The Grand Tour was a NASA program that would have sent two groups of robotic probes to all the planets of the outer Solar System. It called for four spacecraft, two of which would visit Jupiter, Saturn, and Pluto, while the other two would visit Jupiter, Uranus, and Neptune. The enormous cost of the project, around $1 billion, led to its cancellation and replacement with Mariner Jupiter-Saturn, which became the Voyager program.
The concept of the Grand Tour began in 1964, when Gary Flandro of th...