NASA Mars Curiosity Rover: MMRTG likely cause of electronic short

This artist concept features NASA's Mars Science Laboratory Curiosity rover, a mobile robot for investigating Mars' past or present ability to sustain microbial life. 

In this picture, the mast, or rover's "head," rises to about 2.1 meters (6.9 feet) above ground level, about as tall as a basketball player.

This mast supports two remote-sensing instruments: the Mast Camera, or "eyes," for stereo color viewing of surrounding terrain and material collected by the arm; and, the ChemCam instrument, which is a laser that vaporizes material from rocks up to about 7 meters (23 feet) away and determines what elements the rocks are made of.

NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, manages the Mars Science Laboratory Project for the NASA Science Mission Directorate, Washington.

NASA's Mars rover Curiosity resumed full science operations on Saturday, Nov. 23.

Activities over the weekend included use of Curiosity's robotic arm to deliver portions of powdered rock to a laboratory inside the rover.

The powder has been stored in the arm since the rover collected it by drilling into the target rock "Cumberland" six months ago.

Several portions of the powder have already been analyzed. The laboratory has flexibility for examining duplicate samples in different ways.

The decision to resume science activities resulted from the success of work to diagnose the likely root cause of a Nov. 17 change in voltage on the vehicle. The voltage change itself did not affect the rover safety or health.

The vehicle's electrical system has a "floating bus" design feature to tolerate a range of voltage differences between the vehicle's chassis—its mechanical frame—and the 32-volt power lines that deliver electricity throughout the rover. This protects the rover from electrical shorts.

"We made a list of potential causes, and then determined which we could cross off the list, one by one," said rover electrical engineer Rob Zimmerman of NASA's Jet Propulsion Laboratory, Pasadena, Calif. Science operations were suspended for six days while this analysis took priority.

The likely cause is an internal short in Curiosity's power source, the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG).

Due to resilience in design, this short does not affect operation of the power source or the rover.

Similar generators on other spacecraft, including NASA's Cassini at Saturn, have experienced shorts with no loss of capability.

Testing of another MMRTG over many years found no loss of capability in the presence of these types of internal shorts.

Following the decision to resume science activities, engineers learned early Nov. 23 that the rover had returned to its pre-Nov. 17 voltage level.

This reversal is consistent with their diagnosis of an internal short in the generator on Nov. 17, and the voltage could change again.

The analysis work to determine the cause of the voltage change gained an advantage from an automated response by the rover's onboard software when it detected the voltage change on Nov. 17.

The rover stepped up the rate at which it recorded electrical variables, to eight times per second from the usual once per minute, and transmitted that engineering data in its next communication with Earth. "That data was quite helpful," Zimmerman said.

NASA halts work on MMRTG nuclear generator for deep space exploration

MSL’s Pu-238 fueled MMRTG in the laboratory. Credit: NASA

Another blow was dealt to deep space exploration this past weekend.

The announcement comes from Jim Green, NASA's Planetary Science Division Director.

The statement outlines some key changes in NASA's radioisotope program, and will have implications for the future exploration of the outer solar system.

We've written about the impending plutonium shortage and what it means for the future of spaceflight, as well as the recent restart of plutonium production.

NASA is the only space agency that has conducted missions to the outer planets—even the European Space Agency's Huygens lander had to hitch a ride with Cassini to get to Titan—and plutonium made this exploration possible.

Probably the most troubling aspect of the announcement is the discontinuation of procurement by NASA of flight hardware for what was to be NASA's next generation nuclear power-source for exploration, the Advanced Stirling Radioisotope Generator, (ASRG).

This was to replace the Multi-Mission Radioisotope Generator, (MMRTG) that has been in use on spacecraft for decades.

An Advanced Stirling Converter prototype in the laboratory. 

Credit: NASA

The announcement states:

"With an adequate supply of Pu-238… NASA has decided to discontinue procurement of ASRG flight hardware."

"We have given direction to the Department of Energy… to end work on the flight units."

"The hardware procured under this activity will be transferred to the Glenn Research Center to continue development and testing of the Stirling technology."

The announcement cites the current budget-constrained environment that NASA and planetary space exploration finds itself up against.

What the exact future role is of NASA Glenn beyond basic research and development isn't entirely clear, but two ASRG units that were to be flight-ready for missions in 2016 are shelved for now.

The announcement does mention that NASA will continue to utilize flight-proven MMRTG's in the near term, which provide the same approximate power output as the ASRG, albeit with less efficiency.


Plutonium is vital for outer solar system exploration. As you get farther away from the Sun, solar energy ceases to become a viable alternative power source.

Pioneers 10 & 11, Voyagers 1 & 2, Galileo, and Cassini all utilized nuclear RTGs, as does the Mars Curiosity rover and New Horizons mission headed out explore Pluto in during its July 2015 flyby.

NASA's Juno spacecraft scheduled to reach Jupiter in 2016 will be the first-ever mission without an RTG to explore the outer solar system, and it must employ three enormous solar panels to do so.

A labeled cutaway of an ASRG. Credit: 

Wikimedia Commons And although the production of the Pu-238 isotope used in space exploration was a side-benefit of the Cold War, it isn't the same stuff as its isotopic cousin Pu-239, which is used in nuclear weapons. 

Plutonium production in the United States ceased in 1989, and although the U.S. government announced earlier this year that NASA will restart the plutonium production pipeline for space exploration, production levels are only expected to reach 1 to 1.5 kilograms per year.

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