Illustration of Galileo at Jupiter

A rugged, conical-shaped probe will separate from NASA's Galileo spacecraft later this week and head for Jupiter's cloud tops, the first time in history a human-made object will enter the atmosphere of an outer planet of the solar system.

The probe and it scientific payload will be deployed from the main Galileo spacecraft at 10:30 p.m. Pacific Daylight Time on July 12, and fly solo the remaining 82 million kilometers (51 million miles) over five months to Jupiter. Confirmation that the probe has been released will be received on Earth 37 minutes later, at 11:07 p.m. PDT.

The 339-kilogram (747-pound) probe is scheduled to slam into the giant planet's atmosphere on December 7, 1995, then begin a parachute descent into the planet where it will make the first ever measurements within Jupiter's atmosphere.

Designed to survive the highest impact speed ever achieved by a human-made object (171,000 kilometers per hour or 106,000 miles per hour), Galileo's atmospheric probe should provide extraordinary new details about Jupiter's chemical makeup and atmospheric dynamics.

At separation, Galileo will be 663 million kilometers (412 million miles) from Earth and on a flight path headed toward the probe's aimpoint in Jupiter's atmosphere. Before the separation, controllers will line up Galileo's spin axis so that it is pointed along the path the probe will take as it enters Jupiter's atmosphere and spin up the combined spacecraft and probe to 10.5 rpm. The spin stabilizes the probe's attitude, or orientation, in space as it flies toward Jupiter.

In the sequence of events leading to probe release, ground controllers and Galileo's onboard systems are sending a series of commands to prepare the probe for its mission. These include programming the probe's coast timer, an onboard clock that will "wake up" the probe's systems and scientific instruments six hours before it enters Jupiter's atmosphere.

After checks of command, data, power and other subsystems, a built-in cable cutter will sever the umbilical between the atmospheric probe and Galileo. Later, small charges on nuts that secure the probe to Galileo will detonate to free the probe. Three small springs will gently push the probe away from the main spacecraft, sending it on its last leg to Jupiter. Confirmation of the release will be received 37 minutes later, the time necessary for the radio signal to travel back to Earth at the speed of light.

Two weeks later, on July 27, Galileo will fire its main engine to deflect its own course toward an orbit high above Jupiter's cloud tops. The probe and main spacecraft will communicate again on Dec. 7 as the descending probe transmits its data to the Galileo spacecraft. The probe will send data to Galileo orbiter for up to 75 minutes.

The ultimate fate of the probe will be determined by its battery lifetime, or it may succumb to either the high temperatures or the immense pressure of Jupiter's atmosphere. Galileo, meanwhile, will begin two years of close-up studies of Jupiter, its moons, rings and powerful magnetic environment as it orbits the planet.

The overall Galileo mission is managed for NASA's Office of Space Science, Washington, D.C., by NASA's Jet Propulsion Laboratory, Pasadena, CA. Galileo's atmospheric probe is managed by NASA's Ames Research Center, Moffett Field, CA.

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