New and Improved Antimatter Spaceship for Mars Missions
NASA Goddard Space Flight Center
April 14, 2006
Most self-respecting starships in science fiction stories use
as fuel for a good reason - it’s the most potent fuel known. While tons
of chemical fuel are needed to propel a human mission to Mars, just
of milligrams of antimatter will do (a milligram is about
the weight of a piece of the original M&M candy).
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
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.
complete conversion to energy is what makes antimatter so powerful.
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
The NIAC research is a preliminary study to see if the idea is
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.
“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
Mars. This is desirable because nuclear propulsion reduces travel time
Mars, increasing safety for the crew by reducing their exposure to
rays. Also, a chemically-powered spacecraft weighs much more and costs
lot more to launch. The reactor also provides ample power for the
three-year mission. But nuclear reactors are complex, so more things
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
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
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
large chemically-powered rocket has a danger zone of about the same
size, due to the big fireball that would result from its explosion,”
Another significant advantage is speed. The Reference Mission
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
Advanced engines do this by running hot, which increases their
efficiency or “specific impulse” (Isp). Isp is the “miles per gallon”
rocketry: the higher the Isp, the faster you can go before you use up
your fuel supply. The best chemical rockets, like NASA’s Space Shuttle
main engine, max out at around 450 seconds, which means a pound of fuel
will produce a pound of thrust for 450 seconds. A nuclear or positron
reactor can make over 900 seconds. The ablative engine, which slowly
vaporizes itself to produce thrust, could go as high as 5,000 seconds.
One technical challenge to making a positron spacecraft a reality is
cost to produce the positrons. Because of its spectacular effect on
normal matter, there is not a lot of antimatter sitting around. In
space, it is created in collisions of high-speed particles called
rays. On Earth, it has to be created in particle accelerators, immense
machines that smash atoms together. The machines are normally used to
discover how the universe works on a deep, fundamental level, but they
can be harnessed as antimatter factories.
“A rough estimate to produce the 10 milligrams of positrons needed for
human Mars mission is about 250 million dollars using technology that
currently under development,” said Smith. This cost might seem high,
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.
If this is so, perhaps 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.