space probe, space vehicle carrying sophisticated instrumentation but no crew, designed to explore various aspects of the solar system (see space exploration). Unlike an artificial satellite, which is placed in more or less permanent orbit around the earth, a space probe is launched with enough energy to escape the gravitational field of the earth and navigate among the planets. Radio-transmitted commands and on-board computers provide the means for midcourse corrections in the space probe's trajectory; some advanced craft have executed complex maneuvers on command from earth when many millions of miles away in space. Radio contact between the control station on earth and the space probe also provides a channel for transmitting data recorded by on-board instruments back to earth. Instruments carried by space probes include radiometers, magnetometers, and television cameras sensitive to infrared, visible, and ultraviolet light; there also may be special detectors for micrometeors, cosmic rays, gamma rays, and solar wind. A probe may be directed to orbit a planet, to soft-land instrument packages on a planetary surface, or to fly by as close as a few thousand miles from one or more planets. The particulars of trajectory and instrumentation of each space probe are tailored around the mission's scientific and technological objectives; the data provided by a single space probe may require months or even years of analysis. Much has been learned from probes about the origins, composition, and structure of various bodies in the solar system. Scientists trying to understand the earth's weather by constructing theoretical models of global weather systems make use of the knowledge that is gained concerning the atmospheres and meteorology of the planets. Because conditions on other planets are simpler than on earth, scientists can check each of their hypotheses separately in isolation from complicating factors.
The earliest space probes in the U.S. space program were the Mariner series, which investigated Mars, Venus, and Mercury, and the Pioneer series, which explored the outer planets. Pioneer 10 was the first human-made object to entirely escape the solar system. Several Viking space probes voyaged to Mars in the late 1970s, mapping the planet and searching for life. The two Voyager probes, launched in 1977, returned spectacular photos and data from brushes by Jupiter, Saturn, Uranus, Neptune, and their moons, and continued toward and then through the heliosphere, where the effects of the sun's solar wind interact with the interstellar magnetic field, becoming the first space probes to leave the solar system and enter interstellar space in 2012 and 2018. The Magellan spacecraft succeeded in orbiting Venus in 1990, returning a radar map of the planet's hidden surface. The Japanese probes Sakigake and Suisei and the European Space Agency's (ESA) probe Giotto both rendezvoused with Halley's comet in 1986, and Giotto also came within 125 mi (200 km) of the nucleus of the comet Grigg-Skjellerup in 1992. The U.S. probe Ulysses returned data about the poles of the sun in 1994, and the ESA Solar and Heliospheric Observatory (SOHO) was orbited in 1995. Launched in 1989, the Galileo spacecraft followed a circuitous route that returned data about Venus (1990), the moon (1992), and the asteroids 951 Gaspra (1991) and 243 Ida (1993) before it reached Jupiter in 1995 and sent a small probe into the Jovian atmosphere to study its composition. Over the next eight years it orbited Jupiter 35 times, returning data about the planet's atmosphere and also about Jupiter's largest moons, Io, Ganymede, Europa, and Callisto. The joint U.S.-ESA mission Cassini, launched in 1997, explored Saturn, its rings, and some of its moons from 2004 to 2017 and deployed the lander Huygens on the surface of Saturn's moom Titan. The Mars Pathfinder and Mars Global Surveyor, both of which arrived at the red planet in 1997, were highly successful, the former in analyzing the Martian surface and the latter in mapping it. Both the Mars Climate Orbiter and Mars Polar Lander, however, were lost upon their arrival at Mars in 1999, setting NASA's Mars exploration program back by at least two years. In 2003 the ESA's Mars Express achieved orbit around Mars, and although its Beagle 2 lander proved unsuccessful, the orbiter has returned data on the planet. NASA's Spirit and Opportunity rovers landed on the planet shortly afterward in early 2004. In 2008 NASA's Phoenix lander touched down in the planet's north polar region, and in 2012 the NASA rover Curiosity landed near the equator. In 2018 joint ESA-Roscosmos ExoMars Trace Gas Orbiter it began looking for gases indicative of biological or geological processes as it orbited Mars, but the Schiaparelli lander, which was launched with it, crashed into the surface in 2016. NASA returned to Jupiter in 2016 with the space probe Juno, which was launched in 2011 and flew by the earth in 2013; Juno carries instruments designed to examine the nature and properties of Jupiter.
The NEAR (for Near Earth Asteroid Rendezvous)-Shoemaker probe returned data about the asteroid Mathilde as it flew by in 1997 and the asteroid Eros as it orbited it in 1999 and 2000 and then landed on its surface in 2001, returning unparalleled data about a minor planet. Other probes have since studied asteroids, including ESA's Rosetta; Japan's Hayabusa, which returned with samples of the asteroid Itokawa in 2010, and Hayabusa2, which returned with samples of Ryugu in 2020; NASA's Dawn, which orbited and studied Vesta (2011–12) and Ceres (2015–); and NASA's Osiris-Rex, which visited Bennu (2018–). In 2014, Rosetta, which had also conducted a flyby of Mars (2007), went into orbit around Comet 67P and then studied it as it approached the sun, heated up and reacted, and then moved away from the sun. Pluto was visited for the first time in 2015 by NASA's New Horizons (launched 2006); the space probe later (2018) flew by Arrokoth. Located in the Kuiper belt, it became the most distant object to be visited by a space mission. The first space probe to orbit a comet, Rosetta also deployed a lander on the comet and crash-landed into the comet's surface at the end of its mission (2016). Messenger, launched by NASA in 2004, became in 2011 the first space probe to orbit Mercury, and continued to study Mercury until its mission ended in 2015; it also made flybys of Venus (2006–7) and Mercury (2008–9).
See W. E. Burrows, Exploring Space (1990); R. D. Launius et al., NASA and the Exploration of Space (1998); D. Fischer, Mission Jupiter: The Spectacular Journey of the Galileo Space Probe (1999); J. Kluger, Journey beyond Selene: Remarkable Expeditions past Our Moon and to the Ends of the Solar System (1999); S. J. Pyne, Voyager: Seeking Newer Worlds in the Third Great Age of Discovery (2010).
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