Both Tau Zero Foundation founder Marc Millis and JPL’s recently retired Robert Frisbee appear in an article in the Smithsonian’s Air & Space, where voyages to distant places indeed are discussed. Nothing is further from Earth, the article notes, than Voyager 1, which travels at a speed (almost 17 kilometers per second) that would get it across the US in a little under four minutes. Point that spacecraft toward Proxima Centauri and the journey at this speed would take 73,000 years. Clearly, something has to give, and writer Michael Klesius runs through the options.
From Ideas to Engineering
Voyager is actually headed in the vague direction of the constallation Camelopardalis, and won’t come near anything stellar in several hundred thousand years. We’d like to get mission times to a nearby star down to decades so that scientists and engineers working on the project could live to see its outcome.
How to achieve that is a question that has been at the back of Bob Frisbee’s mind for a long time now. To Alpha Centauri in just decades? Years? “We can see the theoretical possibilities of these things happening, but we just can’t get the engineering there,” Frisbee notes in the article, but he points out that this kind of brainstorming was what we used to do when thinking about a moon voyage, and that was a journey we made. It may take several generations of brainstorming but the ideas continue to fly.
Building a Breakthrough Concept
Let’s hope the Air & Space article provokes public discussion as it runs through the background of advanced propulsion studies in the 1990s, when wormholes were seriously tackled and warp drive began to be written up in scientific journals. Miguel Alcubierre’s concept of a spacecraft riding what is essentially a wave in spacetime kicked off a resurgence in breakthrough propulsion that led materially to projects like NASA’s Breakthrough Propulsion Physics, run by Millis until its termination in 2002. Funding issues are always problematic, but Tau Zero continues to probe these matters, and Frontiers of Propulsion Science, co-edited by Millis and Eric Davis, shows that the ongoing conversation is robust indeed.
“I think back to the era of Dirac and Schrödinger and Einstein. When they were having their pivotal meetings and sometimes heated debates, they weren’t being funded for that work. They were just doing it because that’s what they did. And they made significant advances… And I’m thinking to myself, Well, it would be great if we got funding, but even if we don’t, when we talk amongst ourselves and debate things and encourage each other to write papers, we’re going to make progress.”
That kind of progress is what Tau Zero is about. Not that robust funding is out of the picture — we are building a philanthropic model for the foundation that should be able to tap private sector sources (with all the good things that follow from not being channeled through endless layers of bureaucracy). But keeping an eye on the issues and encouraging debate is bound to produce good outcomes, if only in the synergies that result from putting propulsion theorists with good ideas in continuing contact.
Options for Infrastructure
But before we go to the stars, we’ve got to build up our capabilities right here in the system. On that score, the article is also noteworthy for its examination of NERVA, a nuclear thermal rocket design that Klesius describes this way:
It would produce thrust the way chemical rockets do: by heating a propellant—in this case, hydrogen—and ejecting the expanded gas through a nozzle. Instead of heating hydrogen through combustion, however, the nuclear rocket vaporizes it through the controlled fission, or splitting of atomic nuclei, of uranium. Because nuclear fuel has a greater energy density, it lasts a lot longer than chemicals, so you can keep the engine running and continue to accelerate for half the trip. Then, with the speedometer clicking off about 15 miles per second — twice the speed reached by returning Apollo astronauts — you’d swing the ship around to point the other way and use the engine’s thrust to decelerate for the rest of the trip. Even when factoring in the weight of the reactor, a nuclear engine would cut the transit time in half.
NERVA was a promising technology that delivered 850 seconds of thrust — twice the efficiency of chemical rockets — in 1960s-era tests, but the program faded in the 1970s. You’ll find more on NERVA, and on Franklin Chang-Díaz’ work on VASIMR, in Klesius’ article. Neither NERVA nor VASIMR has interstellar potential, but in terms of opening up the Solar System for exploration and infrastructure building, these are solid options to investigate.
An Insurance Plan for Human Survival
I like the Chang-Díaz quote that ends the piece:
“The space program began the day humans chose to walk out of their caves. By exploring space we are doing nothing less than insuring our own survival.”
Indeed. And all the technologies described here point to ways of making the insurance policy pay big dividends. Klesius writes about a fusion-powered 180-day trip to Jupiter, one dependent on breakthroughs in fusion itself and in materials science. Build the infrastructure here in the Solar System and gradually push the envelope outwards. It’s a plan that could pay off one day in making that journey to Proxima Centauri via fusion, antimatter or other means, and crossing the gulf in a single human lifetime.