1979 AD
                    JUPITER ORBIT
                    Of all the objects in our solar system, Jupiter's moon Io is surely
                    one of the most unusual. (Some people call it "EYE-oh"; others
                    say "EE-oh." Both are okay.)
                    Discovered by Italian astronomer Galileo, Io is the biggest of
                    Jupiter's four main moons and is the closest to the giant planet. It
                    is 3,640 kilometers (2260 miles) in diameter, slightly larger than
                    our Moon.
                    The amazing thing about Io is that it has more erupting volcanoes
                    than any body in the solar system. Why?
                    Just before the first Voyager flew by Io, scientists had predicted
                    that Jupiter's gravity would cause Io to heat up. The reason is this:
                    Io moves pretty much in a circular orbit around the planet, but the
                    slight gravity of another Moon, Europa, sometimes tugs on it a
                    little. This disturbs Io's orbit slightly, so it's not quite a circle.
                    This causes Io to move slightly in and out around Jupiter. But
                    Jupiter's enormous gravity squeezes Io, in just the way that our
                    Moon's gravity squeezes Earth's oceans to make the tides. This
                    squeezing makes Io heat up, like a piece of metal bent again and
                    again.
                    The heat causes volcanoes to erupt. In 1979, when the Voyager
                    1 spacecraft flew by Io, a volcano was seen erupting. Between
                    the Voyager 1 and 2 flybys of Io, eight erupting volcanoes have
                              been detected.