Galileo mission overview
FROM NASA PRESS KIT
Posted: September 20, 2003
NASA's Galileo spacecraft was designed to study the large, gaseous planet Jupiter, its moons and its surrounding magnetosphere, which is a magnetic bubble surrounding the planet. The craft was named for the Italian Renaissance scientist who discovered Jupiter's major moons in 1610.
The primary mission at Jupiter began when the spacecraft entered orbit in December 1995 and its descent probe, which had been released five months earlier, dove into the giant planet's atmosphere. Its primary mission included a 23-month, 11-orbit tour of the jovian system, including 10 close encounters of Jupiter's major moons. Although the primary mission was completed in December 1997, the mission has been extended three times since then. Galileo had 35 encounters of Jupiter's major moons - 11 with Europa, 8 with Callisto, 8 with Ganymede, 7 with Io and 1 with Amalthea. The mission will end when the spacecraft impacts Jupiter on Sunday, Sept. 21, 2003.
Venus and Earth flybys
The flight path provided opportunities for scientific observations. Scientists obtained the first views of mid-level clouds on Venus and confirmed the presence of lightning on that planet. They also made many Earth observations, mapped the surface of Earth's Moon, and observed its north polar regions.
Because of the modification in Galileo's trajectory, the spacecraft was exposed to a hotter environment than originally planned. To protect it from the Sun, project engineers devised a set of sunshades and pointed the top of the spacecraft toward the Sun, with the umbrella-like high-gain antenna furled until well after the first Earth flyby in December 1990. Flight controllers stayed in touch with the spacecraft through a pair of low-gain antennas, which send and receive data at a much slower rate.
A special team performed extensive tests and determined that a few (probably three) of the antenna's 18 ribs were held by friction in the closed position. Despite exhaustive efforts to free the ribs, the antenna would not deploy. From 1993 to 1996, extensive new flight and ground software was developed, and ground stations of NASA's Deep Space Network were enhanced in order to perform the mission using the spacecraft's low-gain antennas.
On August 28, 1993, Galileo carried out a second asteroid encounter, this time with a larger, more distant asteroid named Ida. Ida is about 55 kilometers (34 miles) long and 24 kilometers (15 miles) wide. Observations indicated that both Ida and Gaspra have magnetic fields, although Ida is older and its surface is covered with craters. Scientists discovered that Ida boasts its own moon, making it the first asteroid known to have a natural satellite. The tiny moon, named Dactyl, has a diameter of only about 1.5 kilometers (less than a mile). By determining Dactyl's orbit, scientists estimated Ida's density.
The Galileo spacecraft, approaching Jupiter, was the only observation platform with a direct view of the cometıs impact area on Jupiter's far side. Despite the uncertainty of the predicted impact times, Galileo team members pre-programmed the spacecraft's science instruments to collect data and were able to obtain spectacular images of the comet impacts.
Arrival day on December 7, 1995, turned out to be an extremely busy 24-hour period. When Galileo first reached Jupiter and while the probe was still approaching the planet, the orbiter flew by two of Jupiter's major moons - Europa and Io. Galileo passed Europa at an altitude of about 33,000 kilometers (20,000 miles), while the Io approach was at an altitude of about 900 kilometers (600 miles).
About four hours after leaving Io, the orbiter made its closest approach to Jupiter, encountering 25 times more radiation than the level considered deadly for humans.
The wok-shaped probe floated down about 200 kilometers (125 miles) through the clouds, transmitting data to the orbiter on sunlight and heat flux, pressure, temperature, winds, lightning and atmospheric composition. Fifty-eight minutes into its descent, high temperatures silenced the probe's transmitters. The probe sent data from a depth with a pressure 23 times that of the average on Earth's surface, more than twice the mission requirement.
An hour after receiving the last transmission from the probe, at a point about 200,000 kilometers (130,000 miles) above the planet, the Galileo spacecraft fired its main engine to brake into orbit around Jupiter.
This first orbit lasted about seven months. Galileo fired its thrusters at its farthest point in the orbit to keep it from coming so close to the giant planet on later orbits. This adjustment helped mitigate possible damage to spacecraft sensors and computer chips from Jupiter's intense radiation environment.
During this first orbit, new software was installed which gave the orbiter extensive new onboard data processing capabilities. It permitted data compression, enabling the spacecraft to transmit up to 10 times the number of pictures and other measurements that would have been possible otherwise.
In addition, hardware changes on the ground and adjustments to the spacecraft-to- Earth communication system increased the average telemetry rate tenfold. Although the problem with the high-gain antenna prevented some of the mission's original objectives from being met, the great majority were. So many new objectives were achieved that scientists feel Galileo has produced considerably more science than ever envisioned at the project's start 20 years ago. Orbital Tour During its primary mission orbital tour, Galileo's itinerary included four flybys of Jupiter's moon Ganymede, three of Callisto and three of Europa. These encounters were about 100 to 1,000 times closer than those performed by NASA's Voyager 1 and 2 spacecraft during their Jupiter flybys in 1979. Galileo's instruments scanned and scrutinized the surface and features of each moon. After about a week of intensive observation, with its tape recorder full of data, the spacecraft spent the next one to two months-until the next encounter in orbital "cruise"-playing back the information in transmissions to Earth.
Timeline - Times and descriptions of Galileo's descent into Jupiter.
Entry preview - Story on Galileo's demise.
Spacecraft - A technical review of the Galileo spacecraft.
Top 10 - Chart of the leading science achievements by Galileo mission.
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