WASHINGTON — NASA has ruled nuclear power off limits for its next Discovery-class planetary mission and likely will require any such mission going farther than the Moon to carry an experimental laser communications payload, officials said here March 12.

The 25-kilogram payload, which draws 75 watts, would be provided by the Jet Propulsion Laboratory in Pasadena, Calif., as part of an interplanetary laser communications demonstration, Michael New, chief scientist for the Discovery Program at NASA headquarters here, told members of the NASA Advisory Council’s planetary science subcommittee during a public teleconference. 

New acknowledged that the so-called mass and power tax that carrying such a payload would entail “is a lot” for a planetary mission that can cost no more than $450 million, not counting launch. As such, New said, NASA could still decide to drop the requirement to demonstrate JPL’s laser communications package if it ultimately proves unfeasible.  

“We will probably be requiring missions to carry it, and demonstrate it somewhere in flight,” New said. “If it works, great. The mission is welcome to use it for its own data return. If it doesn’t work … the mission should have its own radio system to back it up.”

If NASA decides to eliminate the piggyback payload requirement, the change would show up by the end of 2014 in the final announcement of opportunity for what would be the agency’s 13th standalone Discovery-class mission since inaugurating the program of cost-capped, competitively selected missions with the 1996 launches of the Near Earth Asteroid Rendezvous probe and Mars Pathfinder rover. A draft announcement of opportunity is expected in May, and an award is planned for 2016, when NASA will choose one mission from among two or three finalists to be selected next spring.

While NASA could effectively make the laser communications experiment mandatory for nonlunar Discovery proposals this time around, it is also barring a tried-and-true payload from consideration.

NASA is telling scientists not to propose Discovery missions that require a radioisotope power system — long-lasting nuclear batteries capable of providing spacecraft with electricity when sunlight is too faint to rely on solar arrays.

That is a big turnaround from NASA’s most recent Discovery competition, when scientists were given the option to design their missions around a government-furnished Advanced Stirling Radioisotope Generator, a next-generation nuclear power supply roughly four times more efficient than the flight-proven generators aboard the Mars Curiosity rover and Cassini Saturn probe.

NASA ultimately selected the solar-powered InSight Mars lander over two competing nuclear-powered proposals for the 2016 launch opportunity and has since halted development of the Stirling system. 

With Stirling out of the picture, NASA says there will not be enough plutonium-238 ready at the end of the decade to fuel comparatively inefficient Multi-Mission Radioisotope Thermoelectric Generators for both the Mars 2020 rover and the Discovery 13 mission.

Jim Green, director of NASA’s Planetary Science Division, told the NASA Advisory Council’s planetary science subcommittee March 12 that the Department of Energy (DoE) is having trouble with aging equipment known as a hot press and as a result will only be able to mint enough plutonium-238 pellets for the Mars 2020 rover. 

NASA has been paying DoE about $50 million a year to restart plutonium-238 production at Oak Ridge National Laboratory in Tennessee. The United States stopped producing the nonweapons-grade isotope in the 1980s and has been relying since on a dwindling stockpile supplemented with periodic purchases from Russia.

NASA is now pouring money into repairing DoE’s hot press, which is more than 50 years old, but Green said, “It will take approximately three-and-a-half years to replace that fully and get into production of the pellets.” Once the hot press is back on line, the Mars 2020 mission has first claim on the plutonium pellets it will produce, he said.

Meanwhile, NASA has also put a hard limit on international contributions to the next Discovery mission, saying no more than one-third of the mission’s development cost may be borne by a non-U.S. partner. The same one-third limit also applies to the mission’s science payload, New said.

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Dan Leone is the NASA reporter for SpaceNews, where he also covers other civilian-run U.S. government space programs and a growing number of entrepreneurial space companies. He joined SpaceNews in 2011.Dan earned a bachelor's degree in public communications...