Safety and Radioisotope Power

Safety is a critical priority in NASA’s use of radioisotope power systems, and is integrated into every phase of the design, test, manufacture, and operation of space nuclear systems.

Quick Facts


Multiple layers of safety features are incorporated into the design of the RPS —also known as radioisotope power systems — used by NASA. RPS are designed and tested to contain their nuclear fuel during normal operating conditions and in a range of potential accident conditions.

The United States has flown more than two dozen missions with RPS and one reactor system over the past 50 years. RPS have an outstanding safety and reliability record. They have never caused a spacecraft failure, and decades of effort have been invested in their engineering, safety, analysis, and testing.

NASA has a long history of safe launch and operations of spacecraft carrying RPS, and no member of the public or NASA employee has ever been injured by any previous use of an RPS system or a related launch.

RPS are extremely reliable and dependable sources of spacecraft electrical power and useful heat energy. Every RPS flown by the United States has worked as designed, with many far exceeding their operational design life (the current MMRTG is designed to operate for at least 14 years in flight).

Previous Launch Accidents Involving an RPS

Wreckage on the sea flooer, including a cylinder in the foreground.
Intact SNAP-19 fuel capsule is shown among debris on Pacific Ocean floor resulting from aborted launch of a Nimbus B weather satellite in 1968.
U.S. Department of Energy

Three missions using RPS have been subject to mechanical failures unrelated to the power system that resulted in early aborts of the mission. In each instance, the radioisotope power system performed precisely as it was designed to do.

  • The April 1964 launch of the Transit 5-BN-3 navigational satellite was aborted during its ascent to orbit. Its radioisotope thermoelectic generator (RTG) burned up upon reentry, as intended by its design, and dispersed its plutonium fuel in the upper atmosphere.
  • The May 1968 launch of the Nimbus B-1 weather satellite was aborted shortly after launch. Its RTG contained the plutonium fuel as designed. The fuel container was retrieved intact and the fuel was used on NASA's Nimbus III weather satellite.
  • An RTG intended to operate science instruments on the surface of the moon as part of Apollo 13 returned to Earth in April 1970 following the aborted mission. The Apollo 13 lunar module, "Aquarius," was used successfully as a lifeboat for the three astronauts following damage to their command module — unrelated to the RTG — on the way to the Moon. Following the astronauts' safe return, the lunar module carrying the RTG fell into deep water in Pacific Ocean. No release of radiation from this incident has been detected.

NASA and the Department of Energy place the highest priority on assuring the safety of the general public and their workers during activities that utilize radioactive materials, and at related facilities.

National Environmental Policy Act (NEPA) and Launch Approval

NASA is subject to the National Environmental Policy Act (NEPA), which requires all federal agencies to integrate environmental values into their decision making processes by considering the environmental impacts of their proposed actions and the reasonable alternatives to those actions.

NASA and DOE have demonstrated that potential mission risks are small, through ground-based testing and modeling, the NEPA process, and related risk assessments.