The Juice spacecraft is scheduled to launch to Jupiter and its moons in April. Although she gains momentum four times during maneuvers on Venus, Earth and the Moon, she will be on the road for more than eight years. And if astronauts set out for Mars with current technology, they would have to reckon with a journey time of three years. Because an energy-optimal orbit requires six months of travel time in each direction and at least two years on Mars.
The reason for the duration are the chemical drives that space travel has always relied on. Chemical engines that burn hydrogen with oxygen, for example, have high thrust. They are indispensable for transport from earth to orbit. For long missions in space, however, their efficiency is limited. Nuclear drives, in which a nuclear fission reactor generates the energy, could help. NASA and the US Department of Defense’s Defense Advanced Research Projects Agency (DARPA) announced a collaboration in January with the goal of testing nuclear-thermal propulsion in space by 2027.
With nuclear propulsion “in a few months to Mars”
“In a nuclear-thermal drive, the reactor heats a light fuel such as hydrogen to around 2000 degrees,” explains Professor Uwe Apel, head of the aerospace engineering course at the Bremen University of Applied Sciences. “The gas then flows out of the nozzle at up to 9,000 meters per second, which is twice as fast as with the best chemical engines.” This higher efficiency allows for greater acceleration – a nuclear-thermal thruster would ‘burn’ for several hours on launch from Earth orbit. “You would get to Mars in a few months, stay there for a few months and be back in a few months – a year’s travel time,” explains Apel.
This creates further advantages: Less radiation exposure during the flight, more flexibility in the event of mission aborts and enough electricity, which the reactor also supplies. “You could also dimension the drive in such a way that you can fly at the same speed as with chemical engines, but transport significantly more payload,” explains Apel – an interesting option for unmanned freight transporters.
A second type of nuclear propulsion, the “nuclear-electric”, uses the nuclear reactor purely to generate electricity. This powers a magnetoplasma dynamic or ion engine that ionizes a gas such as argon or xenon and accelerates and ejects it using magnetic or electric fields. The University of Stuttgart is working on the first type with the start-up Neutron Star Systems. Ion thrusters have been used in spacecraft and satellites such as Hayabusa-2, BepiColombo and Starlink. Solar cells have always been the energy source here. If this is replaced by a nuclear reactor, much more energy is available.
The exit speed and thus the fuel efficiency of the engine are even higher than with nuclear-thermal propulsion. However, since the thrust is small, it takes a long time to accelerate. With a manned Mars spacecraft, the acceleration time would “eat up” a lot of flight time, but at the same time would use little fuel. Nuclear-electric drives are particularly advantageous for unmanned missions to distant targets. The “Jupiter Icy Moons Orbiter” (JIMO), which NASA wanted to develop 20 years ago – the project was discontinued in 2005 – would have flown directly to the moons of Jupiter with a nuclear-electric engine in a few years.
Treaties and politics could put the brakes on nuclear propulsion
International agreements such as the United Nations’ “Principles Relevant to the Use of Nuclear Power Sources in Outer Space” from 1992 and the “Safety Framework for Nuclear Power in Space” from 2009 provide the use of nuclear systems in space with a set of rules for maximum safety.
The 1963 Test Ban Treaty, which bans “radioactive deposits outside one’s national territory,” may also be relevant to nuclear-thermal propulsion. It is an “open system” – hydrogen lines run directly through the reactor, which is mounted in front of the outlet nozzle. For Professor Georg Herdrich from the University of Stuttgart, it would be unacceptable to transport such a reactor into orbit with a launch vehicle: “Any launch vehicle can have a false start, which in this case can have very negative consequences for the environment and people.” Alternatively, the drive and reactor should be built in earth orbit – or even better on the moon – and transported there in subcritical form, says Herdrich. He is Germany’s technical representative in the UN working group on nuclear applications in space. The transport of a nuclear-electric engine through the atmosphere is less critical, because here the reactor is a closed system for generating electricity. Such can be built in such a way that it survives critical scenarios.
China and Russia also have plans
If NASA and DARPA want to test a nuclear-thermal drive in space as early as 2027, they cannot avoid transporting the reactor on a rocket. In view of the risks, the mission could encounter political and social hurdles, which is why experts such as Uwe Apel and Georg Herdrich consider the schedule difficult to implement.
But one thing is clear: the major nuclear powers are very interested in using nuclear energy in space. In China there are plans, among other things for the nuclear power supply of a future moon base. And the Russian space agency Roskosmos has been working on the nuclear-electric space tug for over ten years. As a “Zeus” mission, it should fly to Jupiter within a few years from 2030. A 500-kilowatt reactor would drive the 22-ton probe – it would be many times larger than previous Jupiter probes. But sanctions and budget cuts along the war make this plan announced in 2021 unrealistic. Finally, the European space agency ESA is involved in research projects such as “Alumni” and “RocketRoll” with nuclear-thermal and nuclear-electric drives. The University of Stuttgart is also involved in this.
When is nuclear fusion coming in space?
A real game changer for interplanetary travel would be nuclear fusion. Flight times of weeks to Mars and a year to Saturn would be possible. Ambitious plans of the US company PSS envisage a “direct fusion” drive. The University of Stuttgart is also active in the field of basic research. But as long as there are no real breakthroughs in fusion research on earth, fusion drives in space will remain in the realm of desire. Drives based on nuclear fission, on the other hand, will find their way into space travel in the foreseeable future. Although the risks remain and the release of radioactivity – close to the earth, in manned missions also far away from the earth – could have devastating consequences.
(jl)