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There’s one place where the case for nuclear power is undisputable

I was amused to read recently that the US military plans to send nuclear reactors into space, as if space is not already pretty full of nuclear reactors. Unfortunately, neither the US military nor anyone else on Earth has yet worked out how to harness nuclear fusion – the process that powers the sun and all the other stars. The only human use of fusion is in thermonuclear weapons. This more modest plan involves sending ordinary fission reactors into space.

In many ways, the only real surprise is that it isn’t already routine: after all the US and other militaries already use nuclear reactors to power submarines. They do this because all other means of powering a ship require air, or some other plentiful source of oxygen, to burn chemical fuels with. This means that conventionally powered subs need to come to the surface often (or extend a “snort” air-intake mast above it) so as to run their engines and recharge their batteries. Only a nuclear submarine can move fast and far while fully submerged.

No requirement for air obviously makes nuclear reactors suitable for space, too. And reactors have other things in their favour as well. Reactors pack a huge energy density punch, with large amounts of energy delivered from small units and small volumes of fuel. This makes them ideal for space travel where things which don’t weigh a lot or take up a lot of space are always needed.

There are essentially two requirements for energy in spacecraft. One is electrical power, used to run the spacecraft’s systems and also for some forms of propulsion. The other is heat energy, used to blast reaction mass out of a rocket to provide thrust.

Current propulsion methods mostly use chemical fuel and oxidiser both to generate heat and as the reaction mass. Launch rockets typically consist of large tanks of highly flammable hydrogen or kerosene fuel and other tanks of liquid oxygen. Even the vast amounts of energy stored in such a rocket are not enough to put it into orbit, so the rockets have to be stacked on top of each other, with only the final stage actually achieving orbit. Such rocket stacks are tricky things, as more than a dozen rocket failures in 2022 can attest.

Putting even relatively small payloads into space using this kind of propulsion can become ridiculously expensive. The Saturn V stacks which sent the Apollo spacecraft (around 40 tonnes) to the moon massed almost 3000 tonnes. The forthcoming NASA Space Launch System is essentially the same technology. Elon Musk’s Falcon and Starship rockets are at least new designs, and they have the huge virtue that most of the stack comes back to land on Earth for re-use: but they are still bound by the limits of chemical reactions and chemical energy density. And the numbers only get worse for missions beyond Earth orbit.

NASA's Mars rover Curiosity, which is powered and heated by a Radioisotope Thermal Generator (RTG). This is a low powered application of nuclear energy in space
NASA's Mars rover Curiosity, which is powered and heated by a Radioisotope Thermal Generator (RTG). This is a low powered application of nuclear energy in space - NASA Handout/AFP

It is possible to build a nuclear powered rocket, in which nuclear power would heat up the reaction mass – probably hydrogen – rather than a chemical reaction. NASA spent a long time working on this, in the Nuclear Engine for Rocket Vehicle Application (NERVA) programme, which ran from 1958 to 1973. The NERVA rocket could have been used as an upper stage in a Saturn V, which would have made the stack able to lift a lot more payload, or it could have been used for propulsion out in space: NASA considered that it would have made manned Mars missions possible in the 1970s. The heated hydrogen in a NERVA-type rocket propels the spacecraft with double the propellant efficiency of chemical rockets. NASA engineers estimated that a mission to Mars powered by nuclear thermal propulsion would be 20 per cent shorter than a trip on a chemical-powered rocket. In the event, NERVA was cancelled as part of the massive NASA cuts of the early 1970s.

An alternative approach is to use electrical power to squirt the reaction mass out of the rocket. This is now done routinely using so-called “Hall thrusters” powered by solar panels. Only tiny amounts of reaction mass (usually krypton, xenon or argon) are ejected from a Hall thruster of today, but they are ejected at huge velocities and so deliver a lot more push for a given amount of reaction mass. A Hall thruster could never lift itself off the ground, but it is excellent for modifying the orbit of a satellite. Because only very small amounts of thrust are involved, the small amounts of power produced by solar panels are sufficient.

There have been more ambitious plans along these lines. Former space shuttle astronaut and plasma physicist Franklin Chang Díaz has calculated that a ship equipped with his Variable Specific Impulse Magnetoplasma Rocket (VASIMR) propulsion technology – like Hall thrusters, a type of ion engine – powered by a dustbin-sized nuclear reactor of the same kind used in submarines would be able to reach Mars in just 39 days. Chemically powered craft take about seven months.

Some nuclear power is used in space today. The Voyager, Cassini and New Horizons missions were powered with nuclear energy using Radioisotope Thermoelectric Generators (RTGs) as are the new generation of Mars rovers, Perseverance and Curiosity. However, RTGs are not nuclear reactors – they are comparatively low powered devices which convert heat generated by the normal radioactive decay of small volumes of nuclear material (often plutonium) into electricity. These devices can run for decades: the twin Voyager spacecraft that were launched in 1977 are still going strong, having now travelled outside our solar system. It was the desire to send them so far from the Sun which led to the decision to use nuclear rather than solar energy, which obviously diminishes at these distances. Even Mars is a noticeably worse place than Earth for solar power, being in the next orbit out from the Sun.

In fact, there has already been some use of actual nuclear reactors in space, too. A reactor, which uses the neutrons emitted from its fuel to stimulate its fission reactions, can generate much more power than an RTG. The Soviets in the old days sent up satellites equipped with powerful radars, intended to find US aircraft carriers out in the huge expanses of the world’s oceans. Solar panels could never have provided enough juice, nor could RTGs.

This led the Soviets to equip the radar satellites with actual small nuclear reactors. Between 1967 and 1988 the Soviets put 33 nuclear powered radar-ocean-reconnaissance satellites (Rorsats) into space. Most of the reactor cores are still up there: they were parked in high “storage” orbits before the main spacecraft descended at the end of their lives. This didn’t always work: two reactors ended up in the sea and another one scattered itself all over some remote parts of Canada. One of the “stored” satellites appeared to suffer some kind of accident and broke up into a cloud of debris in 2014.

The Soviet Rorsats showed that nuclear reactors can work in space, and now NASA is looking to develop its own versions of such technology. It has a backronymed project called Joint Emergent Technology Supplying On-orbit Nuclear (JETSON) under which Lockheed and other contractors will develop a spacegoing reactor which can generate electricity. Unlike a submarine reactor which uses steam turbines to generate power, this will use its heat to drive Stirling-cycle engines to run generators. JETSON-powered ion ships could make Mars voyages, and JETSON reactors could also be used to provide power for bases and space stations, far more efficiently than huge solar arrays (the current International Space Station is mostly solar panels, and it still doesn’t have a lot of power). NASA is also teamed up with maverick military tech bureau DARPA on the Demonstration Rocket for Agile Cislunar Operations (DRACO) project, intended to deliver a NERVA style nuclear thermal rocket for use in Earth-Moon space.

Use of nuclear power in space often attracts criticism. But the limitations of solar and chemical energy are extremely severe. If the human race is ever going to have a serious presence beyond low Earth orbit – if we’re ever going to move off this planet – we’re going to need nuclear power up there: and lots of it.

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