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Galileo Project

Galileo Project


Spacecraft and Mission





The Galileo spacecraft was a robotic probe sent to Jupiter, the largest planet in the solar system, by the United States National Aeronautics and Space Administration (NASA). The probe was launched on October 18, 1989 and arrived at Jupiter on December 7, 1995 after an elaborate, looping journey through the solar system to conserve fuel. It orbited Jupiter and observed the planet and its moons until it was deliberately crashed into Jupiters atmosphere on September 21, 2003. Galileo greatly enlarged our knowledge of Jupiter and its complex moons.

The Galileo spacecraft was named after the Italian astronomer Galileo Galilei (15641642), who first discovered the four large moons of JupiterIo, Europa, Ganymede, Callistoin 1610. These four moons of Jupiter, its largest, are called the Galilean satellites in

Galileos honor. Studying them was a primary objective of the Galileo project.

Spacecraft and Mission

The Galileo probe was really two spacecraft, an orbiter and an atmospheric descent probe, which were joined until they neared Jupiter. Together they stood some 21 ft (7 m) high. The orbiter was a dual-spin design, meaning that it was divided into two sections, one that spun at three revolutions per minute and another that did not spin. A spinning section was included was to stabilize the spacecraft, which otherwise would have tumbled randomly as it flew through space or else consumed excessive fuel stabilizing itself with rockets.

The larger, spun portion of the spacecraft contained six scientific instruments that could benefit by constantly scanning different parts of the heavens, including devices that imaged the stars and measured electromagnetic fields and charged-particle flows in space. Antennas for communication with Earth were also mounted on the spun portion, facing backward along the main axis of the spacecraft.

On the smaller, despun part of the craft were mounted devices that had to be held steady while acquiring data. These included the main camera (for imaging Jupiter and its moons) and several spectrometers. The atmosphere probe was also mounted on the despun part.


The Galileo project, which commenced in 1977, was a scientific success despite technical setbacks and political controversy. For example, the probe was originally intended for launch from Earth orbit using a stronger rocket than was eventually used. However, after the explosion of the space shuttle Challenger during takeoff on January 28, 1986, NASA said that the original rocket (with its large load of explosive fuel) would not be allowed to go up in the shuttle. Yet a smaller rocket could not send Galileo directly to Jupiter. Mission planners came up with a clever solution: instead of going straight to Jupiter, Galileo circled the inner solar system several times, first flying by Venus (February, 1990), then Earth (December, 1990), then Earth again (December, 1992), picking up enough velocity from these encounters to begin the real trip to Jupiter.

Next, when the spacecrafts main (high-gain) antenna was commanded to open in 1991, it failed. The antenna resembled the ribs of an umbrella, and engineers believe that three of the ribs stuck together instead of popping open. In any case, the failure of the main antenna threatened disaster: it had been designed to return data to Earth at 134 kilobits per second, but the crafts only other antenna, a low-gain unit, had been designed to return data at only 816 bits per second. (In radio communications, gain refers to how directional an antenna is: a high-gain antenna sends or receives along a narrow cone, whereas a low-gain antenna sends or receives over a wide range of angles.)

Again, a clever solution was found: by compressing data using software on Galileo before transmitting to Earth and by improving the sensitivity of the receiving antennas, engineers were able to raise the throughput of the low-gain antenna to 160 bits per second, higher than had been intended for the high-gain antenna.

Galileo was politically controversial before launch because it contained 49 pounds (22.3 kg) of plutonium-238, a highly toxic and radioactive metal, as a power source. (Solar panels were not practical because Jupiter is so far from the sun.) Critics were concerned that an explosion during launch or a mishap during Galileos two high-speed close encounters with Earth would spread the plutonium in Earths atmosphere, perhaps causing numerous cancers. A lawsuit was filed in a Washington, DC, court but did not stop the launch.

The orbiter and the descent probe flew as a single unit until Galileo was only five months away from Jupiter, whereupon the descent probe was separated from the main probe by remote control (July, 1995). Upon arrival at Jupiter, the orbiter fired a rocket to slow itself and enter orbit while the atmosphere probe shot straight into Jupiter like a bullet, only much faster. After being slowed by friction with Jupiters outer atmosphere, the probe opened a parachute and began to sink slowly. As it did so it radioed measurements of pressure, temperature, acceleration, and chemistry to the orbiter, which relayed them to Earth. After an hour of descending through the Jovian atmosphere, the probes electronics were destroyed by the increasing heat and pressure. Jupiter has no solid surface but gradually becomes more dense at greater depths. The probe was eventually melted and then vaporized deep inside the planet.

The Galileo orbiter circled Jupiter for eight years, taking pictures and other measurements of the planet and its moons. In 2003, a year after its cameras had been destroyed by the intense radiation surrounding Jupiter, Galileo was ordered to fire its rocket again and so descend into the Jovian atmosphere, where it burned up. This guaranteed that no hitchhiking germs from Earth could pollute the moons of Jupiter, several of which harbor liquid water and might have native life.


Galileo data showed that the moons Europa, Ganymede, and Callisto contain oceans of liquid water under their solid outer crusts. Scientists believe that these moons are some of the most likely places in the solar system, other than Earth, for life to have evolved. Galileo also documented the extreme volcanic activity on the moon Io, many times greater than that of Earth. It returned vast amounts of scientific information about the structure of the Jovian atmosphere, magnetic fields, and radiation environment. Galileo also performed the first encounter with an asteroid, passing within 1,000 mi (1,600 km) of 951 Gaspra in 1991.



Fischer, Daniel. Mission Jupiter: The Spectacular Journey of the Galileo Spacecraft. New York: Springer, 2001.


Kivelson, Margaret G., et al. Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa. Science. 289 (2000): 1340-1343.

Moore, John. Galileo Gears Up for Swansong as Crash-Landing Looms. Nature. 425 (2003): 229.

Stevenson, David J. Jupiter and Its Moons. Science. 294 (2001): 71-72.

Young, Richard E., et al. Galileo Probe: In Situ Observations of Jupiters Atmosphere. Science. 272 (1996): 837-838.


National Aeronautics and Space Administration. Galileo: Journey to Jupiter. April 14, 2003. <> (accessed November 29, 2006).

Larry Gilman

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