Going Interstellar (collection)

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).

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.