WASHINGTON — The U.S. Environmental Protection Agency (EPA) found no show-stoppers with NASA’s plan to put a nuclear battery aboard the Mars 2020 sample-caching rover, according to a Final Environmental Impact Statement the space agency published on its website Nov. 6.

The NASA-led environmental review will not technically be complete until at least Dec. 19 — the soonest federal regulations allow NASA to post a formal record of its decision to use nuclear material on the mission — but the lack of red flags from the EPA is a signal that the way is essentially clear for NASA to proceed with its plan to power Mars 2020 with a multimission radioisotope thermoelectric generator (MMRTG).

The MMRTG will convert heat from about 4.8 kilograms of decaying plutonium 238 into electricity to power Mars 2020’s seven science instruments, powertrain, sample-caching apparatus and communications array. The rover is slated to launch in July or August 2020 to collect and cache a martian surface sample that could be returned to Earth later by another mission. The backup launch date is August or September 2022.

Mars 2020 will be a modified version of the Mars Science Laboratory’s Curiosity rover, which launched in 2012 and will continue roving the red planet until at least 2016, if it stays healthy. Curiosity also uses an MMRTG and was subject to the same environmental review process Mars 2020 is finishing now.

The only quibble EPA had with NASA’s 317-page Environmental Impact Statement was that the document did not spell out NASA’s intent to involve the EPA in any accident investigation that would be necessary should Mars 2020 be destroyed by a launch failure or unplanned atmospheric re-entry. In comments appended to the statement, EPA said NASA could add such an invitation in the record of decision expected in December. NASA replied that it would do so.

Under the National Environmental Policy Act of 1969, NASA has to conduct an environmental review of any mission that involves launching nuclear material. Among other things, the agency must quantify the risk of radioactive contamination from launch failure and explore whether the mission requires a radioactive power supply.

The risk of contamination due to launch failure is minimal, according to NASA.

There is only a 2.5 percent chance of a Mars 2020 launch failure, the agency calculated, and an even slimmer chance, 0.4 percent, of a launch failure that would breach the rover’s MMRTG and its iridium-encased plutonium fuel capsules, according to NASA’s Final Environmental Impact Statement.

“The average maximum dose to any member of the public from an accident with a release in the launch area would be equal to about 3 months of exposure to natural background radiation for a person living in the United States,” NASA wrote.

NASA Planetary Science Division Director Jim Green told SpaceNews in a Nov. 11 email that a solar-powered Mars 2020 rover could carry the full seven-instrument, $130 million science payload the agency approved in July, but such a rover would have to land closer to the martian equator and might not last as long as the plutonium-powered rover NASA intends to build.

“The main difference is that the MMRTG alternative would be capable of a full Martian year of operations within a broader range of landing latitudes, whereas the two solar-powered options would require narrower landing latitudes, constrained science operations, and potentially reduced mission lifetime,” Green wrote in his email. One martian year is just under two Earth years long.

Although NASA technically has to wait until after posting its record of decision to proceed with an MMRTG-powered Mars 2020, some of the early work on the rover’s nuclear battery is already underway.

For example, the Department of Energy in June gave Aerojet Rocketdyne of Sacramento, California, a $7.5 million contract to prepare for flight one of three MMRTGs that Canoga Park, California-based Boeing Rocketdyne Propulsion and Power built under a 2003 contract. Aerojet Rocketdyne inherited the MMRTG contract in 2012, when its parent, Gencorp Inc., bought Rocketdyne from United Technologies Corp.

The presence of plutonium on Mars 2020 also has ramifications for the type of launch vehicle NASA will be allowed to use. There are several existing or planned launch vehicles capable of lofting the car-sized rover to Mars, but the only one now in operation that is also NASA-certified to carry nuclear material is United Launch Alliance’s Atlas 5. NASA examined the possibility of launching Mars 2020 on that rocket, United Launch Alliance’s Delta 4 rocket or Space Exploration Technologies Corp.’s planned Falcon Heavy, which is slated to debut in 2015.

Neither Delta 4 nor Falcon Heavy is currently approved to launch NASA science missions, let alone the high-risk missions known internally as Class A payloads. An MMRTG automatically makes a mission Class A, Jim Norman, director of the agency’s Launch Services Program here, wrote in a Nov. 12 email.

The cost of using a nuclear power source goes beyond the expense of the hardware and expertise required to install an MMRTG or similar unit on a spacecraft. NASA spokesman Dwayne Brown declined to say how much NASA paid to conduct the Mars 2020 environmental review now nearing completion, but estimates of previous reviews peg the cost of compliance at tens of millions of dollars.

Back in January, Ralph McNutt, a planetary scientist based at the Applied Physics Laboratory in Laurel, Maryland, and a participant in several nuclear-powered space missions including the Pluto-bound New Horizons probe, told the NASA-chartered Outer Planets Assessment Group it cost around $65 million to study whether the public would have been exposed to radiation if NASA’s flagship Saturn orbiter Cassini failed during its 1997 launch.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.