I did an interview a few days ago with Michio Kaku on his radio show “Science Fantastic”. It’ll be broadcast sometime this weekend. It was a strange conversation, at least from my point of view. I’d never given a long radio interview before and the whole thing seemed rushed and stressful. Kaku wanted to emphasize the weirder and more outré aspectes of science fiction; fitting, perhaps, from the program title, but most good science fiction rests on a firm basis of known science.
One good thing did come from the program. Kaku and I were discussing starships. I, perhaps unwisely, told him that my personal favorite idea for a starship was Project Orion, the truly audacious suggestion that we could propel spacecraft using nuclear bombs as propellants. Dr. Kaku raised a very good objection to this, which is what I want to write about here. (As an aside, there is a new NASA program to develop a crewed rocket propelled by conventional engines called “Orion” – don’t get the two confused!)
Before I go on with this topic, let’s discuss nuclear rockets in general: spaceflight enthusiasts have always wanted rocket fuels which have higher energy density than chemical propellants. This is because it takes a lot of energy to get a rocket moving at the high speeds it needs (25,000 mph or more) to reach destinations like the moon or farther. Chemical propellants like combinations of liquid hydrogen and oxygen work, but just barely. From the 1950′s through the 1970′s nuclear energy was seen as a viable fuel alternative – the Nobel laureate physicist Richard Feynman may have been the first person to realize this, but a number of science fiction writers including Robert Heinlein also came to this conclusion.
The problem is that harnessing the large amounts of energy, millions of times higher per kilogram than chemical energy, is difficult. The simplest way to do it is to take a gas (usually hydrogen) and heat it to very high temperatures and pressures using the reactor. The gas escapes through a nozzle and the spaceship is pushed forward. However, materials science limits this. If the temperature gets too high, you start melting the nuclear fuel itself. This limits the fuel ejection speed to about twice what chemical propellants can do.
Project Orion wanted to get around this by building a big spaceship with a “pusher plate” separating the crew from the engines. Nuclear bombs would be ejected behind the ship and blown up. The resulting explosion propelled the ship forward. It seems daft, but a team led by two very good scientists, Ted Taylor and Freeman Dyson, worked out many of the practical details. The effective fuel ejection speeds which you could get were five or six times, maybe even more, than you could get from conventional chemical fuels. It also seems that you could keep the crew safe from radiation, and keep the explosions from destroying the ship.
You can’t build a spacecraft based on this principle today. Trying to make it work would probably violate the 1963 treaty banning above-ground nuclear testing. Dr. Kaku made the very good point that Ted Taylor himself stopped working on bomb design because the small hydrogen bombs which the ship would use would make very good terrorist weapons. This caused me to stop and think things over.
I’m of two minds about Orion. On the one hand, I am very worried about the prospects of nuclear terrorism. I grew up in the DC suburbs, and was terrified as a teenager by the prospect of nuclear war with the Soviet Union. Today isn’t much less scary, as smaller nations have gotten the bomb, and mounting pressures due to the world economy and the environment seem to make war and terrorist acts more and more likely. Developing small nuclear weapons seems foolish in this light.
On the other hand, the Orion idea is the only feasible method using current technology to move a payload at very high speeds – perhaps at speeds of up to a few percent of the speed of light. People have discussed other methods of powering starships, for example fusion reactors and matter-antimatter annihilation. (Yes, people have studied this seriously – it’s not just from Star Trek!) However, neither could be done now: antimatter has an equivalent cost of several trillion dollars a pound, and can only be produced in microscopic quantities today. No one knows how to make a fusion reactor on Earth, let alone put one in a spacecraft, although recent developments may be promising.
If humanity wanted to send a space probe to the Alpha Centauri system, four light years away, the only conceivable means to do it using today’s technology would be something like Orion. It wouldn’t be easy or cheap, and it wouldn’t happen any time soon. It might not be possible, but this method would be the one with the best chance of success. It would also take decades for the spacecraft to reach the system; any government funding a project like this would need to be farsighted in a way which most aren’t today. Whether it would be worth doing is something which our society, or perhaps our society fifty years from now, would have to decide for itself.
The issue here is that any propulsion system for any spacecraft is a weapon in disguise. Conventional rockets are made from highly explosive materials, as we saw in the Challenger disaster. The higher the energy density, the worse it gets: in the extreme case, if you somehow could make a rocket propelled by antimatter, simply turning the engine on too close to Earth could sterilize it! You can’t deweaponize a starship.
The best science fiction investigates the problems created or issues raised by new discoveries or new technologies. I’ve never read a story or seen a movie or TV show which really looks at the societal problems which would be caused by the development of the Orion concept into a truly workable spacecraft. Perhaps it’s time to write one.
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