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Space exploration
Space probes



A space probe is an unpiloted 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 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 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 Earth to determine conditions in space. Changes in course and speed can provide information about atmospheric density and gravity fields.

Early unpiloted 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 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 analyzed the space environment between 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 3,700 miles (6,000 kilometers) 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 program made a series of successful soft landings beginning in 1966. Between 1970 and 1972, three Soviet probes returned lunar soil samples to Earth in small capsules. Two of them sent remote-controlled jeeps called Lunokhods, which traveled 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.

The United States space probe Clementine orbited the moon from February to May 1994. The probe photographed the moon extensively. In addition, Clementine measured the height and depth of mountains, craters, and other features, and gathered data on mascons. In January 1998, another U.S. probe, Lunar Prospector, went into orbit over the moon's poles. The probe found strong evidence of large amounts of frozen water mixed with the soil at both poles.

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. These probes passed inside the orbit of Mercury to measure solar radiation. The Ulysses probe was launched in 1990 by the United States and the European Space Agency (ESA), an association of 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. However, the capsule 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 Mars'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 would have orbited Mars. Contact 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 24.5 inches long, 18.7 inches wide, and 10.9 inches high (63 by 48 by 28 centimeters). Its mass was 11.5 kilograms, equivalent to a weight of 25.4 pounds on Earth.

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, and the probe relayed the information to Earth.

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

In 1996, the United States launched a probe called the Mars Global Surveyor to map the planet's surface. The probe used a laser device to determine the elevation of the Martian surface. That instrument produced maps of the entire surface that are accurate to within 3 feet (1 meter) of elevation. Another instrument determined the composition of some of the minerals on the surface. A camera revealed layered sediments that may have been deposited in liquid water, and small gullies that appear to have been carved by water.

In 2001, the United States launched the Mars Odyssey probe to Mars. The craft carried instruments to help identify minerals on the surface, to search for evidence of water and ice beneath the surface, and to measure radiation that might harm any future human explorers. In 2002, Mars Odyssey discovered vast quantities of ice within 3 feet (1 meter) of the surface, most of it near the south pole.

Contributor: James Oberg, M.S., Spaceflight Engineer; author, UFOs and Outer Space Mysteries.
Source : World Book 2005

Space exploration
What is space?
Getting into space and back
Living in space
Meeting basic needs in space
Communicating with Earth
The dawn of the space age
Space probes
Probes to Venus
Probes to Jupiter and beyond
Probes to comets
Human beings enter space
Apollo: Mission to the moon
Exploring the moon
Returning to Earth
The International Space Station
Space shuttles
Types of shuttle missions