In an article on interstellar propulsion options at Physorg.com, writer Chuck Rahls focuses on three technologies that have been proposed to make a trip to Alpha Centauri possible. Of the three, laser-pushed lightsails are indeed in the running, and have been since Robert Forward realized the implication of the laser while working at Hughes Aircraft. Also employed by Hughes in the company’s research laboratories was Theodore Maiman, who had shown how to make a functional laser in 1960. Forward wrote the concept up as an internal memo at Hughes in 1961, and later went public in the journal Missiles and Rockets. In the same year (1962), he described the idea in an article in Galaxy Science Fiction.
Rahls writes about a laser-driven craft weighing 16 grams making it to the Centauri stars in ten years. It’s a grand concept — Forward came up with it, too, and gave it the wonderful name Starwisp, though he used not lasers but microwaves to drive it — but Geoffrey Landis has convincingly shown that Starwisp could never fly, the intensity of the microwaves needed to accelerate it being sufficient to vaporize the entire spacecraft. Forward knew this and was working on other solutions at his death.
Centauri Dreams also has serious reservations about the second concept addressed here, the Bussard ramscoop. One problem is that enormous speeds are needed just to ‘light’ the ramscoop’s engine. But a more profound issue is that physicists have shown the ramscoop idea to be unworkable because of drag. In fact, Dana Andrews and Robert Zubrin demonstrated in the late 1980s that a spacecraft of Bussard’s design would experience more drag from its enormous electromagnetic ‘scoop’ than thrust. The real beauty of the ramscoop concept is that it generated an equally interesting — and workable — notion: use an electromagnetic sail tens of kilometers in diameter that could be pushed by particle beam, or used in the destination solar system for braking upon arrival.
The third, and perhaps most exciting in today’s terms of Rahls’ technologies is antimatter. Here the options are proliferating, and because we know how to harvest only the minutest quantities of the stuff, we’re finding ways to make fast propulsion systems that use antimatter only as a catalyst, igniting fusion, perhaps, or using it to interact with an uranium-coated sail. The latter concept is Steve Howe’s (I referred to it in these pages just the other day), a proposal so ingenious that any star-minded reader should make haste to the NASA Institute for Advanced Concepts site to download Howe’s “Antimatter Driven Sail for Deep Space Missions.”
NASA’s John Cole told me at Marshall Space Flight Center back in 2003 that the power released by Howe’s design is on the order of 2000 kilowatts per kilogram. “It’s just an enormous figure,” Cole said. So even if Howe’s figures are an order of magnitude off, even two orders of magnitude off, a factor of 100, he is still in realm of where we can have human exploration of the outer planets.”
The Andrews/Zubrin article mentioned above, by the way, is a key work in the development of interstellar concepts. It’s titled “Magnetic Sails and Interstellar Travel,” found as International Astronautical Federation Paper IAF-88-5533 (Bangalore, India, October 1988). If I had to put money on the proposition, I’d bet a particle-driven magnetic sail will be our first true star mission, a robotic probe launched around 2100. But that’s the last bet you’ll get out of me.