When physicist Geoffrey Landis reviewed interstellar concepts at the American Association for the Advancement of Science’s 2002 meeting, his wide-ranging presentation considered where we stand on nuclear propulsion, solar and lightsail technologies, and particle-pushed sails. He also addressed the question of the Bussard ramjet, which would use an electromagnetic scoop to collect atoms from the interstellar medium to fuel a fusion reactor. Finding serious problems here (he cites, among other things, the fact that the scoop technology acts more like a brake than an accelerator), Landis went on to consider an alternative:
“These problems can be alleviated if, instead of using the ambient interstellar medium, fuel is deliberately emplaced in the path of the spacecraft before flight. In this way, the fuel (probably in the form of small ‘pellets’) can be chosen to be the optimum composition…
The ‘runway’ of fuel pellets could be emplaced, for example, by a dedicated craft which drops fuel pellets at predetermined locations along the flight path. Each ‘fuel pellet’ would have to contain more than the fusion fuel; at a minimum the pellets would have to contain passive locator beacons. The interstellar probe would adjust its trajectory as required to ingest and utilize each successive fuel pellet. Alternately, and possibly more realistically, the fuel pellets themselves would each have a small amount of propulsion capability, enough for them to station-keep in a predetermined location, for example, maintaining position along a laser used as a guide-beam.
[Jordin] Kare noted that, at the velocities proposed for interstellar flight, nuclear fusion can be accomplished at the temperatures produced by impact. If a small pellet carried on the vehicle impacts a stationary pellet of fusion fuel, the result of the impact will be ignition of the fusion reaction, and potentially the liberation of a considerable amount of energy…”
When Centauri Dreams talked to Landis in 2003 about the fusion runway concept (Kare calls the idea the ‘Bussard Buzz Bomb’), the physicist elaborated on why he found the concept so intriguing. Kare’s work had indicated that it would take a velocity of 200 kilometers per second to light the starship’s main engines using these techniques. Landis pointed out that a spacecraft making a close pass by the Sun (virtually skimming the solar surface) could reach 600 kilometers per second, with the fuel pellets lined up so that the vehicle could start using them immediately after the solar pass.
How long would a fusion runway have to be to function? The answer depends on your spacecraft. An unmanned robotic probe that could endure intense gravitational forces would require a much shorter runway. If the craft is manned, the pellets would have to be strung out over a tenth of a light year to allow the probe to reach a cruising velocity of 30,000 kilometers per second. That translates into a forty-plus year journey to the Centauri stars.
Landis’ AAAS presentation is available as “The Ultimate Exploration: Approaches to Interstellar Flight,” in Y. Kondo, ed., Interstellar Travel & Multi-Generational Space Ships (Apogee Books, 2003).