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NASA meatballNational Aeronautics and Space Administration
Ames Research Center
Space Science Division and Educational Programs Office
Educational Brief EB-117
Subject: Planetary Science, Astronomy

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Galileo: Probe Into Jupiter

page 4

Searching for Answers: Galileo Entry Probe and Orbiter

Project Galileo will orbit a spacecraft around Jupiter and send a probe deep into Jupiter's atmosphere to solve many of the puzzles about the giant planet. Galileo is a project sponsored by the National Aeronautics and Space Administration. The Jet Propulsion Laboratory has overall project management and responsibility for the Orbiter, mission design, and mission operations. NASA's Ames Research Center is responsible for the Atmospheric Entry Probe built by Hughes Space and Communications Group. Scientists expect that Galileo will provide not only information about Jupiter, but also important evidence about the origin and evolution of the Solar System. In addition it will provide new insights into phenomena that directly relate to our understanding of all the planets including Earth.

an image showing the Probe separating from the spacecraft
Probe release, July 13, 1995

Originally scheduled for launch in January 1982, Galileo was delayed several times by development and scheduling problems in the Space Shuttle and rockets it could carry to boost interplanetary missions out of Earth's orbit. Resulting from the tragic Challenger accident in1986, a less powerful rocket (a solid propellant Inertial Upper Stage, IUS) had to be used to propel Galileo from Earth's orbit. The spacecraft and the IUS were finally launched into low Earth orbit by the Space Shuttle Atlantis on October 18, 1989. Within hours the IUS propelled the Galileo spacecraft out of Earth's orbit.

Because the lower thrust IUS had to be used, reaching Jupiter required a gravity-assist trajectory. The gravity and orbital energy of Venus and Earth boosted the energy of the Galileo spacecraft by close flybys of these two planets (called the Venus-Earth-Earth- Gravity-Assist or VEEGA trajectory). This resulted in a six-year circuitous journey of almost 2.4 billion miles from Earth to Jupiter. The spacecraft used a close flyby of Venus (February 1990) and two close flybys of Earth (December 1990 and December 1992) as gravity slingshots. Without these flybys Galileo would have required twelve times as much propellant at launch to carry the desired payload of scientific instruments to Jupiter. Navigation engineers lined-up each maneuver and slingshot encounter so that Galileo arrived precisely on target for the separation of the Probe from the Orbiter on July 13, 1995. The probe continues on a ballistic path heading straight toward Jupiter for a meteoric plunge into the atmosphere on December 7, 1995. The Orbiter was maneuvered into a separate trajectory that will allow it to enter an orbit around Jupiter on the same date.

a diagram showing the position of Jupiter moving relative to the route traveled by the Galileo spacecraft
Gravity slingshots to Jupiter

The Probe will speed toward Jupiter's swirling cloudtops at 170,000 km/hr (106,000 mph)&emdash;a speed equivalent to flying from San Francisco to Washington D.C. in 100 seconds. The Probe is aimed to strike the atmosphere at an angle of 8.5 degrees to the horizontal. A 1.5 degree shallower angle would cause the Probe to skip off back into space; a 1.5 degree steeper angle would overheat the spacecraft and destroy it. With a successful entry, the Probe will descend through turbulence, violent winds, and three (?) major cloud layers into the hot, dense atmosphere below. It is designed to survive until at least 200 km (125 miles) below the visible cloud tops where the atmospheric pressure is about 25 times that at Earth's sea level As hinted by the question mark above, what the Probe will find during its descent is, to a large extent, a mystery.

On the day the Probe enters the atmosphere, the Orbiter will be busy. It will make a very close flyby of Io (Jupiter's exotic volcanic moon), then record the radio signal from the Probe descending into the atmosphere. Next the Orbiter will fire its rocket engine to become the first spacecraft to enter orbit about Jupiter. This will start the Orbiter's 22 months of high resolution observation of Jupiter's system. The flyby of Io will be the closest flyby of the mission&emdash;a unique opportunity for high resolution study of this enigmatic satellite. Jupiter's radiation belts are particularly strong near the orbit of Io. Consequently the Orbiter's time near Io will be limited to avoid damage to the spacecraft. The Orbiter will listen to the signals from the Probe for up to 75 minutes only before the bulk of Jupiter obscures the entry site from the Orbiter. How much longer the Probe will continue to radio its findings into space, and what its ultimate fate might be, will be unknown.

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This page was created by Tobin A. Snell and Josh Parker.


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