Mars rover gets 'engine' upgrade: Curiosity fueled by nuclear power

Although the technology has been deemed safe, protests have arisen over the use of RTGs, reaching a crescendo during the run-up to NASA's 1997 launch of the Cassini-Huygens mission to Saturn. The tandem spacecraft reached Saturn in 2004. The Cassini orbiter is currently touring Saturn and its moons.

Cassini's RTGs carry between 25 and 33 pounds of plutonium. Unlike the Mars Science Laboratory mission, which hosts 10.6 pounds of plutonium-238 and will head directly for the red planet, Cassini returned to Earth's neighborhood for a gravitational boost on its way to Saturn, providing the anxious with a second potential window for a mishap.

By contrast, the New Horizons mission and – so far – Mars Science Laboratory have seen little in the way of opposition.

Needed: more plutonium

The biggest concern among planetary scientists now is that the Mars Science Laboratory and other long-duration missions planned for the decade may be the last of their breed, according to Ralph McNutt, chief scientist in the Johns Hopkins University Applied Physics Laboratory's space division.

The reason: Plutonium-238 supplies are dwindling, and efforts to produce more are sputtering in an uncertain budget climate.

"If we don't get our act together and get restarted on this, the doors are going to close on a big piece of the future" of space exploration at destinations ranging from the moon to the solar system's outer reaches, he says.

The US stopped producing plutonium-238 in 1988, he says, estimating that between 40 and 70 pounds of the deep-space power source remains in stock.

Efforts to restart production, under way since 2010, have slowed to a crawl over who pays: NASA, the US Department of Energy, or both. Lawmakers in Washington initially opted to have the two agencies share the cost. But recent budgets have funded NASA's contribution, but not the Department of Energy's contribution, to the effort, pegged at between $70 million and $150 million to resume production.

In the meantime, engineers are developing an alternative to the RTG that uses plutonium-238 to drive a Stirling engine to generate electricity. For the same amount of juice, the new design is said to require one quarter of the plutonium current RTGs require.

"That would make better use of existing stocks," says the Idaho national Laboratory's Dr. Johnson in an interview.

Dr. McNutt agrees, but suggests we'll still need RTGs while the new power source is being developed. To use a new power source on a billion-dollar-class mission to Jupiter's moon Europa, for instance, would probably require a 28-year test, since scientists would want to see how it performs over a period two times longer than the duration of the mission.

That could create additional delays – and add costs associated with delays – for an already expensive mission.

The alternative, giving up on plutonium to help generate electricity, would represent its own kind of setback to space exploration, McNutt adds.

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