Planetary Grand Tour

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 the Jet Propulsion Laboratory (JPL) noted that an alignment of Jupiter, Saturn, Uranus, and Neptune that would occur in the late 1970s would enable a single spacecraft to visit all of the outer planets by using gravity assists. The particular alignment occurs once every 175 years. By 1966, JPL was promoting the project, noting it would allow a complete survey of the outer planets in less time and for less money than sending individual probes to each planet.

In 1969, NASA created the Outer Planets Working Group, which favored the concept of two missions that would visit three planets each (including Pluto, which was considered a planet at the time). These missions were referred to as the Grand Tour. One would launch in 1977 and visit Jupiter, Saturn, and Pluto, while the other would launch in 1979 and visit Jupiter, Uranus, and Neptune. This would reduce total mission time compared to a single Grand Tour from over thirteen years to seven and a half years. The Working Group also called for the development of a new spacecraft to carry out the flyby missions. These probes, called Thermoelectric Outer Planets Spacecraft (TOPS), were being designed at JPL and featured an operational life of over ten years.

By 1971, the estimated cost of Grand Tour was $750 to $900 million, plus over $100 million to launch the spacecraft. Congressional pressure, combined with internal competition from the recently approved Space Shuttle program, led to the decision to cancel the project in December 1971. The Grand Tour and TOPS were replaced with a proposal to visit only two planets using a pair of Mariner-derived probes.

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