New and Improved Antimatter Spaceship for Mars Missions
04.14.06
Most self-respecting starships in science fiction stories use
antimatter as fuel for a good reason its the most potent fuel
known. While tons of chemical fuel are needed to propel a human
mission to Mars, just tens of milligrams of antimatter will do (a
milligram is about one-thousandth the weight of a piece of the
original M&M candy).
Nuclear-thermal rocket design Image right: A spacecraft powered by
a positron reactor would resemble this artist's concept of the Mars
Reference Mission spacecraft. Credit: NASA
However, in reality this power comes with a price. 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.
Antimatter is sometimes called the mirror image of normal matter
because while it looks just like ordinary matter, some properties
are reversed. For example, normal electrons, the familiar particles
that carry electric current in everything from cell phones to
plasma TVs, have a negative electric charge. Anti-electrons have a
positive charge, so scientists dubbed them "positrons".
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.
The NIAC research is a preliminary study to see if the idea is
feasible. If it looks promising, and funds are available to
successfully develop the technology, a positron-powered spaceship
would have a couple advantages over the existing plans for a human
mission to Mars, called the Mars Reference Mission.
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.
Print-resolution copy Credit: Positronics Research, LLC
"The most significant advantage is more safety," said Dr. Gerald
Smith of Positronics Research, LLC, in Santa Fe, New Mexico. The
current Reference Mission calls for a nuclear reactor to propel the
spaceship to Mars. This is desirable because nuclear propulsion
reduces travel time to Mars, increasing safety for the crew by
reducing their exposure to cosmic rays. Also, a chemically-powered
spacecraft weighs much more and costs a lot more to launch. The
reactor also provides ample power for the three-year mission. But
nuclear reactors are complex, so more things could potentially go
wrong during the mission. "However, the positron reactor offers the
same advantages but is relatively simple," said Smith, lead
researcher for the NIAC study.
Also, nuclear reactors are radioactive even after their fuel is
used up. After the ship arrives at Mars, Reference Mission plans
are to direct the reactor into an orbit that will not encounter
Earth for at least a million years, when the residual radiation
will be reduced to safe levels. However, there is no leftover
radiation in a positron reactor after the fuel is used up, so there
is no safety concern if the spent positron reactor should
accidentally re-enter Earth's atmosphere, according to the
team.
It will be safer to launch as well. If a rocket carrying a nuclear
reactor explodes, it could release radioactive particles into the
atmosphere. "Our positron spacecraft would release a flash of
gamma-rays if it exploded, but the gamma rays would be gone in an
instant. There would be no radioactive particles to drift on the
wind. The flash would also be confined to a relatively small area.
The danger zone would be about a kilometer (about a half-mile)
around the spacecraft. An ordinary large chemically-powered rocket
has a danger zone of about the same size, due to the big fireball
that would result from its explosion," said Smith.
Another significant advantage is speed. The Reference Mission
spacecraft would take astronauts to Mars in about 180 days. "Our
advanced designs, like the gas core and the ablative engine
concepts, could take astronauts to Mars in half that time, and
perhaps even in as little as 45 days," said Kirby Meyer, an
engineer with Positronics Research on the study.
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