Antimatter seems the boldest — and newest — of propulsion concepts, but in fact Eugen Sänger’s work on the uses of antimatter in rocketry goes back to the 1930s. The German scientist thought it would be possible to reflect gamma rays produced by the annihilation of electrons and positrons to produce thrust. His work wowed the Fourth International Astronautical Congress in 1952, but there was a catch: the gamma rays created by this reaction seemed too energetic to use the way Sänger hoped — they penetrated all known materials and could not be channeled effectively into a rocket exhaust.
Which is why most antimatter designs since have focused on antiprotons. When antiprotons and protons annihilate each other, they produce not only gamma rays but pi-mesons, short-lived particles also known as pions. Many of these are charged as they emerge from the proton/antiproton annihilation, and can therefore be controlled by sending them through a strong magnetic field. Early designs by Robert Forward and David Morgan (Lawrence Livermore National Laboratory) took advantage of these traits even though the technology to produce sufficient antimatter lagged far behind their visionary concepts.
But antimatter researcher Gerald Smith and colleagues have been working on a study for NASA’s Institute for Advanced Concepts that takes us back to positrons, one that could power a human mission to Mars with tens of milligrams of antimatter. Not only would such a design be far lighter than competing chemical and nuclear options, but it would be fast enough to dramatically shorten flight time to the Red Planet; advanced versions might make the trip in as little as 45 days.
Smith’s background in antimatter research needs little elaboration; he is a towering figure in the field. While at Pennsylvania State University, he oversaw two key hybrid designs called ICANN II and AIMStar that used antimatter as a catalyst to induce nuclear reactions. The author of hundreds of research papers, he is also the designer of both the Mark I portable antimatter trap and the current state of the art, the High Performance Antimatter Trap (HiPAT).
He is, in other words, a key figure when it comes to wedding powerful antimatter technologies to practical spacecraft designs. Now head of Positronics Research LLC in New Mexico, Smith has built on the lessons learned from these earlier concepts to promote a new design that seems to offer enormous benefits, if we can produce the antimatter needed to make it fly.
One advantage of positrons is that the gamma rays they generate are about 400 times less energetic than those created by antiprotons, making the spacecraft a far safer place for human crews. According to a description of Smith’s work posted on the NIAC Web site (PDF warning), the positron/electron annihilation “…results in the creation of two soft 511 keV gamma rays. These gamma rays can be easily absorbed to heat a working fluid in a closed, high-efficiency thermodynamic power system, or directly into a propellant.” The NIAC work is ongoing — Smith’s Phase I study was completed in March and he is now making the case for an advanced Phase II project that will examine design variants like the positron reactor shown below.
Image: 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. Credit: Positronics Research, LLC.
The high cost of antimatter is always an issue, but one that may become manageable. Smith is now estimating that the 10 milligrams of positrons a human Mars mission would require could be produced for roughly $250 million. It seems a reasonable assumption that antimatter production costs will continue to go down, just as it is also reasonable to question the wisdom of using staged chemical rockets with launch costs of $10,000 per pound when designs that could undertake far more sophisticated missions are waiting to be developed. Let’s talk more about these notions tomorrow and dig into antimatter’s advantages when it comes to deep space work.