National 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

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Exploration by Spacecraft

Scientists studying the giant planet entered a new era in 1972 when spacecraft began to explore the outer Solar System. Pioneer 10 was the first of four spacecraft to fly by Jupiter. Pioneers 10 and 11 were trailblazers, with Pioneer 11 swinging by Jupiter and continuing on to explore Saturn. They answered two basic questions about exploration of the outer Solar System. They showed that spacecraft could safely pass through the swarm of small planetary bodies (asteroids) in orbits between those of Mars and Jupiter.

Pioneer 10

They also showed that spacecraft could pass relatively unscathed through the intense magnetic field and radiation environment of Jupiter. This showed that spacecraft could safely use a gravitational slingshot technique at Jupiter to reach more distant outer planets.

The Pioneers were followed by Voyagers 1 and 2. These NASA spacecraft made a more detailed sur-vey of Jupiter and Saturn. Voyager 2 continued to Uranus and Neptune so that all the gas giants of the outer Solar System were explored. The Voyagers carried ad-vanced instrumen Volcanic plume on Io discovered by Voyager tation and high resolution cameras and made many important discoveries. At Jupiter, the Voyagers sent to Earth detailed maps of the radiation environment and magnetosphere. The magnetosphere is the region around a planet where the planetUs magnetic field is dominant and holds off the solar wind, the blizzard of charged particles streaming across the Solar System from the Sun. The Voyagers obtained high resolution images of the cloud systems of Jupiter, and detailed images of the planet's big satellites.

Voyager

The Hubble Space Telescope provided a follow-on from the Pioneers and the Voyagers by allowing a new era of long-term, detailed monitoring of the planet from low Earth orbit (LEO). Operating beyond Earth's atmosphere, the space telescope has major advantages over Earth-based telescopes.

Galileo

Galileo is the next great step forward in Jupiter's exploration. TheProbe and Orbiter carry a battery of instruments for increasing our understanding of this giant planetUs atmosphere, as described in this Educational Brief. While the Probe will plunge deep into the planetUs atmosphere, the Orbiter will also gather information about the atmosphere from orbit and will provide detailed observations of the Galilean satellites during close approaches to these bodies. Entering orbit in December 1995, the Orbiter will obtain detailed images of mysterious volcanic Io. Then the spacecraft's 11 science experiments will gather information about the giant planet and details of the four large satellites using slingshot gravitational techniques to navigate through the Jovian system until December 1997. That is when the nominal mission ends. In addition, it will extensively observe the strong magnetic fields and radiation belts of JupiterUs magnetosphere for those many months. The radiation belts are where electrons and protons are trapped in the planetUs magnetic field. Galileo will greatly increase our understanding of the Jupiter system.

Volcanic plume on Io Discovered by Voyager

A spacecraft similar in size to Galileo, called Cassini, is due to be launched toward Saturn in October 1997 making this a most exciting era of exploration of the giants of our Solar System, their atmospheres, complex magnetospheres, and retinues of satellites.

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