Glenn Harlan Reynolds

Opinion columnist

It’s the 21st century. Shouldn’t we have nuclear rockets by now?

Actually, we had them a long time ago. In the 1960s, the United States experimented with two types of nuclear rockets. One is now in the budget again, and I think the other should be.

The first type of nuclear rocket uses a nuclear reactor to heat a reaction mass — hydrogen, or even water — and expels it from a thrust chamber as fast-moving gas. (Chemical rockets, like we use today, depend on a chemical reaction — basically, a controlled chemical explosion — to produce the hot gas.) Back during the 1960s, we experimented with nuclear rockets of this type, known as “nuclear thermal rockets,” under projects Rover and NERVA.

Generally, the problem with rocket engines is that you can either make them highly efficient but with very low thrust (like ion drives) or give them powerful thrust with low efficiency, like a chemical rocket. The only way to have both at once, high efficiency and high thrust, is to go nuclear. Nuclear thermal rockets are about twice as efficient as the best chemical rockets.

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With a nuclear rocket like the ones researched in project NERVA, a trip to Mars could be done in four months. (Interestingly, the astronauts would actually be exposed to less radiation with a nuclear ship, because shortening the trip reduces their exposure to cosmic rays in space.) And with higher thrust and higher efficiency, nuclear rockets would open up much of the solar system. They could even be designed to rendezvous with comets and use cometary ice as fuel for the return trip.

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Now NASA is getting interested in nuclear rockets again. In 2017, it awarded a nearly $19 million contract for development. And this year’s NASA budget contains $100 million for nuclear thermal research, leading to a demonstration in 2024.

As Mark Whittington writes in The Hill, “Nuclear propulsion technology will mean the difference between a deep-space exploration program consisting of sorties that land on Mars and visit other destinations, do a lot of good science, then return, and one that expands human civilization throughout the solar system.”

But I mentioned that there are two kinds of nuclear rockets. Nuclear thermal propulsion is great, but if you want real performance, you have to go with another kind of nuclear propulsion: Orion. Not the boring space capsule of the same name being developed today, but the project of the 1960s that involved eminent physicist Freeman Dyson and famed nuclear weapons designer Ted Taylor. Orion was a spaceship powered by (small) nuclear explosions. Specialized bomblets would go off below a big pusher plate, shoving the spacecraft (hard!) in the other direction.

Orion worked better when it was big and could in theory have attained a fraction of the speed of light, and certainly would have been fast enough to go anywhere in the solar system in a reasonable time. (The Orion team’s motto was “Saturn by 1970.”) All of this is recounted in George Dyson’s fascinating book, Project Orion: The True Story of the Atomic Spaceship.

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The Orion never flew (though a model powered by high explosives called the “Hot Rod” did), but maybe it’s time. Some might argue that nuclear explosives would violate the Outer Space Treaty’s provision forbidding “nuclear weapons or any other kinds of weapons of mass destruction” in orbit, but nuclear explosives aren’t necessarily nuclear weapons. (Asked the difference between the Atlas rockets that launched astronauts into space and the Atlas missiles aimed at Moscow, President John F. Kennedy reportedly gave a one-word answer: “Attitude.”)

The Limited Test Ban Treaty prohibits nuclear explosions in outer space, but among space major powers only the United States and Russia are parties; France and China, for example, are not. And there’s an argument that this treaty doesn’t effectively ban nuclear explosions used for space propulsion.

Well, we’re making progress on the first type of nuclear rocket, and for the moment I guess I’ll just have to be content with that. But when I looked at the page for Dyson’s book on Amazon, I found a review by an influential reader, one Jeff Bezos. Wrote Bezos: “For those of us who dream of visiting the outer planets, seeing Saturn's rings up close without intermediation of telescopes or charge-coupled devices, well, we pretty much *have* to read ‘Project Orion.’ In 1958, some of the world's smartest people, including famous physicist Freeman Dyson (the author's father), expected to visit the outer planets in ‘Orion,’ a nuclear-bomb propelled ship big enough and powerful enough to seat its passengers in lazy-boy recliners. They expected to start their grand tour by 1970. This was not pie-in-the-sky optimism; they had strong technical reasons for believing they could do it.”

Like Bezos, I loved the book, and I’d like to see Orion happen. If only there were a really rich guy with a strong interest in space settlement who felt the same way ...

Glenn Harlan Reynolds, a University of Tennessee law professor and the co-author (with Robert Merges) of "Outer Space: Problems of Law and Policy," is a member of USA TODAY's Board of Contributors.