Illustration of Galileo at Jupiter

NASA's Galileo spacecraft, now homing in on Jupiter, is scheduled to fire its main rocket engine in a critical maneuver early Thursday, July 27, that will put it on course for entry into orbit around the giant planet later this year.

The engine firing for the so-called "orbiter deflection maneuver" will begin at 12:38 a.m. Pacific Daylight Time and last 5 minutes, 6 seconds. (This is the time when Galileo's radio signal reporting the event is received on Earth. The signal travels at the speed of light, and one-way light time from Galileo to Earth will be about 38 minutes the day of the maneuver.)

A press briefing originating from NASA's Jet Propulsion Laboratory to discuss the Galileo orbiter deflection maneuver, the recent release of Galileo's Jupiter atmospheric probe, and the major scientific goals of the mission, will be carried live on NASA Television at 10 a.m. PDT on July 27.

A two-second firing or "wake-up burn" of the main rocket engine was successfully completed early this morning. The short burn allowed flight controllers to check out the overall health of the propulsion system in preparation for the longer engine burn Thursday.

The upcoming rocket firing follows the successful targeting and release of Galileo's Jupiter atmospheric probe earlier this month. The atmospheric probe, with no propulsion or guidance system of its own, will continue to freefall to Jupiter and enter the planet's atmosphere on December 7.

As planned, Galileo is currently on a course that without further action would send the spacecraft along the same Jupiter impact trajectory that the atmospheric probe is flying. This week's engine firing, however, will deflect the spacecraft off that trajectory and put it on course for its close flyby of Jupiter's volcanic moon Io also occurring on December 7.

"With the completion of this maneuver, we're looking forward to the culmination of three weeks of intensive work in targeting the spacecraft for the beginning of its tour of Jupiter," said Galileo Project Manager William O'Neil of NASA's Jet Propulsion Laboratory. "Everything is going perfectly."

The main engine could not be tested or fired prior to release of the atmospheric probe because the probe was mounted in front of the engine nozzle. Thursday's firing also serves as a demonstration of engine operability for an even larger firing of the same engine during the December 7 maneuver that will brake the spacecraft and allow it to be captured into Jupiter orbit. That engine firing, called Jupiter orbit insertion, will be the largest such maneuver of Galileo mission.

The orbiter deflection maneuver will provide a change in the spacecraft's velocity of 62.5 meters per second (about 140 miles per hour). By comparison, the Jupiter orbit entry maneuver will expend 377 kg (831 lbs) of propellant and result in a velocity change of 643 meters per second (1,438 miles per hour).

Galileo was launched in October 1989 aboard Space Shuttle Atlantis from NASA's Kennedy Space Center, FL. Galileo's Jupiter atmospheric probe is managed by NASA's Ames Research Center, Mountain View, CA. The Galileo spacecraft was built and the overall mission is managed by JPL.

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