As ambitious as the Voyager mission is, it was scaled down from a plan to send a quartet of spacecraft to Jupiter, Uranus, Neptune and Pluto in what was billed as the "grand tour" of the solar system. But the plan was nixed, and scientists settled for the Voyager mission.
American University space policy expert Howard McCurdy said it turned out to be a boon.
They "took the funds and built spacecraft robust enough to visit all four gas giants and keep communicating" beyond the solar system, McCurdy said.
The double missions so far have cost $983 million in 1977 dollars, which translates to $3.7 billion now. The spacecraft have enough fuel to last until around 2020.
By that time, scientists hope Voyager will already be floating between the stars.
Mission page: http://www.nasa.gov/voyager
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