Galileo (spacecraft) - astronomy.

Galileo (spacecraft) - astronomy. I INTRODUCTION Galileo (spacecraft), American unmanned spacecraft designed to orbit Jupiter and send a probe into its atmosphere. Launched on October 18, 1989, from the space shuttle Atlantis, the Galileo orbiter followed a circuitous route before reaching Jupiter in 1995. It then moved into orbit around the planet and began monitoring and photographing Jupiter's turbulent atmosphere, four largest moons, and surrounding magnetosphere. An atmospheric probe was released from the orbiter and dropped into the Jovian clouds on December 7, 1995, to take chemical and physical measurements of this heretofore unseen environment. Originally scheduled to launch in 1986 on a more direct, two-year flight to Jupiter, the Challenger shuttle explosion forced the National Aeronautics and Space Administration (NASA) to revise plans and budgets for the mission. NASA engineers instead devised a looping six-year journey in which Galileo would dip into the gravitational fields of Venus and Earth to pick up enough velocity to reach Jupiter. This 38-month Venus-Earth-Earth Gravity Assist ended with the second, and final, Earth flyby before arriving at Jupiter in 1995. II SPACECRAFT The 2,222-kg (4,899-lb) Galileo orbiter has two sections. One section spins at a rate of several times per minute to help stabilize the spacecraft. Instruments to detect low-energy charged particles, high-energy and potentially dangerous charged particles, and cosmic and Jovian dust are mounted on this rotating segment. Other instruments study waves generated in planetary magnetospheres and by lightning discharges. Galileo's magnetometer sensors, designed to measure planetary magnetic fields, are mounted on a boom 11 m (36 ft) in length to escape interference from the spacecraft. A second section of the orbiter is stationary and carries the instruments that require stability: a high-resolution camera system; a near-infrared mapping spectrometer and an ultraviolet spectrometer to help analyze the chemistry of Jupiter's atmosphere; a photo-polarimeter radiometer to measure radiant and reflected energy; and a dish antenna to track the probe as it enters Jupiter's atmosphere while relaying data to Earth. The orbiter is powered by converting the natural radioactive decay of plutonium 238 dioxide into electricity. The Galileo orbiter delivered a 346-kg (760-lb) atmospheric probe to Jupiter and relayed data collected by the probe to Earth. The 86-cm (34-in) diameter probe was powered by a lithium-sulfur battery. It contained instruments to study Jupiter's atmospheric structure, temperatures, clouds, and chemical composition, and to detect the presence of lightning within 12,000 km (8,000 mi) of the entry point. III EARLY MISSION HIGHLIGHTS On its long route toward Jupiter, Galileo first encountered the planet Venus on February 10, 1990. At Venus, Galileo's instruments studied the planet's environment in search of charged particles, collected data for infrared maps of its lower atmosphere, made infrared and ultraviolet spectral observations, and took more than six dozen photos. Fourteen months after its launch, Galileo first passed by Earth and the Moon. Eleven months later, on October 29, 1991, Galileo sped past the asteroid Gaspra and took the first close-up photos of an asteroid. On December 8, 1992, the spacecraft cruised by Earth and the Moon again--this time close enough to find evidence of Earth's polar stratospheric clouds, thought to play a role in the destruction of the ozone layer. Galileo also relayed back footage of the Moon's north side in unprecedented detail. On August 28, 1993, Galileo encountered the asteroid named Ida. Cameras revealed surface details of Ida as small as 40 m (131 ft) across, and found the first-ever asteroid moon, named Dactyl (for the nymph Ida's child by Zeus in Greek mythology). In late July 1994, while still 18 months away from Jupiter, Galileo was able to photograph Jupiter's far side when more than 20 fragments of the Comet Shoemaker-Levy plunged into the planet's atmosphere over a six-day period. Despite Galileo's nearly flawless mission, critical technological failures occurred. In 1991 the spacecraft's umbrella-like, high-gain antenna failed to open completely, and scientists were forced to rely on a smaller antenna that transmits information 100 times slower than the larger antenna. With the data flow from the spacecraft reduced to a trickle, engineers installed new compression software and upgraded hardware on earthbound systems to receive the data more efficiently. Further troubles developed in October 1995 when the onboard tape recorder became stuck in the "rewind" position for 15 hours, wearing out a section of the tape. Engineers were able to instruct the recorder to move more slowly and to avoid the bad section of tape. IV JUPITER ENCOUNTER On July 13, 1995, five months before Galileo's arrival at Jupiter, the atmospheric probe was released from the spacecraft to fly on its own toward the giant planet. The probe entered the Jovian atmosphere on December 7, 1995, at 2:04 PM, Pacific Daylight Time (PDT), and began its fiery descent at a speed of more than 160,000 km/h (100,000 mph), deploying its 2.5-m (8-ft) parachute. Two minutes later the craft dropped its protective heat shields so that the probe could collect data. The probe radioed atmospheric data to the orbiter for relay to Earth. Scientists were surprised when preliminary probe data indicated that Jupiter had much less water than expected, but further research indicated that the probe entered Jupiter's atmosphere in a particularly dry spot. Toward the end of its descent, the probe detected winds of up to 530 km/h (330 mph) with intense turbulence, suggesting that Jupiter's winds are driven by heat escaping from the planet's interior. This differs from planets like Earth, Venus, and Mars, whose winds are driven by solar energy. The probe found less helium, neon, carbon, oxygen, and sulfur than expected. And it discovered that lightning occurs on Jupiter only about one-tenth as often as on Earth. As expected, the probe encountered no solid objects or surfaces during its entire 600-km (373-mi) plunge. After 57 minutes, the extreme temperature and pressure of Jupiter's atmosphere destroyed the probe. The Galileo orbiter continued to transmit data to Earth while orbiting Jupiter and making repeated passes of Jupiter's large moons Ganymede, Europa, Io, and Callisto. Galileo has revealed that the three moons Ganymede, Europa, and Io have fairly strong magnetic fields, which means that the moons probably have cores of liquid metal. Molten metal cores provide heat, which could make the moons hospitable to some forms of life (see Exobiology). Galileo's photographs of Europa and Ganymede suggest that these moons may have vast oceans of liquid water underneath their frozen surfaces. V EXTENDED MISSION Galileo's mission was initially scheduled to end in December 1997, but NASA approved funding for an extended mission devoted to further investigation of Jupiter's atmosphere, a look at Io's volcanoes, and an extensive study of Jupiter's moon Europa. Galileo's observations support the theory that Jupiter's most visible storm, the Great Red Spot, sustains itself on energy gained from the upper atmosphere, perhaps by absorbing the energy of smaller atmospheric disturbances. In 1999 Galileo took detailed photographs of Io's volcanic eruptions; scientists hope to use the observations to learn more about similar volcanic activity that occurred on Earth eons ago. Observations taken by Galileo in 2000 lent strength to the theory that Europa may have an ocean of liquid water. The spacecraft's magnetometer recorded regular changes in the direction of Europa's magnetic field, changes that would result from an underlying material that conducts electricity, such as a salty ocean. Such a material would be affected by the regular changes that occur in Jupiter's magnetic field at Europa's position. Galileo's extended mission ended in early 2000, but with the orbiter still functioning, scientists designed a new extended mission to take advantage of the spacecraft's unexpectedly long life. Galileo repeatedly dove closer to Jupiter than any spacecraft before it, passing through the intense radiation of the planet's magnetic field to take a very close look at Jupiter and its innermost moons, Io and Amalthea. When the spacecraft's fuel ran low in 2003, NASA set Galileo on a collision course with Jupiter to prevent it from crashing into one of Jupiter's moons and contaminating it with bacteria from Earth. Galileo burned up when it entered Jupiter's atmosphere traveling at high speed--about 48 km per second (30 mi per second). Contributed By: Dennis L. Mammana Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.