After the success of the Pioneers, two missions with a new type of spacecraft headed back to the outer solar system
in 1977 - Voyager 1 and 2. Voyager 1 flew by
Jupiter and continued on to Saturn. Voyager 2 did the same, and then completed the "Grand Tour" by visiting Uranus
and Neptune.
The Voyagers made many important discoveries. They sent back detailed pictures of Jupiter's incredible cloud
system, including the Great Red Spot, a giant storm that rotates endlessly like an enormous hurricane.
The Voyagers also took pictures of Jupiter's major moons. Among those images were several showing volcanoes
erupting on the moon Io. Nowhere beyond Earth had active volcanoes been seen before.
On February 8, 1992 the Ulysses spacecraft used
Jupiter's gravity to swing "up" so it could explore the poles of our Sun. Ulysses found that Jupiter's gravity had changed,
and there were fewer volcanoes erupting on Io.
These exciting discoveries by the Voyager and Ulysses spacecraft made scientists wish to study Jupiter more closely.
The Galileo mission is the fulfillment of that wish.
