While the bulk of space exploration initially was directed at the earth-moon system, the focus gradually shifted to other members of the solar system. The U.S. Mariner program studied Venus and Mars, the two planets closest to the earth; the Soviet Venera series also studied Venus. From 1962 to 1971, these probes confirmed the high surface temperature and thick atmosphere of Venus, discovered signs of recent volcanism and possible water erosion on Mars, and investigated Mercury. Between 1971 and 1973 the Soviet Union launched six successful probes as part of its Mars program. Exploration of Mars continued with the U.S. Viking landings on the Martian surface. Two Viking spacecraft arrived on Mars in 1976. Their mechanical arms scooped up soil samples for automated tests that searched for photosynthesis, respiration, and metabolism by any microorganisms that might be present; one test suggested at least the possibility of organic activity. The Soviet Phobos 1 and 2 missions were unsuccessful in 1988. The U.S. Magellan spacecraft succeeded in orbiting Venus in 1990, returning a complete radar map of the planet's hidden surface. The Japanese probes Sakigake and Suisei and the European Space Agency's 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 put into orbit in 1995. Launched in 1996 to study asteroids and comets, the Near Earth Asteroid Rendezvous (NEAR) probe made flybys of the asteroids Mathilde (1997) and Eros (1999) and began orbiting the latter in 2000. The Mars Pathfinder and Mars Global Surveyor, both of which reached Mars in 1997, were highly successful, the former in analyzing the Martian surface and the latter in mapping it. The ESA Mars Express, launched in 2003, began orbiting Mars later that year, and although its Beagle 2 lander failed to establish contact, the orbiter has sent back data. Spirit and Opportunity, NASA rovers, landed successfully on Mars in 2004.
Space probes have also been aimed at the outer planets, with spectacular results. One such probe, Pioneer 10, passed through the asteroid belt in 1973, then became the first object made by human beings to escape the solar system. In 1974, Pioneer 11 photographed Jupiter's equatorial latitudes and its moons, and in 1979 it made the first direct observations of Saturn. Voyagers 1 and 2, which were launched in 1977, took advantage of a rare alignment of Jupiter, Saturn, Uranus, and Neptune to explore all four planets. Passing as close as 3,000 mi (4,800 km) to each planet's surface, the Voyagers discovered new rings, explored complex magnetic fields, and returned detailed photographs of the outer planets and their unique moons. Launched in 1989, the Galileo spacecraft followed a circuitous route that enabled it to return data about Venus (1990), the moon (1992), and the asteroids 951 Gaspra (1991) and 243 Ida (1993) before it orbited Jupiter (1995–2003); it also returned data about the Jupiter's atmosphere and its largest moons (Io, Ganymede, Europa, and Callisto). The joint U.S.-ESA Cassini mission, launched in 1997, began exploring Saturn, its rings, and some of its moons upon arriving in 2004. It deployed Huygens, which landed on the surface of Saturn's moom Titan in early 2005.
