Illustration of Galileo at Jupiter

NASA's Jupiter-bound Galileo spacecraft will take the next steps Friday and Saturday in shaping its trajectory for a December flyby of the Earth.

Acting on a computer program developed by the Galileo flight team at the Jet Propulsion Laboratory and sent to the spacecraft via the Deep Space Network, the spacecraft will slow by about 25 mph (from its more than 66,000 mph in solar orbit today). It will use tiny rocket thrusters, pulsing them a total of nearly 3,000 times. Because Galileo is spinning, each pulse must be precisely timed to push the spacecraft in the proper direction. It will take many hours to build up the total correction required.

The Galileo team has designed a series of maneuvers as part of the complex process of flying the spacecraft to Jupiter. Because there was not enough launch energy to fly directly to Jupiter, as the Voyagers did in 1977-1979, Galileo uses the planetary gravity-assist technique, as Voyager 2 did in going to Saturn, Uranus, and Neptune, to gain the needed velocity.

Galileo successfully completed its first gravity assist, a flyby of the planet Venus, last February, after two precise trim maneuvers. A flyby of Earth in December 1990 and another in December 1992, each fine-tuned by several precise maneuvers, will complete the gravity-assist process.

On reaching Jupiter in late 1995, Galileo will use a similar combination of gravity and rocket thrust to swing the spacecraft into orbit around the planet and then change orbits many times to allow close examination of Jupiter's major satellites for nearly two years.

Galileo's mission is to study Jupiter's atmosphere, using an atmospheric probe as well as remote sensing, its magnetosphere and its satellites.

The Galileo Project is managed for NASA's Office of Space Science and Applications by the Jet Propulsion Laboratory.

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