Projects Daedalus and Icarus—The BIS follows Up
Now that Dyson and Taylor had opened the “Interstellar Door,” other groups began their own studies. The British Interplanetary Society (BIS), which had studied Moon flight decades before the Apollo Project, was ideally situated to conduct a follow-on study to Orion. British researchers Alan Bond and Anthony Martin directed this study, dubbed Project Daedalus, during the 1970’s. The original Daedalus, a mythological Athenian architect, had escaped imprisonment in Crete with the aid of flapping wings handily crafted from goose feathers.
It was soon determined that the modern Daedalus, although inspired by the Orion conceptual breakthrough, would be a bit different. Several problems were acknowledged with the Orion concept. One was scale—an Orion starship (such as the pulsed thermonuclear rocket shown schematically in Figure 2) would be huge even if its payload were small. This was due to the size of the equipment necessary to deflect the copious particles emitted by even a small thermonuclear blast. Another issue was psychological—how would the crew and passengers of a starship react to a megaton-sized explosion going off every few seconds, at a distance of only a kilometer or so? Finally, it is difficult to conceive of any real-world scenario in which nuclear superpowers would allow use of their arsenals in such a constructive endeavor.
Figure 2. Artist concept of a Project Orion nuclear pulse spacecraft. (Image courtesy of NASA.)
Daedalus evolved as a kid brother to Orion. Instead of using the dramatic thermonuclear-pulse drive, it used a somewhat tamer approach—inertial fusion. Small micropellets of fusion fuel were to be ejected into a combustion chamber equipped with strong magnetic fields. Instead of ignition by a fission trigger, these pellets were to be heated to fusion temperatures and condensed to fusion densities by an array of focused laser or electron beams.
Researchers involved in the effort spent a good deal of time considering fusion fuel cycles. They rejected the deuterium-tritium (D-T) and deuterium-deuterium (D-D) fusion reactions under active consideration for terrestrial energy production. Although cleaner than fission, the copious thermal neutrons produced by these reactions would rapidly irradiate the spacecraft. Instead, they settled on a reaction between a low-mass form of helium (Helium-3) and deuterium. The products of this reaction are electrically charged particles—these are relatively easy to focus and expel with the aid of powerful magnetic fields.
Although the Helium-3/D reaction is the second easiest to ignite after D-T, it has one significant drawback. Helium-3 is very, very rare in the terrestrial environment. Starship designers were faced with four alternatives to obtain the necessary tens of millions of kilograms of this substance.
1. They could pepper the surface of the Earth or Moon with breeder reactors, which produce more nuclear fuel than they consume to produce it.2. Since Helium-3 is a trace component of the solar wind of ions ejected from our Sun, some form of superconducting electromagnetic scoop could mine the solar wind for this isotope—but high temperatures in the inner solar system might render superconducting scoops difficult to build and maintain.3. Tiny amounts of He-3 had been deposited in the upper layers of lunar soil as evidenced by samples returned by Apollo astronauts—but at that time nobody knew how the He-3 concentration varied with depth in lunar soils and how feasible lunar mining might actually be.4. What they opted for was the fourth alternative: He-3 is found in the atmospheres of giant planets. Perhaps a series of robotic helium mines suspended by balloons in the upper atmosphere of Jupiter would be the answer.
Although the Daedalus engine could in concept be used to accelerate and decelerate a “thousand-year ark,” the initial application was expected to be robotic probes that could be accelerated to about 10% the speed of light (0.1c) and then fly through the destination star system. In the 1970s, it was (erroneously) suspected that the second nearest star—a red dwarf called Barnard’s star at a distance of about six light years from the Sun—had Jupiter-sized planets. So Barnard’s Star was selected for the hypothetical star mission.
Project Daedalus resulted in and inspired many papers published in dedicated issues of JBIS (The Journal of the British Interplanetary Society). In 2010, a follow-up BIS study called Project Icarus (after the son of mythological Daedalus who approached the Sun too closely and fell to his death in the Aegean) commenced.
Directed by another British researcher, Kelvin Long, Project Icarus aims to continue and update the Daedalus study. The target star is currently Alpha/Proxima Centauri. Not only is this the nearest star system to our Sun at a distance of about 4.3 light years (roughly 40 trillion miles) but the two central Centauri stars are sun-like and separated enough that multiple terrestrial planets may exist in stable orbits.
It is acknowledged that using fusion rockets to accelerate to and decelerate from 0.1c will require an enormous amount of fuel, but an un-decelerated probe that crosses the interstellar void in 50 years and then flies through the destination star system in just a few hours is not acceptable. It would be difficult to justify the expense and the effort for only a few hours worth of data. So Icarus researchers are considering non-rocket deceleration techniques. Approaches include reflecting the very tenuous interstellar plasma and/or the stellar wind(s) of the destination star(s) and using a light sail directed towards the destination star for terminal deceleration.
To again interject physics humor: the rest is simply a matter of engineering….
The Fusion Ramjet
Robert Bussard contributed to many aspects of fusion research. But when he finally achieved his fifteen minutes of fame in an episode of Star Trek: The Next Generation, his surname was pronounced “Buzzard.” What a pity for a true space visionary!
Bussard’s most famous contribution to the study of thermonuclear propulsion in space is the interstellar ramjet, which he considered in 1960. Although the Bussard interstellar ramjet may never be technologically feasible, it does represent one of the very few physically possible modes of interstellar transport that could be capable of near-light speed velocities.
In its pure form (Figure 3), the interstellar ramjet is both simple and elegant. Ahead of the spacecraft, some form of scoop projects an electromagnetic field with a diameter measured in thousands of kilometers. Interstellar protons and electrons, called a “plasma,” are directed towards the scoop by the specially tailored electromagnetic field. The plasma enters the ship and is directed to a fusion reactor at its core. Inside this reactor, plasma density and temperature are high enough to fuse protons and produce helium and energy. The energized helium exhaust is expelled from the rear of the spacecraft. As with any rocket, the reaction to the exhaust accelerates the spacecraft forward.
Figure 3. The Bussard interstellar ramjet would use interstellar hydrogen scooped from deep space propellant mass. (Image courtesy of NASA.)
The interstellar ramjet requires no on-board fuel. Both energy and reaction mass come from the local interstellar medium. In their epochal and very popular book Intelligent Life in the Universe (Holden-Day, San Francisco, 1966), the American astronomer Carl Sagan and his Russian co-author I. S. Shklovskii demonstrated the awesome potential of an ideal interstellar ramjet by showing how it could accelerate to nearly the speed of light—and cross the universe within the lifetime of the on-board crew (while, with thanks again to Special Relativity, billions of years elapsed on Earth).