Interstellar theorist Richard Obousy (Baylor University) has some thoughts about William and Arthur Edelstein’s ideas on flight near the speed of light. As discussed in these pages on Friday, the Edelsteins, in a presentation delivered at the American Physical Society, had argued that a relativistic rocket would encounter interstellar hydrogen in such compressed form that its crew would be exposed to huge radiation doses, up to 10,000 sieverts in the first second. Because even a 10-centimeter layer of aluminum shielding would stop only a tiny fraction of this energy, the Edelsteins concluded that travel near lightspeed would be all but impossible.
Obousy, who handles the Project Icarus Web site, has his own credentials related to high speed travel, authoring a number of papers like the recent “Casimir energy and the possibility of higher dimensional manipulation” (abstract) that press for continued work into breakthrough propulsion. And when he talked to astrophysicist Ian O’Neill about warp drive concepts last week, Obousy said that we are in the process of laying down “…a mathematical and physical framework for how such a device might function, given the convenient caveat of a ‘sufficiently advanced technology.'” The device, he said, is purely theoretical as of now and we have no evidence that it could be built.
But should we keep investigating? On that score, Obousy has no doubts. With regard to shielding, he argues that metamaterials that bend radiation around objects are a place to begin, offering a conceivable barrier against the kind of radiation the Edelsteins are talking about. All of which makes for lively reading, as does Obousy’s continuing work on the Project Icarus team. Icarus is the descendant of Project Daedalus, the 1970’s era starship design created by the British Interplanetary Society. And while the Icarus guidelines focus on fusion as the propulsion method of choice, Obousy’s interests extend not just to warp drive but also to antimatter possibilities.
The latter is of interest because of its huge energy density, drawing on the abundant energy available within all matter. A single kilogram of matter contains 9×1016 J of energy. “[I]n simpler terms,” says Obousy, “about five tonnes of antimatter would theoretically be enough to fuel all the world’s energy consumption for a single year.” But as he notes in this entry on the Project Icarus site, storage is a huge problem. Positively charged positrons exert a Coulombic force of repulsion against each other, one of the reasons we can store only tiny amounts with current technology.
Ideal storage involves neutral antimatter — antimatter with no net charge — which points to antihydrogen (a stable atom containing a single positron and an antiproton) as a solution. Storing antihydrogen in the form of a Bose Einstein Condensate is one possibility for packing more of the stuff in less space.
As to the vast cost of antimatter production, Obousy has this to say:
With regards to the question of production, current methods utilized at CERN are prohibitively expensive and generation of antihydrogen in quantities that would be valuable to spaceflight would cost trillions of dollars. Despite this, it’s important to recognize that CERN is not a dedicated antimatter production facility and that antihydrogen production is a remarkable, yet tertiary goal of the facility. According to recent research, a low-energy antiproton source could be constructed in the USA at a cost of around $500M over a five year period, and would be an important first step for mass production of antimatter. However the overall roadmap for antimatter propulsion would involve timescales closer to 50 years.
If we start talking near-future uses of antimatter, though, tiny quantities could be put to work in projects like Steven Howe’s antimatter sail, which would use antihydrogen to initiate a fission reaction in a small, uranium-coated sail. Howe developed this idea for NASA’s now defunct Institute for Advanced Concepts. The antimatter, which drifts from storage unit to sail, causes fission as it encounters the uranium, producing neutrons and fission fragments that leave the sail at enormous speeds. NASA’s John Cole, who studied the antimatter sail idea while at Marshall Space Flight Center, told me in 2003 that the sail could develop specific power on the order of 2000 kilowatts per kilogram, enough to drastically shorten human missions to the outer planets even if Howe’s estimates are an order of magnitude off.
Or could antimatter be used as a trigger for fusion? Obousy is interested in the prospect:
Although a spacecraft propelled by antimatter may be many decades away, it maybe possible to use antimatter in the near future to catalyze nuclear fusion reactions using antimatter. Only very small quantities would be required and this might provide an alternative method for liberating energy from fusion. Because Icarus must use current, or near technology, it is possible that Icarus will utilize this form of propulsion…
And he adds:
Clearly a multitude of technological hurdles must be overcome before antimatter use becomes routine in space exploration. However, the fundamental theoretical issues have been proved. Antimatter exists, antihydrogen can be created technologically, antihydrogen can be stored. The rest is progress.
For more on the potential uses of antihydrogen in propulsion, see Nieto et al., “Controlled antihyrogen propulsion for NASA’s future in very deep space,” NASA/JPL Workshop on Physics for Planetary Exploration, 2004 (available online).