An antimatter spaceship for Mars?

If you're a science fiction reader, you know that spaceships are using antimatter to travel through space. Now NASA is working on such a spaceship to go to Mars in 45 days using only 10 milligrams of anti-electrons -- or positrons -- for the round trip mission.

If you're a science fiction reader, you know that spaceships are using antimatter to travel through space. Now NASA is working on such a spaceship to go to Mars in 45 days using only 10 milligrams of anti-electrons -- or positrons -- for the round trip mission. These positrons will emit gamma rays with about 400 times less energy than the ones emitted by antiprotons used in previous designs. Such a rocket would be much safer because it would reduce the time to travel to Mars and because there should be no leftover radiation after the fuel is used. There are still some remaining issues, such as the cost -- $250 million for 10 milligrams -- and the storage of antimatter which would have to be contained with electric and magnetic fields. But it's permitted to dream, isn't?

If such a small quantity of antimatter can propel a spaceship to Mars -- and even further -- why hasn't been tried before?

Some antimatter reactions produce blasts of high energy gamma rays. Gamma rays are like X-rays on steroids. They penetrate matter and break apart molecules in cells, so they are not healthy to be around. High-energy gamma rays can also make the engines radioactive by fragmenting atoms of the engine material.
The NASA Institute for Advanced Concepts (NIAC) is funding a team of researchers working on a new design for an antimatter-powered spaceship that avoids this nasty side effect by producing gamma rays with much lower energy.

Here is a short explanation.

When antimatter meets matter, both annihilate in a flash of energy. This complete conversion to energy is what makes antimatter so powerful. Even the nuclear reactions that power atomic bombs come in a distant second, with only about three percent of their mass converted to energy.
Previous antimatter-powered spaceship designs employed antiprotons, which produce high-energy gamma rays when they annihilate. The new design will use positrons, which make gamma rays with about 400 times less energy.

Below is a diagram of a rocket powered by a positron reactor (Credit: Positronics Research, LLC). And here is a link to a larger version.

A diagram of a rocket powered by a positron reactor

Positrons are directed from the storage unit to the attenuating matrix, where they interact with the material and release heat. Liquid hydrogen (H2) circulates through the attenuating matrix and picks up the heat. The hydrogen then flows to the nozzle exit (bell-shaped area in yellow and blue), where it expands into space, producing thrust.

Such an engine would be safer for the astronauts and for the environment for several reasons: it would reduce the travel time to Mars, increasing safety for the crew by reducing their exposure to cosmic rays; the reactor would not be radioactive after its fuel is used; and there should be no risk for the public even if the reactor exploded during its launch because "gamma rays would be gone in an instant."

So when will see such spaceships? It's hard to tell because some technical and financial issues still need to be addressed and solved.

"A rough estimate to produce the 10 milligrams of positrons needed for a human Mars mission is about 250 million dollars using technology that is currently under development," said Dr. Gerald Smith [of Positronics Research, LLC, in Santa Fe, New Mexico]. This cost might seem high, but it has to be considered against the extra cost to launch a heavier chemical rocket (current launch costs are about $10,000 per pound) or the cost to fuel and make safe a nuclear reactor. "Based on the experience with nuclear technology, it seems reasonable to expect positron production cost to go down with more research," added Smith.
Another challenge is storing enough positrons in a small space. Because they annihilate normal matter, you can't just stuff them in a bottle. Instead, they have to be contained with electric and magnetic fields. "We feel confident that with a dedicated research and development program, these challenges can be overcome," said Smith.

But if these technical hurdles are overcome, "the first humans to reach Mars will arrive in spaceships powered by the same source that fired starships across the universes of our science fiction dreams."

Sources: NASA news release, April 14, 2006; and various web sites

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