Space nuclear propulsion is a technology that draws energy from fission instead of traditional chemical reactions, thus providing virtually unlimited energy density and opening the door for crewed missions to Mars and deep space science. NASA is looking at two types of nuclear propulsion systems – thermal and electric.

Nuclear thermal propulsion provides high thrust and twice the propellant efficiency of chemical rockets. The system works by transferring heat from the reactor to a liquid propellant. That heat converts the liquid into a gas, which expands through a nozzle to provide thrust and propel a spacecraft.

Nuclear electric propulsion systems use propellants much more efficiently than chemical rockets but provide a low amount of thrust. They use a reactor to generate electricity that positively charges gas propellants and pushes the ions out through a thruster, which drives the spacecraft forward.

Milestones:

• Via a request for proposals released in February 2021, NASA and DOE asked industry for preliminary reactor design concepts for a nuclear thermal propulsion system. In July 2021, the government team selected three industry teams to explore different approaches. Future contracts will generate more detailed reactor designs and build preliminary testing hardware.
• DOE is testing, developing, and assessing the feasibility of new fuels that use low-enriched uranium for space applications.
• Idaho National Laboratory is advancing and testing fuel composites at its Transient Reactor Test (TREAT) facility to examine how they perform under the harsh thermal and radiation environments needed for nuclear thermal propulsion.
• Since 2016, NASA and its partners focused on reducing risk for nuclear thermal propulsion. This effort included fuel element manufacturing and testing; engine performance and feasibility analysis; developing a safe, affordable engine ground test approach; developing a cost and schedule estimate for a full system; and demonstrating successful long-term storage of liquid hydrogen.
• A 2016 memorandum of understanding between NASA and DOE serves as the basis of this interagency work. An October 2020 NASA-DOE memorandum of understanding expands on it, establishing working groups that focus on developing interagency collaborations for space nuclear power and propulsion.

Fast facts:

• Nuclear thermal propulsion has been on NASA’s radar for more than 60 years.
• Nuclear thermal propulsion could be an enabling technology for future crewed missions to Mars, largely due to its performance advantages over conventional chemical propulsion systems. Under NASA’s current Moon to Mars exploration approach, human exploration of Mars is targeted for as early as the 2030s.
• Nuclear thermal propulsion could allow for more flexible abort scenarios, allowing crew to return to Earth at multiple times, if needed, including immediately upon arrival at Mars.
• Materials inside the fission reactor must be able to survive temperatures above 4,600 degrees Fahrenheit.
• To keep the round-trip crewed mission duration to about two years, NASA is looking at nuclear-enabled transportation systems to facilitate the shorter-stay class of missions, taking advantage of optimal planetary alignment for a low-energy transit for one leg of the trip, and using the new technology to make the higher-energy transit for the other leg.
• NASA is looking at two types of nuclear propulsion systems – thermal and electric – for human missions to Mars, specifically for stays of up to 50 days in the vicinity of Mars (30 days on the surface) and an overall mission duration of no more than two years. To meet this mission duration, a nuclear electric system would need a large chemical stage to meet the overall thrust capability needed for a human Mars mission.

Partners:

• NASA’s space nuclear propulsion project is led by the agency’s Space Technology Mission Directorate and funded through its Technology Demonstration Missions program based at NASA’s Marshall Space Flight Center in Huntsville, Alabama. The project, also at Marshall, collaborates with DOE to advance the key technologies needed for future human missions to Mars.
• DOE facilities supporting NASA’s space nuclear propulsion project include Idaho National Laboratory, Oak Ridge National Laboratory, and Los Alamos National Laboratory.
• In addition to Marshall, NASA centers supporting nuclear thermal propulsion activities include NASA’s Glenn Research Center in Cleveland and NASA’s Stennis Space Center in Mississippi. 
• Nuclear thermal propulsion development is also supported by the Massachusetts Institute of Technology, University of Alabama Huntsville, Aerojet Rocketdyne, BWX Technologies, UltraSafe Nuclear Corporation, the Aerospace Corporation, Analytical Mechanics Associates, and Geocent.
• Each supporting entity brings its own unique expertise and capabilities to contribute to the goal of realizing a high-performance fission-based propulsion system to enable extended human exploration of the solar system.