Great Red Spot with moon Io
This image was taken by NASA's Cassini spacecraft on Dec. 1, 2000. It reveals Jupiter's Great Red Spot (GRS) in detail. Jupiter's Great Red Spot is similar to a hurricane on Earth. The gas giant, which was first which was first observed by Galileo Galilei in 1610, is so massive that three Earths would fit inside of it. The composition of Jupiter's atmosphere is similar to that of the sun, mostly hydrogen and helium. In addition to showing the planet, this photo also shows Jupiter's large moon, Io (to the left).