One advantage of the ramjet is shielding from interstellar dust. Although micron-sized interstellar dust grains are very rare in the local interstellar medium, dust impacts at near the speed of light would have an effect worse than a stationary ship being impacted by multiple shotgun blasts. Such impacts may limit non-ramjet interstellar cruise velocities to a few percent of the speed of light. But since the flow of collected protons deep within the ship’s electromagnetic scoop field will collide with and atomize the fragile dust grains, ramjets will not be so limited in terms of cruise velocity.
A number of science-fiction authors have featured the interstellar ramjet in their stories. Perhaps most notably, the ramjet appeared more than once in Larry Niven’s Tales of Known Space, and propelled the crew of Leonora Christine on an impossible but entrancing voyage in Poul Anderson’s Tau Zero.
But, alas, rigorous scientific skeptics began to chip away at this most exciting concept. It was found that most electromagnetic scoops are efficient drag brakes, reflecting interstellar ions rather than collecting them. But this was not the most serious problem—by the mid-1970’s few believed that human technology could ever tame the proton-proton thermonuclear reaction. Even the catalytic carbon cycle may forever be beyond our capabilities.
There is a way around this, but it does not seem practical for high-speed flight. As tabulated by Eugene Mallove and Gregory Matloff in The Starflight Handbook (Wiley, NY, 1989), both deuterium and Helium-3 exist in the interstellar medium (and the solar wind) at concentrations of a few parts per hundred thousand. If it is possible generate electromagnetic scoop fields hundreds of thousands of kilometers across, collect the fusion fuel from the hydrogen ions, and fuse the deuterium and Helium-3, some form of ramjet might be possible. But it will be a far cry from the dream ships of Bussard, Sagan and Schlovskii, Anderson, and Niven.
Fortunately, the ramjet idea was too attractive to abandon. So a number of less capable alternatives to the proton-fusing ramjet have been proposed. Some of them might just work.
Many journal articles have been written in recent decades about interstellar propulsion using thermonuclear rockets or ramjets. Most of these articles have appeared in Acta Astronautica, an organ of the International Academy of Astronautics published by Elsevier Ltd. In Oxford UK and in The Journal of the British Interplanetary Society, published by the British Interplanetary Society in London.
A number of books have been written that review and describe the results of the technical papers. One of these, The Starflight Handbook (by Eugene Mallove and Gregory Matloff and published by Wiley in 1989) was designed to appeal to both technical and non-technical audiences.
A somewhat more recent, but more technical compendium is Prospects for Interstellar Travel (by John H. Mauldin for the American Astronautical Society and published by Univelt in San Diego CA in 1992).
The third and most up-to-date of the books considered here is the second edition of Deep Space Probes (by Gregory L. Matloff in 2005 for Springer-Praxis in Chichester, UK).
PROJECT ICARUS
A Theoretical Design Study for an Interstellar Spacecraft
Dr. Richard Obousy
“Standing on the shoulders of giants” definitely describes the task being undertaken by Richard Obousy and his colleagues as they work to design a realistic interstellar spacecraft based on state-of-the-art engineering. The shoulders upon which they stand belong to the Project Daedalus team that performed a similar study in the 1970s for The British Interplanetary Society. Led by Alan Bond, Project Daedalus became the standard by which all interstellar spacecraft concepts to follow were judged.
Named for Icarus, Daedalus’ son who flew too close to the Sun and fell to his death, Obousy’s international team is designing a craft that will hopefully avoid its namesake’s mistakes and harness the power of the sun to someday give us the stars—Project Icarus.
Kepler, launched in 2009, is the first NASA mission designed explicitly to search for planets orbiting other stars. On Saturday 19th February 2011, the project scientist for Kepler, Dr. William Borucki, estimated that there are at least fifty billion exoplanets in our galaxy. Perhaps more tantalizing is the probability that five hundred million of these alien worlds are inside the habitable zones of their parent stars. So just how many of these exoplanets contain life? Unfortunately, there’s no good answer to that question, but given such vast numbers of potentially habitable worlds, the question is, Where is ET?
This is, of course, the famous Fermi Paradox, an apparent contradiction between the high estimates of the probable existence of extraterrestrial civilizations and the disconcerting lack of evidence for such civilizations.
Many assumptions regarding the ability of an alien civilization to effectively colonize other solar systems are based on the premise that interstellar travel is, in fact, technologically possible. However, one early proposed solution to the Fermi Paradox was that interstellar travel on timescales of tens, or hundreds, of years is impossible. Project Daedalus, a study conducted in the 1970s by members of The British Interplanetary Society, was a bold effort to examine this very question. The project was essentially a feasibility study for an interstellar mission, using capabilities appropriate to the era, with credible extrapolations for near-future technology.
One of the major objectives was to establish whether interstellar flight could be realized within established science and technology. The conclusion was that it is feasible. Although our current understanding of the laws of physics rules out the possibility of superluminal travel, it does appear that there are no major theoretical barriers to the construction of rapid, sublight, interstellar ships.
The final Daedalus design had a total dry mass of greater than 2600 hundred tons, of which 450 tons was the science payload. The propellant mass was fifty thousand tons of Deuterium and Helium-3. The latter component of the fuel is incredibly rare on Earth. However, it is found in abundance on the gas giants of the solar system. Thus, a component to the Daedalus project entailed mining Helium-3 from Jupiter. Because of the huge mass of the spacecraft, and the necessary Jovian mining aspect of the mission, the Project Daedalus study group determined that such a spacecraft could probably only be constructed as part of a solar-system-wide economy with abundant resources at its disposal. Daedalus, from the perspective of 1970s science, was deemed to be effectively unavailable in the near-term future.
This would place its earliest likely construction date somewhere circa 2200. However, numerous technologies have advanced since the 1970s, including microprocessor technology, materials science, nanotechnology, fusion research and also our knowledge of the local interstellar neighborhood. It seemed timely then to revisit the Project Daedalus study, and the successor initiative, Project Icarus, officially began in September 2009 at a meeting in London at the headquarters of the British Interplanetary Society (BIS). This theoretical engineering design study is a project under the umbrella of the Tau Zero Foundation and The BIS.