Nuclear-Powered Mars Rovers Planned in '09


20th May 2002

NASA engineers are hopeful they can develop nuclear-powered long-distance Mars rovers for launch in 2009. But the agency's new space nuclear power initiative probably won't produce a nuclear-electric propulsion system in time to enable an earlier arrival at Pluto than the gravity-assisted trip already planned.

Viking landers were powered by two RTGs each, covered by windscreens in the right foreground and left edge of this image. A near term program is just getting underway to develop a new generation of radioisotope thermoelectric generators (RTGs) that convert the heat from decaying plutonium fuel into electricity.

NASA also is starting work on space nuclear reactors to dramatically increase power output down the road. Electricity produced by the nuclear devices would power space-probe instruments beyond what is possible with solar power and energize ion drives and other propulsion systems that could accelerate continuously, outstripping the speeds delivered by today's chemical rockets.

The first use of that technology is planned for the 2009 Mars Smart Lander/Mobile Laboratory, which would move across the surface indefinitely in search of promising sites for future missions to explore. NASA is already assembling a team drawn from four of its field centers, its Washington headquarters and the U.S. Energy Dept. to develop new RTGs that can function in planetary atmospheres. The U.S. hasn't built an atmospheric RTG since those it developed for the Viking landers of the mid-1970s, but the approach is straightforward.

"We have to be able to handle the venting differently, and the radiators have to be designed differently," said Chris Scolese, deputy associate NASA administrator for space science, who is overseeing the nuclear
power effort. "In space we have almost 0 deg. K to radiate to. In an atmosphere you have to deal with convective cooling, your fins are different, and the environment that you package it in is slightly different. The internals of it are the same. It's really the radiators that are different."

Headed by Glenn Research Center in Cleveland with requested funds of $79 million in Fiscal 2003, the power portion of the nuclear initiative is also gearing up to boost the electricity output from RTGs. In addition to enhancing the direct conversion of heat to electricity, the program will examine mechanical techniques like Stirling engines.

"The goal is to try and get ourselves up to 20-30% conversion efficiency, as opposed to 7-10% conversion efficiency," Scolese said. "The nice thing about that is we get either more power for a standard-size RTG, or we can use less plutonium and get the same amount of power."

The power produced by RTGs--one or two kilowatts on a spacecraft--isn't enough to supply an electric propulsion system with sufficient thrust to speed a probe to a distant target and then slow it to orbital speeds for an extended survey. Scolese said a power source able to generate "tens of kilowatts" is called for, and that means space nuclear reactors. NASA has mapped a nuclear electric propulsion program funded at $46.5 million in Fiscal 2003 that would start work on a notional system with a Brayton-cycle reactor able to generate a kilowatt of power for every 50 kg. of weight, and a proof-of-concept ion engine with a specific impulse of 9,000 sec.

Marshall Space Flight Center in Huntsville, Ala., would head the effort, with "significant participation" by Glenn Research Center. Early plans called for a systems readiness review in 2003 and a preliminary design review in 2005, leading to flight demonstration programs after that.

"That is purely a research effort at this stage of the game," Scolese said. "There are so many factors that we need to understand and study that we are not ready to commit to a particular design or a particular deployment time."

While NASA is already working on nuclear technology for Mars exploration, it remains to be seen exactly how the nuclear initiative will play into the agency's deep-space work. NASA and the planetary science community are awaiting the results of a National Academy of Science study aimed at setting priorities for outer planets exploration, typically expressed as a competition between those who want to explore Pluto and the other objects at the outer edge of the solar system, and those who want to put an orbiter around Jupiter's moon Europa to see if there is indeed a liquid-water ocean under its frozen surface that could support life.

Backers of the "New Horizons" mission to Pluto plan to use the last remaining RTG in the U.S. inventory to power their probe's instruments at the outer reaches of the solar system, which the mission would reach with a gravity assist from Jupiter following its 2006 launch. But the project is running on unrequested congressional authorizations in the face of opposition from the White House and NASA. Administrator Sean O'Keefe has argued repeatedly that it would make more sense to develop nuclear power and propulsion for a wide range of missions than to spend the next decade and a half on a project that will only produce a flyby of Pluto, Charon and Kuiper Belt objects.

Even if Congress funds the Administration's total space nuclear power request of $125.5 million in Fiscal 2003, there "probably" wouldn't be enough time to mount a nuclear-propelled mission that would reach Pluto any sooner than New Horizons, according to Scolese. However, NASA has also requested $62.5 million in Fiscal 2003 to continue an in-space propulsion program that includes advanced electric propulsion systems with a possible Pluto application using solar power inside the orbit of Jupiter, where sunlight is still strong enough for meaningful solar-electric propulsion.

"WHAT WE WOULD DO to accomplish that, if we could do it, would be to use solar electric to give us the added velocity that we would get from a Jupiter swing-by," Scolese said. "So in the transit from Earth to roughly Jupiter's orbit we would use an ion engine like Deep Space-1 and just keep on accelerating that whole time, and then when we got to the point where sunlight was inadequate to drive the engine we'd jettison that stage. What has to be worked out are the orbital mechanics, does it work with New Horizons or do you need another spacecraft, those sorts of things."

NASA has encountered stiff opposition from environmental groups in the past over plans to launch RTGs, most recently on board the Cassini Saturn probe in 1997. When pitching his space nuclear power research effort, O'Keefe cites the experience of the U.S. Navy in operating nuclear reactors safely over long periods.

"We need to engage this debate with the environmentalists to be sure that we are doing it right, doing it in a way that ensures public safety," O'Keefe responded to a warning from Sen. Barbara Mikulski (D-Md.) on the nuclear safety issue.


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