Solar System Exploration: 1970-2000

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Solar System Exploration: 1970-2000

Overview

Between 1970 and 2000, solar system exploration included major missions to most of the planets. The United States sent out many unmanned spacecraft that studied the planets, their moons, and even asteroids. The Soviet Union's planetary missions included the Venera spacecraft series, some of which landed on Venus and sent back pictures from the surface. The Soviets, the Japanese, and the European Space Agency all sent probes to study Halley's Comet in 1985-86.

Background

By 1970 humans had landed spacecraft on the moon but not on any planet. By 1999, however, spacecraft had landed on and mapped Mars and Venus, our nearest planetary neighbors. Mariner 10 visited Mercury, and the Voyager missions visited all four of the gas giant planets. The Galileo probe spent nearly four years studying Jupiter and its four largest moons. Even some of the smallest bodies in the solar system, asteroids and comets, were studied by unmanned missions.

Impact

In 1974 Mariner 10 visited Mercury, the closest planet to the sun. Mercury's surface is heavily cratered and barren, and scientists originally thought it was similar to the moon. Mariner 10 data, however, revealed that Mercury began as a mostly molten planet that was deformed by the tidal influences of the Sun. As it cooled and shrank, its surface cracked and shifted as its shape became rounder. Mariner 10 revealed the cracks and ridges that resulted, and also indicated that Mercury must have a huge iron core not far below the surface.

Mariner 10 was also the first spacecraft to get close-up pictures of the thick atmosphere that surrounds Venus and hides its surface. Pioneer Venus 1 began orbiting the planet in 1978, using radar to map the surface through the clouds. It also discovered significantly more sulfur dioxide in the planet's atmosphere than scientists had measured from Earth. The sulfur dioxide levels slowly dropped, and astronomers concluded that a volcano had erupted on Venus just before Pioneer 1 arrived, pumping sulfur dioxide into the atmosphere. Pioneer Venus 2 sent four small probes through the atmosphere to measure it. One survived its landing and sent back data from the surface for an hour.

The Soviet Venera series of spacecraft visited Venus in the 1970s and sent back photographs and other data. Several landed on the surface, where they briefly studied surface rocks and took photographs before being crushed by the pressure and heat of the atmosphere.

The Magellan missions in 1990 and 1994 used radar to map Venus's surface in great detail. Magellan was able to see features as small as 400 feet across. It revealed an utterly dry surface containing large volcanic mountaintops and few craters. The tectonic forces that shape Earth are nonexistent on Venus, which has a single thick plate rather than many thinner plates that can move across its surface.

Earth's moon was the object of two missions in the 1990s. In 1994 Clementine mapped most of the moon's surface. It was followed by lunar prospector, which orbited the moon for 1.5 years, returning data about the moon's resources, magnetic field, and gravity. The prospector was able to confirm the existence of water ice in permanently shaded regions near the moon's south pole.

Probes to Mars in the 1960s examined mostly the southern hemisphere and seemed to discover a cratered, dead world. Mariner 9, which landed in a global dust storm in 1971, studied the northern hemisphere after the dust began to settle, and discovered four huge volcanoes, winding valleys that resembled dried riverbeds, and an enormous canyon complex that was named Valles Marineris after the spacecraft.

The Viking missions in the 1970s included two orbiters and two landers. The latter established a base from which data was returned about the surface of Mars and the weather in its thin atmosphere. Viking included an experiment that scooped up a small sample of the Martian soil and looked for indications of living things; it found none. However, if water had ever existed on the surface of Mars, as the Mariner results seemed to indicate, it was possible that primitive life had existed there as well. In the 1990s, NASA examined the planet's geological history, in particular the possible past existence of surface water, with several missions.

In July 1997, Pathfinder bounced to a landing and released the Sojourner rover. Pathfinder and Sojourner sent back data for nearly three months, well beyond their expected lifetimes. They returned a wealth of scientific data about the rocks on the martian surface, including a particular area that looks like it was once a floodplain. In March 1999, Global Surveyor began a Mars mapping mission. It has sent back stunning pictures revealing previously unseen details of the martian surface.

The Voyager missions to the outer planets began in the 1970s. Voyagers 1 and 2 neared Jupiter in 1979. The Voyagers studied weather systems in Jupiter's atmosphere, including rotation of the Great Red Spot, and observed lightning and auroras in the Jupiter's clouds. The Voyagers also observed the four largest moons of Jupiter, discovering the first nonterrestrial volcano as it erupted on the moon Io, and discovered three new moons. The discoveries made by the Voyager spacecraft were expanded by the Galileo mission in the 1990s.

The Galileo mission, beginning in December 1995, studied Jupiter and its moons for four years. After a two-year primary mission, its two-year extended mission included many flybys of its four largest moons. As a daring finale to the extended mission, Galileo made two very close passes of the volcanic moon Io, coming within 200 miles (322 km) of the surface. Io harbors the hottest volcanoes in the solar system, and the latest images show both active volcanoes and past lava formations. Galileo greatly extended our knowledge of Jupiter's moons, including evidence suggesting the presence of subsurface oceans on Europa and Callisto, and a thin atmosphere and magnetic field at Ganymede (the largest satellite in the solar system and so far the only one known to have its own magnetic field).

Voyagers 1 and 2 continued on to Saturn after studying Jupiter, arriving in 1979 and 1981, respectively. They examined the weather systems on Saturn (which were similar to those on Jupiter) and mapped the rings in exquisite and initially baffling detail. Scientists were able to unravel the evidence and figure out some of the intricate mechanisms by which the gravity of Saturn and some of its moons produced features like the braided effect observed in some of the rings.

Voyager 1 was scheduled to take many photographs of Saturn's largest moon, Titan. However, Voyager showed that the moon has a thick cloud layer that prohibits visual observation. The atmosphere itself turned out to be interesting, probably resembling that of the Earth in its earliest years. This makes Titan a possible candidate for the future development of simple life forms like those that first arose on Earth.

Voyager 2 continued on to Uranus, arriving in 1986. Uranus itself presented a bland appearance, with a thick atmosphere covering a warm ocean surrounding a hot rocky core. Voyager 2 was able to measure the temperature in different areas of the planet's atmosphere, but the most exciting find of its Uranus mission was the surface of the moon Miranda. Miranda is tiny, and scientists expected it to be too small for the kind of geological forces that create interesting surface features. However, Miranda shows some of the most dramatic topography in the outer solar system, with strange grooves, white markings, and a very distinctive chevron pattern.

In 1989 Voyager 2 arrived at Neptune, which, like Jupiter and Saturn, shows weather patterns in its atmosphere. Neptune's atmosphere is a deep blue, a sign that it contains large amounts of methane. Neptune has darker blue spots (some with white clouds) that contain giant storm systems. In examining these weather systems, Voyager found Neptune to be the solar system's windiest planet. It also revealed that Neptune's moon, Triton, is one of the very few moons (along with Titan) that have an atmosphere, although a very thin one. Voyager 2 also made the surprising discovery that this icy moon appears to have geysers of nitrogen or methane gas that erupt nearly five miles up from the surface before being carried sideways by the wind.

It wasn't until 1978 that astronomers discovered that Pluto had a satellite, the moon Charon, and even then the only evidence they had was a bulge on one side of images of Pluto. This tiny, distant, and icy world is still largely a mystery. No spacecraft have yet visited Pluto, though the Hubble Space Telescope has provided pictures that show Pluto and Charon as two distinct bodies, as well as patterns of light and dark areas on the surface of Pluto.

In 1985-86 the return of Halley's Comet to the inner solar system gave scientists an opportunity to study a comet closely. The Soviet Vega mission took photographs of the nucleus at the center of the comet, and the Giotto mission flew only 400 miles (644 km) away, returning images of dark, carbon-rich rock. Giotto was redirected to Comet Grigg-Skjellerup, passing within 125 miles (201 km) of that comet's center in 1992. Early in the twenty-first century the Stardust mission is scheduled to fly close to the Comet Wild 2 and return of a small sample of the comet's gases and dust to Earth.

In 1991, on its way to Jupiter, Galileo observed the asteroid Gaspra, providing the first close-up view of one of these irregularly shaped chunks of rock. Galileo also flew by the asteroid Ida and discovered that it had a tiny companion moon, the first time such a thing had been observed.

The twentieth century saw dramatic advances in our knowledge of the solar system. As the century drew to a close, new solar system missions were beginning. The Cassini probe is on its way to Saturn, Stardust is on its way to a comet, and NASA is planning a series of Mars missions for the early years of the twenty-first century.

MARY HROVAT