Before revision: The two probes of the Voyager program, In 1964, Gary Flandro of the Jet Propulsion Laboratory proposed a mission called Grand Tour, taking advantage of the fact that an alignment of planets in the late 1970s would enable a single spacecraft to visit all of the outer planets, including Pluto, by using gravity assists.
After revision: In 1964, Gary Flandro of the Jet Propulsion Laboratory proposed a mission called Grand Tour, taking advantage of the fact that an alignment of planets in the late 1970s would enable a single spacecraft to visit all of the outer planets, including Pluto, by using gravity assists.