In the 1960s, mission designers recognized that a unique opportunity was going to present itself more than a decade later. Starting in the late 1970s, the giant gaseous outer planets -- Jupiter, Saturn, Uranus and Neptune -- would line up in such a way that single spacecraft might hop from one to the next, using the gravity of each one to keep speeding it on its way. Taking advantage of this alignment -- which occurs only once every 175 years -- NASA approved the Voyager Project, designed to send twin spacecraft to the outer solar system.
Image Credit: NASA/JPL
