A round trip to Mars could stretch past three years. Astronauts would reach the planet, then have to loiter there for more than a year waiting for the planets to line up for the flight home. NASA wants the crew’s time in transit cut to about two years, and in February 2021 it asked industry for help with the engine that might do it.
On 12 February 2021, NASA, working with the Department of Energy, released a request for proposals for preliminary reactor designs for a nuclear thermal propulsion system. The agencies plan to fund several efforts to test different approaches. Later contracts would produce more detailed reactor designs and preliminary test hardware.
Two kinds of nuclear engine
NASA is studying two. Nuclear electric propulsion uses a reactor to make electricity, which charges a gas such as xenon or krypton and pushes the ions out of a thruster. The thrust is low, but the propellant goes much further than in a chemical rocket, so a spacecraft can keep accelerating for a long time and fly a Mars mission on a fraction of the propellant.
Nuclear thermal propulsion works differently. A reactor heats a liquid propellant, the liquid turns to gas, and the gas expands through a nozzle. It gives high thrust and twice the propellant efficiency of chemical rockets.
Which will carry the first astronauts is undecided. “We’re exploring both nuclear electric and nuclear thermal propulsion options for crewed Mars missions,” said Anthony Calomino, NASA’s nuclear technology portfolio lead in the Space Technology Mission Directorate. “Each technology has its unique advantages and challenges that need to be carefully considered when determining the final preference.”
The hard part is heat
Calomino called the reactor behind a thermal system “a significant technical challenge.” Most of the engine runs at modest temperatures, but materials touching the reactor fuel have to survive more than 4,600 degrees Fahrenheit. NASA and DOE have worked with industry on an approach, and companies will now draft preliminary designs to meet the requirement.
NASA and DOE are also developing fuels that use low-enriched uranium. Samples go into research reactors at Idaho National Laboratory’s Transient Reactor Test facility and at the Massachusetts Institute of Technology Nuclear Reactor Laboratory. Non-nuclear tests run in simulated reactors at Marshall.
Who is involved
NASA’s Marshall Space Flight Center in Huntsville, Alabama, leads the space nuclear propulsion project. The DOE team includes people from Idaho National Laboratory, Los Alamos National Laboratory and Oak Ridge National Laboratory. The directorate’s Technology Demonstration Missions program pays for the development.
The agency has looked at nuclear thermal propulsion for more than 60 years. Its immediate priority is different. “While NASA’s immediate priority is returning humans to the Moon with the Artemis program, we are also investing in ‘tall pole’ technologies that could enable crewed missions to Mars,” said Jim Reuter, the directorate’s associate administrator.
Only robotic explorers have gone to Mars so far, and none had to come back.
