Robots to the Outer Solar System

In 1965, Gary Flandro was a student at the California Institute
of Technology. While doing a summer project on the topic of space travel, he realized that the giant planets Jupiter, Saturn, Uranus, and Neptune were all coming around to the same side of the Solar System. This meant that a spaceship could use the grav- ity of each planet to pull it along, speeding it to the next planet. It would take less than half as much time to visit all four planets as anyone had previously calculated. Flandro called this the “Grand Tour,” and his summer project became part of NASA’s plan to explore the outer planets in the 1970s.