President Trump’s pick to lead Nasa, the billionaire Jared Isaacman, believes the next leap in space exploration will be driven by nuclear technologies that could dramatically shorten the journey to Mars.
Nasa has been using radioactive materials to power spacecraft since the 1960s. The energy source for deep-space probes, and for Mars rovers such as Perseverance, has been the radioisotope thermoelectric generator (RTG), which turns the heat from the natural decay of plutonium-238 into electricity. These workhorse systems can run reliably for decades.
Jared Isaacman
PATRICK T FALLON/AFP VIA GETTY IMAGES
Launched in 1977 and now nearly 16 billion miles from Earth, Nasa’s Voyager 1 spacecraft is humanity’s most distant emissary. It still reports home from interstellar space, powered by three RTG units.
But the power they supply is modest — enough to run sensors or heaters, not to push a spacecraft along. If humanity wants to cruise to Mars or haul heavy cargo across the solar system, Isaacman argued this week that we would need to think along different lines.
Nasa is exploring two options: nuclear thermal propulsion (NTP) and nuclear electric propulsion (NEP).
In an NTP system, a compact fission reactor splits atoms to produce intense heat. That heat warms a lightweight propellant — typically liquid hydrogen — which then rushes out of a nozzle to create thrust. The hotter the hydrogen gets, the faster it shoots out, and the more efficiently the rocket flies.
According to Dr Nathaniel Read, of the Nuclear Energy Group at the Department of Engineering at the University of Cambridge, the result could be an engine twice as efficient as today’s best chemical rockets. Less mass devoted to fuel would mean more room for crew and supplies, or extra speed.
The US Department of Energy estimates that an NTP system could cut the voyage time to Mars — currently up to nine months — by nearly 25 per cent, reducing a crew’s exposure to space radiation.
Read suggests even faster journey times may be possible, reducing the need to wait for certain optimal alignments of the planets — a tactic that currently saves fuel but forces launches into narrow time windows. And by giving spacecraft more power in reserve, NTP systems may allow astronauts to turn around and head home should something go wrong at any point during their outbound trajectory, a safety advantage not available with conventional propulsion.
NEP works differently: the heat from fission is converted into electricity, which ionises, or positively charges, a gaseous propellant. An “ion engine” pushes out the charged particles at extremely high speed.
According to Read, the technologies are likely to be suited to different types of missions. An NEP will produce a relatively small amount of thrust. But it should be incredibly efficient, allowing for spacecraft to accelerate gently for months on long trips.
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“If you’re not so interested in speed, if you’re just sending stuff, then you might find that NEP is a better proposition overall,” Read said. “With a crewed-manned Mars mission, you really want to get there quickly — because the astronauts are being exposed to low gravity and radiation from the sun and from deep space. So NCP is probably the better candidate.”
Alternatively, the systems could be combined, he said. “You could do short burns with NTP to do the orbital transfers [such as leaving Earth’s orbit and entering the trajectory toward Mars] and then in the mid-course, rather than just drifting, you use nuclear electric propulsion as well – and that can reduce travel times even more substantially.”
Nasa had aimed to demonstrate nuclear-thermal propulsion in space under its Draco programme, a joint effort with the Defence Advanced Research Projects Agency that aimed for a first test flight before the end of this decade. “They seemed confident that they could do it: lots of ground testing had been done,” Read said. Earlier this year, changing budget priorities led both agencies to scrap the project.
However, at a confirmation hearing before the Senate this week, Isaacman — whom Trump renominated, having previously rescinded his endorsement because of links to Elon Musk — argued that if the US wanted to beat China to a lasting presence on the moon and to send humans to Mars, “we must expand and accelerate investments into nuclear propulsion and surface-power programmes”. In interviews, he has hinted at something akin to a mini-Manhattan Project for nuclear electric propulsion.
But the ambition goes beyond propulsion. Nasa is aiming to develop a nuclear reactor for the lunar surface under its Fission Surface Power programme. The goal is a compact fission module that could power a lunar base continuously, including during the two-week lunar night, when solar panels will not work.
China, working with the Russian space agency Roscosmos, has revealed plans for a lunar nuclear plant of its own by 2035. Its reactor would drive energy-hungry life-support systems and other infrastructure. A nuclear space race is in the offing.
In the future, nuclear propulsion may not necessarily involve reactors. In the late 1950s and early 1960s, engineering teams working on Project Orion, which was backed by the US military and briefly by Nasa, proposed a spacecraft propelled by a rapid succession of nuclear bombs detonated behind a massive “pusher plate”.
The theoretical performance was staggering: cruise velocities orders of magnitude beyond chemical rockets. But the 1963 Partial Test Ban Treaty, which bans nuclear explosions in space, snuffed out the concept.