In the coming decades of space exploration, astronauts will return to the moon, the first manned missions to Mars, and robotic missions to the outer solar system (among others). These missions will use innovative technologies that enable faster transits, longer stays and sustainable living far away from Earth. To that end, NASA and other space agencies are exploring nuclear applications, particularly when it comes to power and propulsion. Many of these proposals have been in the books since the early space age and have been thoroughly validated.
On Tuesday, January 24, NASA and the Defense Advanced Research Projects Agency (DARPA) announced they were initiating an interagency agreement to develop a nuclear-thermal propulsion (NTP) concept. The proposed nuclear missile is known as the Cislunar Agile Operations Demonstration Missile (DRACO), which would allow rapid transit missions to Mars (weeks instead of months). This three-phase program will culminate with an in-orbit demonstration of the DRACO, which is expected to take place in early 2027.
Since the early space age, NASA and other space agencies have considered several nuclear spacecraft proposals. These fall into two categories: nuclear thermal and nuclear electric propulsion (NTP/NEP). For NTP, a nuclear reactor heats deuterium or tritium (hydrogen-2 or -3) propellant to create plasma, which is then passed through nozzles to create thrust. In an NEP rocket, a reactor powers a Hall effect thruster that ionizes and accelerates inert gas (like xenon) to create thrust. While NEP provides higher specific impulse (Isp) or longer periods of thrust, NTP provides greater thrust.
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Artistic concept of a bimodal nuclear thermal rocket in low earth orbit. Photo credit: NASA
In recent years, several proposals have been made for NTP systems that could shorten transit times to Mars to less than 100 days—some as much as 45 days! Faster and more efficient transport technology is critical for manned missions to Mars and is consistent with NASA’s Moon-Mars goals. With conventional rockets, the journey to Mars would take six to nine months, and the missions can only be launched every 26 months (coinciding with a Mars opposition). During these transits, astronauts are exposed to increased solar and cosmic radiation.
They will also spend all of their time in microgravity, which takes a significant toll on human physiology. Finally, longer voyages require more supplies and storage space, which is limited aboard the Orion starship, which serves as its crew’s command room, dormitory, and dining room. A more powerful propulsion system allows for larger spacecraft that can carry larger science payloads and provide more power for instrumentation and communications. As Administrator Bill Nelson said in a recent NASA press release:
“NASA will work with our long-time partner DARPA to develop and demonstrate advanced nuclear thermal propulsion technology as early as 2027. With the help of this new technology, astronauts could travel in and out of space faster than ever before – an important skill in preparing for manned missions to Mars. Congratulations to NASA and DARPA on this exciting investment as we ignite the future together.”
Under the terms of the agreement, NASA’s Space Technology Mission Directorate (STMD) will lead the engineering development of the nuclear heat engine that will be integrated into the spacecraft built by DARPA. DARPA will lead the overall program as contracting authority, overseeing missile system integration and procurement, permitting, scheduling and other considerations. NASA and DARPA will work together to assemble the engine ahead of its space demonstration as early as 2027. DARPA Director Dr. Stefanie Tompkins said:
“DARPA and NASA have a long history of fruitful collaborations in advancing technologies for our respective goals, from the Saturn V rocket that brought humans to the moon for the first time, to robotic maintenance and refueling of satellites. The realm of space is vital to modern commerce, scientific discovery, and national security. The ability to make leapfrogging advances in space technology through the DRACO thermal nuclear missile program will be critical to more efficiently and rapidly transporting material to the Moon and eventually humans to Mars.”
Artist’s rendering of four KRUSTI generators powering an outpost on the surface of Mars. Photo credit: NASA/STMD
NASA’s earlier efforts to develop nuclear technologies for space exploration include the Nuclear Engine for Rocket Vehicle Application (NERVA), which was successfully tested in 1964 and 1969. Radioisotope thermoelectric generators (RTGs) have been tested in space since 1961 and were part of the surface experiments of the Apollo missions. Since then, Multi-Mission Radioisotope Thermoelectric Generators (MMRTG) have powered robotic probes such as the Viking, Voyager, Galileo, Cassini and New Horizons missions, as well as the Curiosity and Perseverance rovers.
NASA, the Department of Energy (DOE), and commercial and industrial partners are also working to implement nuclear technologies for multiple mission profiles. These include NASA’s Fission Surface Power project, which is expanding the Kilopower Reactor Using Sterling Technology (KRUSTY) project to develop nuclear reactors that could power long-term missions to the Moon, Mars and beyond. In June, NASA and the DOE award three commercial design efforts to develop nuclear power plant concepts that could be deployed on the surface of the Moon and later Mars.
This year, NASA’s Innovative Advanced Concepts (NIAC) program awarded Phase I contracts to several proposed nuclear technologies. These include a hybrid fusion/fast fission reactor that would power a mission to Europa, a nuclear-thermal engine that could enable missions to Mars in just 45 days, and a miniature nuclear battery that could enable CubeSat missions to the outer solar system . Said Jim Reuter, Associate Administrator for STMD.
“With this collaboration, we will leverage our expertise gained from many previous nuclear power and propulsion projects in space. Recent advances in aerospace materials and engineering are enabling a new era for nuclear technology in space, and this flight demonstration will be a major achievement to create space transportation capability for an Earth-Moon economy.”
Further reading: NASA
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