The Planetary Grand Tour was an ambitious plan to send unmanned probes to the planets of the outer solar system. Conceived by Gary Flandro of the Jet Propulsion Laboratory in the late 1960s, the Grand Tour would have exploited the alignment of Jupiter, Saturn, Uranus, Neptune, and Pluto, an event that would occur in the late 1970s, and not recur for 175 years. A probe sent to Jupiter could use that planet as a gravitational slingshot to extend its trajectory to planets further out in the Solar System.
The original proposed mission design had four probes. The first two, with proposed launch dates in 1976 and 1977, were to fly by Jupiter, Saturn, and Pluto. The other two, with proposed launch dates in 1979, were to fly by Jupiter, Uranus, and Neptune.
The vehicles were to have been designed with multiple redundant systems to ensure reliability over missions lasting up to 12 years.
NASA budget cuts eventually doomed the Grand Tour missions in 1972, as well as later proposals for a "mini grand tour". However, many elements of the Grand Tour were added to the Voyager program. The two Voyager probes, launched in 1977, were originally meant to fly by Jupiter and Saturn. The Voyager 2 mission used the fortunate alignments of the outer planets and was extended to include close flybys of both Uranus and Neptune. Voyager 2's mission has specifically come to be regarded as the "Grand Tour."
Voyager 1 could have been sent to Pluto after Saturn, but was instead sent on a trajectory which brought it close by Titan, eliminating the Pluto flyby. Voyager 2's trajectory could not be bent to bring the probe by Pluto after the Neptune flyby in 1989.
Pluto is scheduled for exploration by the New Horizons spacecraft set to fly by the dwarf planet and its five known moons in 2015.
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