With at least 61 moons (as of September 2003) and several rings circling it, Jupiter is the largest planet in our solar system. This giant planet is surrounded by an enormous magnetic field called the magnetosphere, which has a million times the volume of Earth's magnetosphere. Its immense, complex atmosphere includes the Great Red Spot, a 300-year-old storm that is almost the size of three Earths.

NASA's Galileo spacecraft orbited Jupiter from December 1995 to September 2003, providing a stream of discoveries about the planet, its four largest moons and the surrounding environment. The moon Europa shows evidence of a deep ocean of melted water under an icy crust. Closer to Jupiter, the moon Io is continuously resurfacing itself with fresh volcanic eruptions. Io's internal heat comes from tidal flexing due to Jupiter's gravitational pull. Galileo also dropped a probe into Jupiter's atmosphere to make the first in-place studies of the planet's clouds and winds. Galileo was preceded by NASA's Pioneer and Voyager spacecraft, which flew past Jupiter in the 1970s. Future missions that may follow up on Galileo's discoveries about Europa, which make that moon a tempting target for investigating the possibility of extraterrestrial life.

Spacecraft
Launch: October 18, 1989
Mass: 2,223 kilograms (5,956 pounds), fueled
Science instruments: Pointed instruments camera, near-infrared mapping spectrometer, photopolarimeter-radiometer, ultraviolet spectrometer/extreme ultraviolet explorer; unpointed instruments magnetometer, energetic particle detector, plasma investigation, plasma wave subsystem, dust-detection subsystem, heavy ion counter, radio science; descent probe atmospheric structure, neutral mass spectrometer, helium abundance, nephelometer, net flux radiometer, lightning and radio emissions/energetic particles, doppler wind experiment

Overview

Even as the Voyager spacecraft completed their initial reconnaissance of the outer solar system in the late 1970s and 1980s, mission planners looked ahead to a next generation of spacecraft that would orbit the giant outer planets to study them in greater detail. The first fruit of that effort was Galileo, an orbiter bound for Jupiter.

The sophisticated spacecraft features two sections joined together by a spin bearing somewhat like a lazy susan. Half of the spacecraft contains pointable instruments such as cameras, and is held fixed in relation to space. The other half of the spacecraft contains instruments that measure magnetic fields and charged particles, and slowly rotates in order to optimize their measurements. Finally, Galileo also carried a descent probe designed to drop into Jupiter's turbulent atmosphere.

Galileo was launched October 18, 1989, from Kennedy Space Center, Florida, carried into Earth orbit in the cargo bay of Space Shuttle Atlantis. It was then propelled onto its interplanetary flight path by a two-stage solid-fuel motor called an Inertial Upper Stage. Although earlier plans called for Galileo to use a more powerful upper stage so that it could fly directly to Jupiter, the final flight took it by other planets first so that it could gain energy from the gravity of each. Galileo flew past Venus on February 10, 1990, and then twice past Earth -- once on December 8, 1990, and again on December 8, 1992.

Also en route to Jupiter, Galileo flew close to two asteroids, the first such visits by any spacecraft. It encountered the asteroid Gaspra on October 29, 1991, and the asteroid Ida on August 28, 1993. During the latter part of its interplanetary cruise, Galileo was used to observe the collisions of fragments of Comet Shoemaker-Levy with Jupiter in July 1994.

Galileo arrived at Jupiter on December 7, 1995, entering orbit and dropping its instrumented probe into the giant planet's atmosphere. Since then it has made about two and a half dozen orbits of Jupiter, usually flying close to one of its four major moons during each loop around the planet.

Galileo discovered strong evidence that Jupiter's moon Europa has a melted saltwater ocean under an ice layer on its surface. The spacecraft also found indications that two other moons, Ganymede and Callisto, have layers of liquid saltwater as well. Other major science results from the mission include details of varied and extensive volcanic processes on the moon Io, measurements of conditions within Jupiter's atmosphere, and discovery of a magnetic field generated by Ganymede.

Mean Distance from Sun: 778,412,010 km (4.84 x 10⁸ miles) or 5.20 astronomical units

Volume (Earth = 1): 1,316

Mass: 1.90 x 10²⁷ (66.97 x 10²⁷ ounces) or 317.8 (Earth = 1)

Density: 1.33 gm/cm³

Surface gravity: 2.34 (Earth = 1)

Rotation period (length of day in Earth days): 0.41 Earth day (9.8 Earth hours)

Revolution period (length of year): 11.86 Earth years

Natural satellites: Four largest are Ganymede, Callisto, Io, Europa. Two of next largest are Amalthea and Himalia. As of 2004, Jupiter has 63 known moons.