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A space probe is an unmanned device sent to explore space. A probe may operate far out in space, or it may orbit or land on a planet or a moon. It may make a one-way journey, or it may bring samples and data back to the earth. Most probes transmit data from space by radio in a process called telemetry. 

Lunar and planetary probes that land on their targets may be classified according to their landing method. Impact vehicles make no attempt to slow down as they approach the target. Hard-landers have cushioned instrument packages that can survive the impact of a hard landing. Soft-landers touch down gently. Penetrators ram deeply into the surface of a target.

How a space probe carries out its mission. Probes explore space in a number of ways. A probe makes observations of temperature, radiation, and objects in space. A probe also observes nearby objects. In addition, a space probe exposes material from the earth to the conditions of space so that scientists can observe the effects. A probe may also perform experiments on its surroundings, such as releasing chemicals or digging into surface dirt. Finally, a probe's motion enables controllers on the earth to determine conditions in space. Changes in course and speed can provide information about atmospheric density and gravity fields.

Early unmanned explorations. Beginning in the 1940's, devices called sounding rockets carried scientific instruments into the upper atmosphere and into nearby space. They discovered many new phenomena and took the first photographs of the earth from space. 

The 1957 launch of Sputnik 1 marked the beginning of the space age. Sputnik 1 carried only a few instruments and transmitters, but it paved the way for the sophisticated probes that would later explore space. 

Many early satellites probed uncharted regions of space. During the late 1950's and the 1960's, the Explorer satellites of the United States and the Kosmos satellites of the Soviet Union analysed the space environment between the earth and the moon. United States Pegasus satellites recorded the impacts of micrometeorites. During the early 1970's, Soviet Prognoz satellites studied the sun.

Lunar probes. In 1958, both the United States and the Soviet Union began to launch probes toward the moon. The first probe to come close to the moon was Luna 1, launched by the Soviet Union on Jan. 2, 1959. It passed within about 6,000 kilometres of the moon and went into orbit around the sun. The United States conducted its own lunar fly-by two months later with the probe Pioneer 4. The Soviet Luna 2 probe, launched on Sept. 12, 1959, was the first probe to hit the moon. One month later, Luna 3 circled behind the moon and photographed its hidden far side. 

The Soviet Union began to test lunar hard-landers in 1963. After many failures, they succeeded with Luna 9, launched in January 1966. The U.S. Surveyor programme made a series of successful soft landings beginning in 1966. Between 1970 and 1972, three Soviet probes returned lunar soil samples to the earth in small capsules. Two of them sent remote-controlled jeeps called Lunokhods, which travelled across the lunar surface. 

Beginning in 1966, the United States sent five probes called Lunar Orbiters into orbit to photograph the moon's surface. The Lunar Orbiters revealed the existence of irregular "bumps" of gravity in the moon's gravitational field caused by dense material buried beneath the lunar seas. These areas of tightly packed matter were called mascons, which stood for mass concentrations. If the mascons had not been discovered, they might have interfered with the Apollo missions that sent astronauts to the moon.

                                                              APOLLO SPACECRAFT

Solar probes. Beginning in 1965, the United States launched a series of small Pioneer probes into orbit around the sun to study solar radiation. Many of these probes were still operating more than 20 years after launch. In 1974 and 1976, the United States launched two German-built Helios probes, which passed inside the orbit of Mercury to measure solar radiation. The Ulysses space probe was launched in 1990 by the United States and the European Space Agency, an association of 14 European nations. In 1994, Ulysses became the first probe to observe the sun from an orbit over the sun's poles.

Probes to Mars. The Soviet Union launched the first probes aimed at another planet, two Mars probes, in 1960. However, neither probe reached orbit. After more Soviet failures, the United States launched two Mariner probes toward Mars in 1964. Mariner 4 flew past the planet on July 14, 1965, and sent back remarkable photographs and measurements. The probe showed that the atmosphere of Mars was much thinner than expected, and the surface resembled that of the moon. 

In 1971, the Soviet probe Mars 3 dropped a capsule that made the first soft landing on Mars, but it failed to return usable data. That same year, the U.S. probe Mariner 9 reached Mars and photographed most of the planet's surface. Mariner 9 also passed near and photographed the planet's two small moons, Phobos and Deimos. 

Two U.S. probes, Viking 1 and Viking 2, landed in 1976 and operated for years, measuring surface weather and conducting complex experiments to detect life forms. The probes found no evidence of life. 

In 1992, the United States launched the probe Mars Observer. In 1993, NASA lost contact with the probe three days before it was scheduled to go into orbit around Mars. Communication was never restored, and the probe was presumed lost. 

The United States launched the Pathfinder probe in December 1996. The probe landed on Mars on July 4, 1997. Two days later, a six-wheeled vehicle called Sojourner rolled down a ramp from the probe to the Martian surface. The vehicle was only 63 centimetres long, 48 centimetres wide, and 28 centimetres high. Its mass was 11.5 kilograms. 

The vehicle used a device called an alpha proton X-ray spectrometer to gather data on the chemical makeup of rocks and soil. Sojourner transmitted this information to Pathfinder, which relayed it to the earth. 

Scientists on the earth controlled Sojourner. However, because radio take about 10 minutes to travel from the eart to Mars, the scientists could not control Sojourner in real time--that is, as it moved. To avoid obstacles, Sojourner used a number of automatic devices.

Probes to Venus. The Soviet Union launched the first probes toward Venus in 1961, but these attempts failed. The first successful probe to fly past Venus and return data was the U.S. Mariner 2, on Dec. 14, 1962. Mariner 5 flew past Venus in 1967 and returned important data. Mariner 10 passed Venus and then made three passes near Mercury in 1974 and 1975. 

Soviet attempts to obtain data from Venus finally succeeded in 1967. Venera 4 dropped a probe by parachute, and it transmitted data from the planet's extremely dense atmosphere. In 1970, Venera 7 reached the surface of Venus, still functioning. Between 1975 and 1985, several other probes landed and conducted observations for up to 110 minutes before the temperature and pressure destroyed them. In 1978, the United States sent two probes to Venus, Pioneer Venus 1 and 2. Pioneer Venus 1 was an orbiter. Pioneer Venus 2 dropped four probes into the planet's atmosphere. 

Probes that orbited Venus generated rough maps of the planet's surface by bouncing radio waves off the ground. Pioneer Venus 1 used radar to map most of the planet's surface to a resolution of about 80 kilometres. This means that objects at least 80 kilometres apart showed distinctly on the map. In 1983, two Soviet probes carried radar systems that mapped most of the northern hemisphere of Venus to a resolution of 1.5 kilometres. The U.S. probe Magellan reached Venus in 1990 and mapped almost the entire surface to a resolution of about 100 metres.

Probes to Jupiter and beyond. Probes to the outer planets--Jupiter, Saturn, Uranus, Neptune, and Pluto--must meet special challenges. Radiation belts near Jupiter are so intense that computer circuits must be shielded. The dim sunlight at the outer planets requires lengthy camera exposures. And the vast distances mean that radio commands take hours to reach the probes. 

The United States launched its first probes to Jupiter, Pioneer 10 and Pioneer 11, in 1972 and 1973. After observing Jupiter, Pioneer 11 was redirected toward Saturn, arriving there in 1979. It was renamed Pioneer-Saturn. From 1979 to 1981, sophisticated Voyager probes provided much more detailed data on Jupiter and Saturn. These probes continue to explore space. Voyager 2 flew past Uranus in January 1986 and Neptune in August 1989. The probes sent back spectacular photos of the outer planets and their rings and moons, and recorded huge amounts of scientific data. Active volcanoes were found on Io, a moon of Jupiter, and geysers were discovered on Triton, a moon of Neptune. Other moons exhibited bizarre formations of ice and rock. 

The Galileo space probe, launched on a mission to Jupiter by the United States in 1989, was far more sophisticated than earlier planetary probes. It consisted of two parts--an atmosphere probe and a larger orbiting spacecraft. Galileo reached Jupiter in 1995. 

Thus, by 1989, the only planet not yet visited was Pluto.

                                                                FLIGHT OF VOYAGER 2

Probes to comets. Two Soviet probes flew past Venus and dropped instruments into the planet's atmosphere, then intercepted Halley's Comet as it passed by the sun in 1986. In 1985, the European Space Agency launched its first interplanetary probe. The probe, called Giotto, passed closer to the comet's nucleus than any other probe. Giotto returned dramatic close-up images of the comet. Japan also sent two small probes. After several years of inactivity, Giotto was reactivated to fly past the comet Grigg-Skjellerup in July 1992. 

The United States did not send a probe to Halley's Comet because of budget limitations. However, NASA scientists realized that a small probe already in space could be diverted to explore another comet. The International Sun-Earth Explorer 3 satellite had spent several years in space between the earth and the sun. In 1983, its course was shifted into interplanetary space, and it was renamed the International Cometary Explorer. On Sept. 11, 1985, it passed a comet named Giacobini-Zinner, becoming the first probe to reach a comet.