NASA has signed an agreement with the U.S. Defense Advanced Research Projects Agency (DARPA) to develop a nuclear rocket that could shorten travel time to Mars by about one quarter compared to traditional chemical rockets. But before nuclear technology is launched into space, there are risks that need to be addressed to ensure public safety.
Nuclear rockets are not a new idea. Experiments were conducted in the 1950s by the U.S. military and later NASA but never put into space. Now, with the prospect of sending humans to Mars in the 2030s, the idea is being revived in an effort to shorten the roughly seven months it takes a conventional rocket to get to Mars. This might be a boon for future astronauts who face a seven-month, one-way journey using current technology.
The idea is to use a small fission reactor to heat up a liquid fuel to very high temperatures, turning it into a hot gas that would shoot out a rocket nozzle at high velocity, providing thrust.
The design of a nuclear rocket means they typically would produce less thrust than a chemical rocket, but nuclear engines could run continuously for weeks, constantly accelerating, ultimately reaching higher velocities in a tortoise-and-hare kind of way.
Nuclear propulsion is expected to be twice as fuel-efficient as chemical rockets, largely because they can heat the gas they use for thrust to a higher temperature than chemical combustion, and hotter gas means more energy.
A quicker trip to Mars provides huge benefits. Astronauts would be exposed to less cosmic radiation during the journey. The psychological pressures of living in a confined space far from home would be reduced. Supplies and a rescue mission could be delivered more quickly. These rockets could also open up the outer solar system so trips to Jupiter and its large family of icy moons could eventually be within reach.
While the technology of nuclear propulsion is certainly feasible, it may not be readily embraced by the public. The accidents at Chernobyl, Three Mile Island and Fukushima have left many people skeptical about nuclear safety. And there will be risk.
A nuclear rocket wouldn’t be used to launch a spacecraft from the Earth’s surface — it would be designed to run in space only. It would have to launch into orbit on a large chemical rocket — so the public would have to accept the risk of launching a nuclear reactor on a standard rocket filled with explosive fuel.
And rockets have and will malfunction catastrophically, in what with black humour rocket scientists sometimes call RUD — “rapid unscheduled disassembly.”
No one wants to see nuclear debris raining down on the Florida coast or Disneyland, and that’s not the only possible scenario. An accident in orbit could potentially drop radioactive material into the atmosphere.
These safety concerns need to be addressed before any nuclear rocket leaves the ground.
We’ve used nuclear, just not a reactor
Nuclear technology in another form has been used since the very beginning of the space program, just not for propulsion. Radioisotope Thermoelectric Generators (RTGs) have provided power to deep space probes for instruments, radios and cameras on a range of missions.
They’re particularly useful for missions into deep space, like Voyager, Cassini and New Horizons which ventured too far from the sun for solar panels to be effective. They are also powering two rovers currently driving around on Mars: Perseverance and Curiosity.
RTGs are much simpler and lower-powered devices — and, crucially, are not nuclear reactors. Instead they convert the heat generated by the radioactive decay of a small amount of nuclear material (often plutonium) into electricity. These devices can run for decades. They twin Voyager spacecraft are still powered by RTGs that were launched in 1977 and are now outside our solar system.
Though there have been objections to their use since the 1980s, RTGs have proven relatively safe. The U.S. has seen several accidents, including one in 1968 when a launch failure of a Nimbus-1 weather satellite threw its RTG into the ocean. It was recovered intact and the fuel was reused on a later mission.
But there have been more serious accidents. Canadians may remember an incident from 1978, when a Soviet reconnaissance satellite scattered 50 kg of uranium from its nuclear thermal generator over 124,000 square kilometres of Canada’s North.
But a fission reactor is a much more complicated device involving higher temperatures, coolants and more nuclear fuel.
Nuclear rockets hold great potential for the next generation of spacecraft that could enable humans to explore deeper into space.
However, the engineers face a challenge to ensure that all checks and balances have been made to reassure the astronauts who will fly these machines — and people on the ground — that they can be operated safely before the technology is adopted.
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