Planetary Grand Tour

The Planetary Grand Tour was an ambitious plan to send unmanned probes to the outermost planets of the solar system. Conceived by Gary Flandro of the Jet Propulsion Laboratory, 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 176 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, launched in 1976 and 1977, would fly by Jupiter, Saturn, and Pluto. The other two, launched in 1979, would 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. But Voyager 2 used the fortunate alignments of the outer planets and was able to add 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 Pluto. Voyager 2's trajectory could not be bent to bring the probe by Pluto after the Neptune flyby in 1989.

Pluto, no longer regarded as a planet after the 2006 redefinition, is scheduled for exploration by the New Horizons spacecraft set to rendezvous with the dwarf planet and its three moons in 2015.

References

* See "National Geographic", August 1970, for proposed Grand Tour project information.