On October 18, 1989, the space shuttle Atlantis lifted off from Kennedy Space Center in Florida on mission STS-34 with the Galileo spacecraft on board. The shuttle astronauts released Galileo in Earth orbit, then the craft used its two-stage inertial upper stage rocket to boost itself toward Venus.
The spacecraft swung by Venus once and Earth twice as part of "gravity assist" maneuvers. These are maneuvers in which a spacecraft flies in close enough to a planet to get a boost from the orbital momentum of a planet traveling around the Sun. NASA compares a gravity assist to throwing a ping pong ball to skim along the top of one of the moving blades of an electric fan. The blades circle the fan's motor at a high rate of speed. The ping pong ball gets close enough to the blade to pick up momentum and shoot off in a different direction. Using gravity assists during space flight saves on fuel. This is particularly important for long journeys to the outer solar system.
By July 1995 Galileo was nearing Jupiter. It released the probe, which was about four feet in diameter and three feet long, and the probe began a five-month plunge toward the planet. On December 7, 1995, the orbiter was in position when the probe began its final descent at more than 105,000 miles per hour. For nearly an hour the heavily protected probe transmitted data about Jupiter's atmosphere, temperature, and weather. It was finally destroyed by the intense heat and pressure surrounding the planet. It had penetrated 124 miles into the violent atmosphere.
The orbiter spacecraft spent the next eight years in orbit around Jupiter. It conducted numerous flybys of the moons Europa, Ganymede, and Callisto, using its eleven scientific instruments to collect data about radiation, magnetic fields, charged particles, and cosmic dust.
The Galileo orbiter was originally designed for a two-year mission. It ended up lasting for fourteen years. In September 2003 NASA scientists destroyed the spacecraft by purposely plunging it into Jupiter's atmosphere. The orbiter was running low on propellant. The scientists feared that it could run out of fuel and crash into one of Jupiter's moons. This could contaminate environments that might possibly contain water and life forms.
The Galileo mission was hugely successful. The spacecraft traveled more than 2.8 billion miles during its long journey. It flew by two asteroids, Gaspra and Ida, on its way to the planet and watched Comet Shoemaker-Levy 9 impact Jupiter while it was in orbit there. Galileo captured thousands of detailed images of the planet and its largest moons and collected a wealth of data about these celestial objects.
Major findings attributed to, or confirmed by, the Galileo mission include:
- There is an intense radiation belt around Jupiter
- The surface of the moon Io is constantly being reshaped by heavy volcanic activity
- There is evidence of liquid water oceans beneath the icy surface of the moon Europa and possibly Callisto
- The moon Ganymede has its own magnetosphere and probably its own magnetic field
- Ganymede is heavily cratered from impacts of comets and asteroids and has icy plains, mountains, and basins likely caused by geologic forces
- Ganymede has a thin ionosphere (electrically charged atmosphere)
FIGURE 8.3
The Cassini spacecraft - Ganymede, Europa, and Io all appear to have metallic cores
The possibility of liquid water is considered very strong for the moon Europa and somewhat likely for the moon Callisto. These conclusions are based on analysis of geological and magnetic data collected by Galileo.
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