GALILEO

unmanned American spacecraft that explored the planet Jupiter and its moons in the mid- and late 1990s. Its 11 instruments were designed to provide observations more detailed and extensive than those recorded by the Pioneer and Voyager spacecraft, as well as to gather data on the composition and behaviour of Jupiter's atmosphere, its moons and rings, and the magnetic field surrounding the planet. Galileo also carried a probe designed to enter Jupiter's atmosphere. The Jovian system is considered in some respects to be a miniature model of the early solar system and could provide scientists with a better understanding of planetary formation and evolution. Asteroid Ida and its satellite. Seen at a distance of about 10,870 km (6,755 miles), Ida shows the Originally scheduled for launch on the Challenger space shuttle in May 1986, Galileo was carried into space by the Atlantis on Oct. 18, 1989. Because of the delay, the probe could not travel directly to Jupiter, instead following a spiral trajectory that took it once past Venus (Feb. 10, 1990) and twice past Earth (Dec. 8, 1990, and Dec. 8, 1992). During its voyage, the satellite transmitted pictures and data about Venus, the Earth's Moon, and the asteroids Gaspra (Oct. 29, 1991) and Ida (Aug. 28, 1993), discovering a satellite around the latter. Vortices in Jupiter's southern hemisphere. The oval on the left is a cyclonic vortex, rotating in a On July 13, 1995, Galileo released a 339-kg (747-pound) probe intended to plunge into Jupiter's atmosphere and transmit data back to Galileo, which would in turn transmit the data to Earth. The probe descended into Jupiter's atmosphere near the planet's equator on Dec. 7, 1995, the same day that Galileo became the first artificial satellite to go into orbit around the planet. Equipped with six instruments, the probe gathered data on the Jovian atmosphereits temperature, pressure, and density, its chemical composition, its cloud layers, and radiation and electrical activity. It transmitted data for about 57 minutes before being destroyed by intense atmospheric heat and pressure. During the next two years, Galileo made flybys of three of the larger moons (the intense radiation surrounding Io prevented the spacecraft from passing too closely to that satellite) and transmitted images and other data about Jupiter and its 16 satellites. Despite the failure of its primary high-gain antenna and difficulties with its main onboard memory (a four-track tape recorder), the spacecraft provided scientists with large quantities of data, as well as dramatic images of objects like Jupiter's Great Red Spot and white ovals. After its primary mission ended in December 1997, a second two-year mission focusing on the satellites Europa and Io was begun.