Space Galileo: Jupiter and the Galilean Satellites

The Mission

Galileo deployed from Shuttle Atlantis, Oct 18, 1989. picture courtesy NASA

As the first missions at the gas giants had been mostly flyby missions, Galileo was the first spacecraft ever to orbit a gas giant. Craft, until now, had just passed by the planet, continuing their route further after the flyby. Such orbiting craft first appeared in missions turned towards the inner solar system like the Viking missions at Mars or Magellan at Venus. Galileo had been named for Galileo Galilei, the famed Italian astronomer who was the first ever to use a telescope (as a part of that, he had discovered the main, four satellites of Jupiter). The Galileo craft had been designed by the Jet Propulsion Laboratory (JPL), as it was carrying an atmospheric probe which had been developed by NASA's Ames Research Center. This probe was to dive into Jupiter's atmosphere at arrival. Galileo had as a mission a comprehensive study of Jupiter and of the Galilean satellites, that is the four main Jupiter's moons

The mission, first known as Jupiter Orbiter/Probe (JOP) had received Congressional approval for flight in 1977, targeting a Space Shuttle launch in late 1981. Delays in the Shuttle Program had led to incremental slips of the launch first to 1984 and then to 1986. The upper stage to send it from Earth orbit toward Jupiter changed from the Inertial Upper Stage (IUS) to the high-energy liquid-hydrogen fueled Centaur, back to the less powerful IUS, and then back to the Centaur. The January 1986 Challenger accident on the other hand, put all Shuttle flights on hold, and a thorough safety review led NASA to cancel the Shuttle Centaur combination in June 1986. Galileo was reprogrammed for launch using the IUS on a Shuttle flight in 1989. Using the less-powerful IUS meant Galileo would need three gravity-assist maneuvers, one at Venus and two at Earth, to achieve the velocity required to reach Jupiter, the so-called VEEGA (Venus-Earth-Earth Gravity Assist) trajectory. This increased travel time from two to six years, and since it was traveling closer to the Sun than previously anticipated, engineers installed additional thermal shielding on the spacecraft.Galileo launched from Cape Canaveral on October, 18, 1989, riding in the payload bay of the Space Shuttle Atlantis, whence it was to start its flight. That was another premiere -- and until now a unique occurrence -- for a planetary mission. Once the Space Shuttle on orbit, Galileo was thus expelled from it, as it was propelled one hour hour after deployment from the Shuttle, and was flying free of the rocket stage 47 minutes later, as he undertook a 6-year journey using two Earth, and one Venus flybys. Galileo successfully flew at Venus by Feb. 10, 1990 -testing its instruments and discovering lightnings- then the Earth on Dec. 8, 1990 -detecting chemical signatures in atmospheric trace elements associated with life form activity. A second flyby of the Earth took place on Dec. 8, 1992, which made that the spacecraft now had the required velocity to head toward Jupiter. Galileo made also two first close-up observations of two asteroids, 951 Gaspra on Oct. 29, 1991, and 243 Ida on Aug. 28, 1993 (making the surprising discovery that it had a tiny companion, Dactyl, the first known moon orbiting around a asteroid). Galileo's unique position in the solar orbit, still 148 million miles away from Jupiter, provided it with a excellent vantage point to observe a target of opportunity, the collision of comet Shoemaker-Levy 9 with Jupiter between July 16-22, 1994. Although a high-gain antenna failed to deploy when commanded in April 1991, engineers managed to get the data through the low-gain antenna only via various communications enhancements. The craft eventually reach Jupiter on December, 7th 1995

The Orbital Tours From 1996 to the Mission Extension

On July 13, 1995, Galileo deployed its atmospheric probe, still 150 days and 50 million miles from its encounter with Jupiter. On Dec. 7, the probe made first contact with Jupiter's upper atmosphere as it returned excellent science data via the orbiter for 58 minutes during a descent of 97 miles. It was eventually vaporized in the deeper layers of the Jovian atmosphere. The orbiter fired its main engine on Dec. 7, 1995, becoming the first artificial satellite of Jupiter with a initial orbital period of 198 days. The Galileo orbiter had used the attraction of Io to slowdown and it was captured by Jupiter's gravity. Beginning in June 1996 Galileo started to perform its nominal 2-year mission. The craft journeyed 11 orbits around Jupiter. Each of them was bringing it at close encounters at Callisto, Ganymede or Europa, with Io studied at each orbit at medium range only because of the hazardous radiation environment there. Such flybys provided a wealth of data about Jupiter major satellites, and about the Jovian system generally. The success of the mission led to an extension of two years (1998-1999) which was named the "Galileo Europa Mission" (GEM). Galileo performed 8 more orbits about Europa studying and characterizing the moon for possible implication for exobiology. The second year was dedicated to Io. The probe explored and mapped Io's plasma torus and it had two close encounters with the moon

Galileo survived Io harsh environment. The Galileo Millennium Mission extension added first one more year of observations then continued until the mission's end by 2003. Mission managers deactivated the spacecraft's cameras on Jan. 17, 2002, after they had sustained irreparable damage from Jupiter's strong radiation environment. Beginning in March 2003, ground controllers contacted Galileo once a week to verify its status. In December 2000, it received the visit of the Cassini-Huygens mission as the probe was flying by Jupiter en route to Saturn. NASA eventually decided to have Galileo crashed into Jupiter's atmosphere, as the craft had became out of control due to fuel depletion and as it was presenting a risk for Europa: any terrestrial contamination might have been damageable to possible forms of life. Galileo continued to broadcast and it made science during its final plunge, on September, 21st 2003. Galileo flew into Jupiter's atmosphere just South of the equator. Galileo mission had lasted 14 years, traveling 2.8 billion miles, and 8 years in orbit around Jupiter, having completed 35 orbits around the giant planet

Data Overview

Galileo returned a stream of data which have brought a further better knowledge of Jupiter and of the four Galilean satellites. In terms of instruments, the Galileo orbiter carried 10 science instruments, including a camera system designed to return images of Jupiter's satellites at 20 to 1,000 times better resolution than Voyager's best, partly due to closer fly-by distances. The probe itself carried seven instruments for measuring various parameters of the Jovian atmosphere during its rapid plunge, and seventeen principal investigators representing six countries led a team of more than 100 scientists involved with Galileo

to an overview of Galileo data