Planetary Exploration, Future of

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Planetary Exploration, Future of

The first artificial satellites launched into Earth orbit were part of an international scientific program called the International Geophysical Year. They returned data on Earth and its space environment. Before long, the United States and the Soviet Union began sending spacecraft to study the Moon and, later, other planets.

Sending Spacecraft to the Planets

The U.S. Ranger spacecraft (1961-1965) were designed to crash into the Moon, transmitting television images right up to the moment of impact. The Surveyor spacecraft (1966-1968) soft-landed on the Moon, verifying that the lunar surface would support an Apollo lander, taking pictures of the surface surroundings, and performing the first crude geochemical analyses of lunar rocks. A series of Lunar Orbiter spacecraft photographed the Moon, developed the film onboard, scanned the developed images, and transmitted the scans to Earth. The Orbiter photographs constituted the primary database for planning the Apollo landings and comprised the only global set of pictures available to scientists for over twenty-five years.

A Soviet Zond spacecraft (1965) returned the first pictures of the far-side of the Moon, the side that always faces away from Earth. Although of poor quality, the pictures showed features that the Soviets were allowed to name through international agreements. Thus, there are names such as Gagarin and Tsiolkovsky for craters on the farside. The Soviet Luna series of spacecraft performed several landings on the Moon, returning pictures and other data. Three of those spacecraft, Luna 16 (1970), Luna 20 (1972), and Luna 24 (1976), acquired lunar surface material by drilling and returned the samples to Earth. No other robotic spacecraft has ever collected extraterrestrial material and returned it to Earth.

The United States explored the inner planets (sometimes called the terrestrial planets) with the Mariner series of spacecraft (1962-1973). The tiny Pioneers 10 (1973) and 11 (1974) were sent hurtling past Jupiter and Saturn, taking crude pictures and measuring magnetic fields and charged particles. These Lilliputian explorers are still flying far beyond the planets, returning data on the farthest reaches of the Sun's influence over interstellar space. Following Pioneer were the larger and more capable Voyagers 1 (September, 1977) and 2 (August, 1977), which completed remarkable journeys, passing all of the large outer planets—Jupiter, Saturn, Uranus, and Neptune. The Voyager mission took advantage of a rare alignment of the outer planets that allowed the spacecraft to receive gravitational boosts at each planet, which were necessary to complete the journey to the next planet.

Until recently, the Soviet Union was the only other nation to attempt planetary exploration. Although all Soviet missions to Mars have failed, the Soviets achieved a unique and amazing success by landing two Venera spacecraft (1981) on the surface of Venus. These landers survived the hellish surface environment long enough to return pictures and send back geochemical data on surrounding rocks.

Exploring a Planet in Stages

The exploration of a planet by spacecraft can be characterized in terms of several stages or levels of completeness: reconnaissance , orbital survey, surface investigation, sample return, and human exploration. The first stage, reconnaissance, is accomplished through a flyby of the planet. As the spacecraft passes, pictures are taken, measurements of the environment are made, and navigation data is accumulated for future missions.

In the second stage, orbital survey, a spacecraft is placed in orbit around the planet. A planetary photographic database is accumulated and remote sensing observations are conducted. Depending on the sensors aboard the spacecraft, data may be collected on planetary surface composition; atmospheric composition, structure, and dynamics; the nature of the gravity field, which can yield information about the internal structure of the planet; and the nature of the magnetic field. A series of orbiters may be flown over time, observing different phenomena or improving the level of detail and resolution of the data.

Eventually, a landing is made on the planet for surface investigation. Two Viking landers settled onto the surface of Mars in 1976 to test for signs of biological activity in the Martian soil. Actual soil samples were placed in special chambers on the lander. Similarly, certain geochemical or geological measurements cannot be made remotely. A lander can also observe the planet at small scales that cannot be imaged from orbit. In some cases, a rover can leave the lander and explore the surroundings. Such was the case of the small rover named Sojourner on the Pathfinder mission to Mars (1996).

As scientists accumulate knowledge about a planet, they seek answers to questions of increasing complexity. At some point, the measurements required are too complex and demanding to be carried out on a robotic spacecraft of limited capability. A sample return mission can bring pieces of the planet to laboratories on Earth, where the most qualified experts using equipment of the highest sophistication can examine them. Preservation of the scientific integrity of the sample has the highest priority—during collection on the planet, during transit to Earth, and after delivery to a special facility for curation. If the samples are cared for appropriately, they become treasures for future scientists with ever more advanced analytic techniques.

The final stage of study is human exploration. Astronaut explorers, aided by robotic assistants, can observe, experiment, innovate, and adapt to changing conditions in ways that cannot be duplicated by machines. Besides, why should robots have all the fun?

Future Exploration of the Solar System

By the beginning of the twenty-first century, reconnaissance had been completed for the inner solar system and for the giant planets of the outer solar system. Orbital surveys have been accomplished at the Moon (Lunar Orbiters, Clementine, Lunar Prospector), Venus (Magellan), Mars (Mariner 9, Viking, Mars Global Surveyor), and Jupiter (Galileo). The Cassini spacecraft was en route to Saturn, with exploration of that planet expected to begin in 2004. Some asteroids have been photographed by passing spacecraft, and the NEAR Shoemaker spacecraft engaged in a rendezvous with the asteroid Eros. Samples have been returned only from the Moon. Meteorites collected on Earth are samples from (unknown) asteroids, from the Moon, and from Mars. Human exploration has occurred briefly and only on the Moon.

The National Aeronautics and Space Administration (NASA) has future plans for more orbital surveys of Mars as well as landings and sample return missions. Human exploration of Mars is being discussed, as is further human exploration of the Moon, without definite commitments. The European Space Agency (ESA), as well as individual European nations, will join NASA in exploring Mars. European and Japanese spacecraft will visit the Moon. India and China are discussing possible Moon missions. Some private companies have plans to land on the Moon through profit-seeking ventures. ESA is planning an orbital survey of Mercury. The Japanese space agency, the Institute of Space and Aeronautical Science, is working on a sample return from an asteroid.

Planetary exploration is becoming an international activity. Equally exciting is the prospect of planetary missions sponsored by institutions other than the traditional government agencies. The future may hold surprises for all of us.

see also Apollo (volume 3); Apollo Lunar Landing Sites (volume 3); Earth—why Leave? (volume 4); Exploration Programs (volume 2); Mars Missions (volume 4); Reconnaissance (volume 1); Robotic Exploration of Space (volume 2).

Wendell Mendell