSTEMS
Since Robert Goddard's work in the early part of the twentieth century, most rockets have been fueled by chemical reactions, burning either solid or liquid propellants. In the last few decades of the twentieth century, NASA began experimenting with alternate spacecraft propulsion systems, searching for smaller, lighter, faster, and more efficient ways to move spacecraft around the Solar System. These included solar sails, ion drives, and an electromagnetic drive that uses a large cloud of ionized gas to pull a spacecraft with it away from the Sun. Of these systems, the most efficient would be the solar sail. Constructed of a huge sheet of mylar or some other light plastic, a solar sail would be driven by the pressure of photons hitting it, driving it through the solar system. It is most efficient because, once beyond the atmosphere, it would sail forever, requiring no further fuel or propellant. Ion drives, already demonstrated on some small spacecraft, use electricity to generate a stream of ions, or charged atoms. These are driven from the ship in a stream, thrusting the craft in the opposite direction. Ion drives are more efficient than chemical rockets but still require a fuel supply. NASA's hope is that one of these drives, or something not yet conceived, might prove the key to opening the Solar System to manned exploration by making interplanetary trips faster and much less expensive. Until then, NASA is likely to continue developing and testing advanced propulsion systems on their unmanned spacecraft.