NASA's most ambitious Mars exploration mission yet is set for launch on Saturday, and the rover has one hot energy source.

Gone are the fly's-wing-like solar panels that provided electricity for earlier rovers Spirit and Opportunity.

Instead, the rover for the Mars Science Laboratory, Curiosity, carries 10.6 pounds of plutonium-dioxide pellets. The system is designed to allow the vehicle to operate where sunlight is so weak or the spacecraft's mission is so demanding that solar panels are impractical.

"You can operate with solar panels on Mars, you just can't operate everywhere," said Stephen Johnson, who heads the space nuclear systems technology division at the US Department of Energy's Idaho National Laboratory in Idaho Falls, in a statement. "This gives you an opportunity to go anywhere you want on the planet, not be limited to the areas that have sunlight, and not have to put the rover to sleep at night."

The power source is known as a multimission radioisotope thermoelectric generator (RTG), and other versions have been incorporated into spacecraft designs since the 1960s. In January 2006, NASA launched an RTG on the New Horizons spacecraft, which is now more than half way to its planned 2015 flyby of Pluto. Even the Mars rovers Sojourner, Spirit, and Opportunity have carried small plutonium-fueled heaters to keep the vehicles' electronics warm.

The RTG on Curiosity uses the contrast between heat of plutonium's radioactive decay and the chilly temperatures on Mars to generate electricity.

What are the launch risks?

The form of plutonium used, plutonium-238, is unsuitable for nuclear explosive. The primary risk, according to physicists, comes if finely ground plutonium is inhaled or ingested along with with food.

The units housing the plutonium-dioxide pellets have undergone rigorous tests under conditions one might expect to see if a rocket carrying an RTG-bearing spacecraft has to be destroyed before it reaches orbit, NASA and US Department of Energy officials have said.

By some accounts, six missions since the 1960s, including the aborted Apollo 13 mission and Russia's Mars 96 mission, have ended with RTGs burning up high in the atmosphere or plunging into the ocean after surviving reentry. While early models released radioactive material as craft burned up on reentry, later models appear to have survived reentry and impact intact without releasing radioactive material.

NASA's environmental-impact statement for this launch puts the risk of a release from a launch-area accident at 1 in 420. But the agency's calculations put the risk of adverse health effects to any single individual from an accident near the launch site at less that 1 in 1 million.

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.