NERVA concept (Credit: aemann)

Nuclear propulsion systems for rockets have been studied by NASA since the early 1960's under the Nuclear Engine for Rocket Vehicle Application (NERVA) program, subsequently cancelled in 1972. In 2003, the nuclear space propulsion idea was revived by the Prometheus Project still under development. Although the nuclear propulsion option looks like a prime candidate for the future Mars mission, its disadvantages (mainly extreme radioactivity) led people like Dr. Gerald A. Smith, founder of Positronics Research in Santa Fe, New Mexico, to suggest a bold new alternative – antimatter. First predicted by the British physicist Paul Dirac in 1928 (and experimentally confirmed 4 years), antimatter is comprised of antiparticles that annihilate when they come in contact with ordinary particles, producing a burst of energy in the form of energetic photons. NASA's Institute for Advanced Concepts (NIAC) recently funded Dr. Smith's research to examine the potential applications of antimatter as a fuel for a manned mission to Mars.

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Q: Were you the first to come up with the idea for a positron-based propulsion system?

PRLLC solid state anti-matter spacecraft design (Credit: Positronics LLC)

A: The first positron engine was proposed by a German engineer, Eugen Saenger, in 1953. This was the classic photon rocket, but the photons (gamma rays) had to be made to reflect in order to give thrust. Unfortunately, there was no way to deflect the gamma rays, then or now. We are different in that we make the gamma rays interact, producing ablative residue, which generates thrust. Compared to antiprotons, positrons are very advantageous: no residual radioactivity, low energy gamma rays make for a compact engine (energy confinement is much simpler), and costs for making positrons are many orders of magnitude less (due to technology of electron accelerators versus proton accelerators). As for the original idea, I can say with 95% confidence that we were the first to tackle the real issues of positron propulsion. Saenger did the early work on dynamical computations of a true photon rocket, but did not deal with the real issues of how to get thrust out of his photons. Sanger deserves the credit for the "big idea", we for solving the physics and engineering problems. 

Q: You mentioned that positrons emit less powerful gamma rays than antiprotons. Is the energy produced by positrons still sufficient for a useful propulsion system?

Positron engine design (Credit: Positronics LLC)

The low energy of the positron annihilation gamma rays make these very easy to contain and turn into propulsive energy. But, it takes 1836 times more positrons to get the same amount of energy as one antiproton. The antiproton annihilation energy is very hard to contain and turn into propulsive energy. In fact, the only way I know to use antiprotons is to make them create nuclear fission reactions in materials like uranium, which results in one of the nasty sides of nuclear fission, namely the presence of radioactive isotopes created by the engine.

Q: Would you describe in a few words the three positron-based propulsion concepts you have come up with, how they work, and what their main advantages and disadvantages are?

1. Solid core - Energy is transferred to a propellant in tungsten metal matrix heated by annihilation gamma rays.

Advantages - Well understood technology.
Disadvantages - Performance limited by melting temperature of tungsten.

2. Gas core - Energy is transferred to liquid/gas propellant directly heated by annihilation gamma rays.

Advantages - Improvement over solid core, not limited by melting temperature.
Disadvantages - Flowing multi-fluid is unstable at boundaries, may ionize and create plasma. 

3. Solid Ablation - Energy is transferred to a material that ablates off surface of a pusher plate.

Advantages - Simplicity in design, no obvious technology limits.
Disadvantages - Half of the gamma rays do not strike the pusher plate, maximum efficiency 50%.  

Solid Ablation spaceship concept (Credit: Positronics LLC)

Q: How do you intend to deal with the two major problems of antimatter propulsion systems - the creation of antimatter and finding a way to store it for long periods of time?

Computer animation of International Linear Collider (ILC) (Credit: Desy.de)

You cannot hold 10 mg of bare positrons in a magnetic trap. The "space charge" forces are enormous and the "positron plasma" blows itself apart. But, with electrically neutral atoms containing positrons, this is not a problem. Our work with positronium is on-going. My sponsors implore me to not discuss details at this time. Suffice it to say we have had some very encouraging results. 

Positron-based concept for the Mars mission (Credit: Positronics LLC)