Galileo sets off for Jupiter

On this day in 1989, the space shuttle Atlantis carried a spacecraft called Galileo into orbit and sent it on its way to Jupiter. Six years later, it became the first spacecraft ever to circle one of the giant outer planets.

Why it mattered

Galileo found signs that three of Jupiter's moons, Europa, Ganymede and Callisto, may have salty water hidden under their surfaces.