Just how hard would it be to build a true interstellar craft? I’m not talking about a spacecraft that might, in tens of thousands of years, drift past a star by happenstance, but about a true, dedicated interstellar mission. Those of you who’ve been following my bet with Tibor Pacher on Long Bets (now active, with terms available for scrutiny on the site) know that I think such a mission will happen, but not any time soon. And the proceedings of the Joint Propulsion Conference, held last month in Hartford, go a long way toward explaining why the problem is so difficult.

Wired looked at the conference results in a just published article, the most interesting part of which contained Robert Frisbee’s speculations about antimatter rocketry. Two things have been clear about antimatter for a long time. The first is that producing sufficient antimatter is a problem in and of itself, one that may keep us working with tiny amounts of the stuff for some time to come. Even so, interesting mission concepts, like Steve Howe’s antimatter-energized sail, have grown out of the studies that have been performed on possible hybrid systems.

As to antimatter itself, while the annihilation of matter and antimatter releases vast amounts of energy, controlling the result is even more difficult than producing antimatter in quantity in the first place. Proton/anti-proton annihilation is preferable to electron/positron because the gamma rays produced by the latter can’t be directed to produce thrust, a problem Eugen Sänger wrestled with fifty years ago. But the former is a possibility because the reaction products (pions) can be directed and confined electromagnetically. The idea here is to transfer some of that vast energy of annihilation to a propellant working liquid.

Even so, our rocket still has problems. Check our friend Adam Crowl’s recent piece on antimatter for several good links and some musing on the relatively poorer performance with antimatter than one might have expected (an exhaust velocity of 0.33 c may itself be a surprise, but take a look at this Frisbee presentation). Frisbee (NASA, Jet Propulsion Laboratory) has been studying the interstellar conundrum for a long time, with particular attention to antimatter. The design he presented at the conference, a stack of linked components designed to keep radiation away from crew or payload, is summed up by Wired this way:

At the rocket end, a large superconducting magnet would direct the stream of particles created by annihilating hydrogen and antihydrogen. A regular nozzle could not be used, even if made of exotic materials, because it could not withstand exposure to the high-energy particles… A heavy shield would protect the rest of the ship from the radiation produced by the reaction.

A large radiator would be placed next in line to dissipate all the heat produced by the engine, followed by the storage compartments for the hydrogen and antihydrogen. Because antihydrogen would be annihilated if it touched the walls of any vessel, Frisbee’s design stores the two components as ice at one degree above absolute zero.

So far, so good. We then include basic spacecraft systems in front of the tanks of propellant and then our payload. But theory meets a grim reality in the numbers: Frisbee is talking about an 80 million metric ton starship (the Space Shuttle weighs in at 2,000 metric tons), with another 40 million metric tons each of hydrogen and antihydrogen. The payoff is a forty year mission to Alpha Centauri.

At least it’s designed as a rendezvous mission. A forty year flyby to the Centauri stars would be moving at something better than a tenth of lightspeed once it gets up to cruise. Even if exquisitely targeted, such a probe would operate within 1 AU of the target system (let’s say Centauri B) for something less than three hours. Ponder the challenge presented by collecting imagery and data from Centauri planets in such a scenario.

What to do? These results reinforce much that we already knew about the difficulty of coming up with an interstellar mission design that is remotely affordable, and everything comes down to energy. As noted by Wired, interstellar theorist Brice Cassenti (Rensselaer Polytechnic Institute) comes up with a minimum value of the current energy output of the entire world to send a probe to the Centauri system, a figure Cassenti is quick to note could easily swell to 100 times that value.

It’s useful to ponder the size of the challenge as we continue to scout for concepts that can overcome these problems. The dual track that interstellar studies takes continues to work this way: 1) Push concepts constructed under the parameters of known physics to their utmost, to see where they might lead. Antimatter rockets, laser sails, pulsed fusion and their ilk all fall under this category. 2) Investigate potential concepts that might extend our knowledge of known physics. Here we turn to studies like those sponsored by, among others, NASA’s now defunct Breakthrough Propulsion Physics project. The Tau Zero Foundation hopes to bring philanthropic support to both approaches.

No one can say whether interstellar missions will ever be feasible. What we can insist is that studying physics from the standpoint of propulsion science may tell us a great deal about how the universe works, whether or not we ever find ways of extracting propulsive effects from such futuristic means as dark matter or dark energy. And if it turns out that our breakthroughs fail to materialize, the potential of multi-generational missions supported by human crews still exists. They will be almost inconceivably demanding, but nothing in known physics says that a thousand-year mission to Centauri is beyond the reach of human technology within a future we can still recognize.

How big would an interstellar mission be? Let me close by quoting Robert Frisbee himself, from a presentation he gave at the 2003 iteration of the Joint Propulsion Conference:

In the long term, it will represent a Solar System civilization’s defining accomplishment in much the same way we look to the past accomplishments of humanity, like the Pyramids, Stonehenge, the great medieval Cathedrals of Europe, the Great Wall of China and, not so long ago, a space program called Apollo.