As Mars has become a likely destination for manned space missions, the search has begun for new breeds of spaceships that are more efficient, more powerful, and more speedy than those that have served us so far. Nuclear-powered ships are one possibility being considered.
The technology for one nuclear propulsion system that already exists -- the nuclear thermal rocket or NTR -- was first tested in the 1960s by NASA and the Atomic Energy Commission in the form of a rocket called NERVA (Nuclear Engine for Rocket Vehicle Applications).
NTRs work by pumping supercold (-400 degrees F) liquid hydrogen into a chamber that is heated to 5,000 degrees F by a nuclear reactor. Once inside the chamber, the hydrogen's temperature instantly jumps several thousand degrees. The atoms accelerate and explode out the engine's nozzle, creating enormous thrust and speed. A different nuclear design that is called a particle bed reactor engine is capable of creating the same effect even more efficiently. It is being developed at Brookhaven National Laboratory on Long Island.
Using nuclear power, a Mars-bound ship could quickly reach a speed of seven miles per second. At that rate, it could make the 35-million-mile trip to the red planet in ninety days rather than six months or more.
Nuclear fuel could also significantly cut the weight of a Mars ship. Because one pound of nuclear fuel contains ten million times more power than a pound of liquid fuel, a nuclear ship would have to carry far less fuel than a conventional ship, making it easier and less expensive to launch. Some of these reductions would be offset by the weight of shielding that would have to be built into the ship to protect a crew from the radiation of a nuclear engine, but the difference would still remain significant.
Nuclear propulsion looks promising, but, just in case it doesn't have a future, Robert Zubrin at Martin Marietta in Denver, Colorado is looking for ways to reduce the amount of fuel a chemically powered ship would have to carry. He has come up with something called in-situ propellant. His idea is to send a robotic mission to manufacture fuel on Mars so that a ship on a later round trip from Earth would only have to carry enough fuel to get there. He has already built a demonstration showing how to turn carbon dioxide, the main component of the Martian atmosphere, into liquid rocket fuel.
