Europa Clipper Would Wash Out Other Nuclear-powered Missions

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BETHESDA, Md. — If NASA sends a nuclear-powered probe to Jupiter’s moon Europa, it would launch no sooner than 2024, and effectively rule out other nuclear missions to the outer solar system before then by tying up the specialized infrastructure required to produce plutonium-powered spacecraft batteries, a senior NASA official said here.

“If the Europa mission goes nuclear, it needs four or five [Multi-Mission Radioisotope Thermoelectric Generators],” Curt Niebur, a program scientist at NASA headquarters in Washington, said in a July 23 interview here during a meeting of the NASA-chartered Outer Planets Assessment Group. “That’s quite a few. If Europa needs that many, that sucks up all the output for the production line between now and 2024. There’s no more left.”

NASA is studying both nuclear- and solar-powered concepts for a dedicated Europa mission that would launch in the early 2020s. NASA’s leading Europa concept is a nuclear-powered probe known as Clipper. Neither Clipper nor any other Europa mission has been officially funded, but Niebur and his boss, John Grunsfeld, NASA associate administrator for science, have said the $15 million the White House requested in April for Europa work in 2015 is an important sign the administration is serious about exploring the icy moon.

Estimated to cost roughly $2 billion, Clipper was designed by a team at NASA’s Jet Propulsion Laboratory in Pasadena, California, to orbit Jupiter for at least four years in order to perform 45 flybys of the ice-encrusted moon, coming as close as 25 kilometers to Europa’s surface as it searches for signs of organic material and perhaps life. Europa’s salty, subterranean oceans are believed by many to harbor conditions friendly for life.

MMRTG schematic. Credit: NASA
MMRTG schematic. Credit: NASA

A Multi-Mission Radioisotope Thermoelectric Generator, or MMRTG, is a nuclear battery that generates electricity by converting heat given off by the decay of radioactive plutonium-238 pellets. An MMRTG weighs about 45 kilograms, produces around 100 watts of power, and allows spacecraft to stay powered up in places such as the outer solar system where sunlight is too weak for most solar-powered spacecraft.

Clipper will likely need such a power source, but the U.S. Department of Energy, which owns and operates all the equipment needed to refine plutonium-238 and press it into pellets usable by an MMRTG, now plans to shut down its aging pellet-stamping hot press at the Los Alamos National Laboratory in Los Alamos, New Mexico, after 2015, when the department plans to produce one last batch of pellets for the single MMRTG needed for Mars 2020, a sample-caching rover based on Curiosity and slated to launch in 2020.

Len Dudzinski, program executive for radioisotope power systems at NASA headquarters, said in an interview here that the Department of Energy “won’t promise us to be able to support Europa without a new hot press.” NASA, not the Department of Energy, is on the hook to pay for the new equipment.

The Department of Energy has 24 pellets on hand, and Mars 2020 needs 32, plus a few spares, Dudzinski said. The existing hot press, while not in great shape, can handle that, Wade Carroll, the department’s deputy director of space and defense power systems, told SpaceNews in a July 23 interview.


The 24 pellets in the Department of Energy’s inventory were produced in step with development of the Advanced Stirling Radioisotope Generator (ASRG), a plutonium-powered spacecraft battery roughly four times more efficient than the MMRTG. NASA canceled ASRG development for budget reasons in 2013, around the time the Department of Energy noticed problems with the Los Alamos hot press, Carroll said. Specifically, Carroll said, the hot press was leaking water through its baseplate, requiring technicians to halt pellet pressing to perform repairs.

The base plate “is integral to the bottom of the glovebox in which the hot press operates, so it cannot be replaced,” Carroll wrote in a July 24 email to SpaceNews. So, while the equipment is operational, the Department of Energy prefers only to operate it for as long as it takes to finish pressing pellets for Mars 2020.

That process will likely start in 2015, Carroll said. When it is done, the press will be taken offline until a new one can be installed. The Department of Energy hopes the new machine will be online by 2017. After that, the department could press enough fuel to prepare one flight-ready MMRTG a year, Carroll said.