About 400 years ago, Italian scientist Galileo Galilei (15 February 1564 to 8 January 1642) looked through his telescope at the planet Jupiter and "perceived ... that beside the planet there were three starlets, small indeed, but very bright ..." Eventually Galileo would discover a total of four moons, unknown before the investion of the telescope, orbiting around Jupiter. His discovery of the moons we know call the Galilean satellites -- Io, Europa, Ganymede and Callisto -- eventually changed how humanity viewed the heavens.
We've come a long way from Galileo's primitive telescope. In 1989, NASA launched an interplanetary explorer named in honor of the great scientist. The Galileo spacecraft (shown below) explored the Jovian system for 14 years, sending back a wealth of scientific data that forever changed our understanding of our solar system. The spacecraft images resolved features with a diameter of 12 meters (39 feet). Compared with the view through a 17th Century telescope, Galileo's robotic nakesake's observations improved on his by factors of 100,000 to 1,000,000.
The spacecraft was the first to fly past an asteroid and the first to discover a moon of an asteroid. It provided the only direct observations of a comet colliding with a planet.
Galileo was the first to measure Jupiter's atmosphere with a descent probe and the first to conduct long-term observations of the Jovian system from orbit. It found evidence of subsurface saltwater on Europa, Ganymede and Callisto and revealed the intensity of volcanic activity on Io.
Galileo plunged into Jupiter's crushing atmosphere on Sept. 21, 2003. The spacecraft was deliberately destroyed to protect one of its own discoveries - a possible ocean beneath the icy crust of the moon Europa.
Learn more about the Galileo spacecraft on the Galileo Legacy Site.
Last Updated: 2 February 2011