Despite the fact that a useful – and rare – alignment of the large outer planets will make a “Grand Tour” possible, NASA has only thus far funded a stripped-down version of the ambitious original Grand Tour plan, a pair of Mariner Jupiter/Saturn ’77 unmanned space probes (later renamed Voyager). Jet Propulsion Laboratory admits that scientists and mission planners have drawn up a “Uranus option” to extend the mission of one of the vehicles to reach Uranus four or five years after a Saturn encounter and gravity assist, and are making modifications to one of the vehicles to permit this contingency. (NASA has yet to approve continuing the MJS’77 program long enough to reach Uranus.) Mission planners also admit that a visit to Uranus could give the vehicle another gravity assist toward Neptune, while admitting that the odds of the vehicle surviving a journey to Neptune with its ability to gather images and scientific data intact would require “a miracle.”
