In revising the Artemis Program plans in the last month, NASA announced an additional mission involving a nuclear-electric interplanetary spacecraft heading to Mars. The switch to nuclear power over chemical rockets is one I have advocated for a long time. Eliminating the enormous fuel payload needed to get to Mars is one of the advantages of going nuclear. The other could be improving transit times, although for this upcoming Mars mission, the journey is expected to take a year.
What’s driving NASA to revisit the nuclear option? Likely, it is the renewed space race with Russia recently announcing plans to deploy a nuclear rocket option and China continuing to make progress with its space program. With either of these two space rivals, NASA does not want to be playing from behind. It already did that in the 1960s before the Apollo Program vaulted past the Soviet Union to achieve the Moon landing in 1969.
NASA Has a Nuclear Past and Present
Radioisotope Thermoelectric Generators (RTGs)
NASA’s history with nuclear power for spacecraft has involved using RTGs and other thermoelectric nuclear power sources. RTGs use radioactive plutonium-238’s natural decay to generate thermoelectric power. The technology has been perfected over time, and it has proven to be very reliable.
Today, RTGs continue to power the two Voyager spacecraft that have long left the Solar System after touring the Ice Giants of the Outer Solar System. RTGs powered the two Viking spacecraft that landed on Mars in 1976. These systems provided power for Galileo as it circled Jupiter, and Cassini during its multi-year mission to the Saturnian system. New Horizons, the spacecraft that visited Pluto and continues to explore the Kuiper belt, is powered by an RTG. Today, two RTG power plants keep Curiosity and Perseverance going on the Martian surface.
RTG technology, however, is not suitable to power a rocket. For that, NASA has looked to alternative technologies starting in the 1960s, up until now.
NERVA and DRACO
The 1960s saw NASA and the Atomic Energy Commission launch a project they called NERVA, a nuclear-thermal-propulsion (NTP) rocket using a solid-core fission reactor to heat hydrogen gas and expel at high speed through a nozzle. The project was cancelled not because it wouldn’t work, but rather because of budgetary concerns.
Its replacement, more recently, has been called DRACO, a demonstration nuclear rocket technology. DRACO also uses a fission reactor and hydrogen fuel. Until recently, it was the technology NASA intended for its first nuclear rocket. Now the project has been mothballed.
Space Reactor-1 (SR-1)
The new nuclear-powered rocket is called the SR-1 Freedom, with a mission plan to go to Mars. The SR-1 mission will carry the Skyfall payload to Mars. The Skyfall capsule will descend to the Martian surface by parachute and deploy three helicopters, built as successors to Ingenuity, the helicopter that accompanied the Perseverance rover and flew 72 missions for almost three years before rotor damage grounded it.
NASA describes the SR-1 as a nuclear-powered interplanetary tugboat. To meet a 2028 launch date, NASA intends to use the already-built Power and Propulsion Element (PPE) that was to be used by the now-defunct Lunar Gateway and modify it to integrate a 20+ KiloWatt nuclear fission reactor. Lunar Gateway was to be powered by solar panels. Repurposed for the SR-1, the reactor will generate electricity to power xenon plasma ion thrusters to provide thrust.
Where NERVA and DRACO converted power from their fission reactors to directly superheat hydrogen gas and expel it to achieve thrust, SR-1 will use a more indirect approach with the reactor generating electricity to feed ion thrusters, in other words, a NEP, a nuclear-electric propulsion system.
The timelines are tight. The Mars launch window is to have SR-1 ready for December 2028. Besides assembling the SR-1, which will involve the nuclear reactor, a power conversion system and the ion thrusters, the U.S. Department of Energy will have to provide nuclear certification.
SR-1 will take a year to get to Mars. So, this is not a high-speed option to be used by crewed missions to Mars in the future. Besides the increased mass and the need for shielding to protect the crew from cosmic rays, a variant of the SR-1 could be used. For example, a 100 to 500 KiloWatt fission reactor paired with a larger power conversion system and more ion thrusters could shorten transit times to 3 to 4 months. A successor to the SR-1 could include return propulsion systems to ensure a margin of safety for its crew.
What the SR-1 Freedom mission will do is be a proof of concept that should quickly render chemical rockets for long-duration missions beyond the Moon obsolete.
Are Fusion-Powered Rockets in Our Future?
Is it possible to see fusion-powered rockets in the next 30 years? Controlled fusion here on Earth is still going through birth pains. A commercial fusion reactor will likely be a reality within the next ten years. A direct fusion drive to power a rocket is another matter.
Why go there? Because nuclear fusion in space is far easier than doing it here on Earth, and because nuclear fusion engines would need only tiny amounts of fuel to generate the thrust needed to shorten a mission to Mars to 1 or 2 months, and to Pluto in less than 4 years. Fusion would be the king of propulsion technologies.
