Aeronautics and Space Administration
Galileo: Probe Into Jupiter
On January 7, 1610 in Padua, Italy, Galileo Galilei aimed his newly invented telescope at Jupiter, the giant planet of the Solar System. He saw three star-like objects arranged in a line with the planet, one on the west and two on the east. A few nights of watching showed him there were four objects changing position about Jupiter. Galileo soon realized that these "stars" were small planetary bodies revolving around Jupiter like the Moon orbits Earth. He had discovered four big satellites of Jupiter. Later they were named Io, Europa, Ganymede, and Callisto, after four of Jupiter's lovers in Greek mythology. As a group they are known as the Galilean satellites of Jupiter. Astronomers and spacecraft scientists later discovered other smaller satellites of Jupiter. A retinue of 16 satellites makes the Jovian system a miniature of the Solar System.
It is fitting that the National Aeronautics and Space Administration's (NASA's) most ambitious Jupiter mission should be named after the Italian astronomer. Project Galileo, launched in 1989 on a six-year trip to Jupiter consists of a Jupiter Orbiter and a Jupiter Probe. The Orbiter will remain in orbit around the giant planet gathering data about the planet itself, its magnetosphere, and its satellites. The Probe will plunge into Jupiter's atmosphere at about 2:15 p.m. PST on December 7, 1995 and provide scientists with a wealth of information about that atmosphere and its cloud systems.
As more powerful telescopes were built, astronomers in their mountaintop observatories discovered tantalizing clues about the uniqueness of Jupiter. The planet's diameter is 11 times that of Earth. From Newton's laws of gravity and the orbits of the four large satellites, astronomers determined that Jupiter is massive but has a low density. This implies that the planet consists mainly of hydrogen. Jupiter, along with Saturn, Uranus, and Neptune, is a giant fluid planet, a gas giant. It is unlike the much smaller icy and rocky planets of which Earth is the largest. Jupiter spins much faster than Earth, so that its day lasts only 9 hours and 55 minutes compared with Earth's 24-hour day. The rapid spin and Jupiter's fluid nature give rise to a bulging equator that is quite apparent in telescopic views and photos of the giant cloud-shrouded planet. When viewing Jupiter, you only see clouds. No solid surface exists. Also, these clouds are very different from those of Earth.
When we view Earth from space, patterns of clouds are visible but a solid surface is also clearly seen. On Jupiter the cloud patterns are east-west bands consisting of dark belts of descending atmosphere and bright zones of ascending gases. Jupiter also has atmospheric spots, ovals, barges, and plumes, some of which may be like storms on Earth. The most famous and largest of these is the Great Red Spot.
A Voyager image of cloud-banded giant