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Interstellar Spaceflight Realities

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.

Comments on this entry are closed.

  • tmayes1999 April 25, 2006, 11:32

    The drag arguement against interstellar ramjets, or bussard drives
    is not valid * because it is based on faulty and incorrect assumptions. Also a ramjet can use an electric field ,rather then an electromagnetic field to collect propellant.

  • Eric James May 21, 2006, 10:48

    The momentum/drag of all the fusion mass being inducted into the system would be pretty immense (relative to expected propulsion), but a scoop that actively conveys the material into the reactor (rather than passively collecting it) would eliminate the drag. In fact, it would essentially pull itself along slightly. Perhaps some sort of manipulated EM fields would do the trick? Although in this case, the energy expenditure would probably exceed energy production.

  • hiro August 23, 2006, 16:56

    how much energy does it produce when we use both fusion and anti-fusion at the same time?

  • Administrator August 24, 2006, 15:32

    Not sure what you mean by anti-fusion unless it’s a reference to antimatter/matter annihilation. And if the latter, then antimatter trumps fusion in a major way, although there are studies for using antimatter to induce fusion reactions, these having the benefit of needing comparatively small amounts of antimatter.

  • hiro September 20, 2006, 15:15

    we can make anti-hydrogen atoms right? In the future, i think we also can produce anti-tritium as well. If we have enough amount of matter for fusion reaction, then we can create a thermonuclear bomb which is made from antimatter. After that, let both the normal fusion and antimatter fusion bombs explode, they will produce huge enery from the fusion AND matter-antimatter reaction. However, the math is nontrivial, so i’m not sure how much energy it creates.

  • Matt January 15, 2007, 1:38

    The main problem with antimatter is not that we couldn’t make anti-deuterium or anti-tritium, but that it would take something like two million years to make a gram. For travel purposes we would need billions of kilograms.

    A major hurdle in the production of antimatter is that no theory has ever been able to offer up a plausible explination of why there is a preponderance of matter over antimatter. With no understanding of how it arises, attempting to reproduce it is haphazard.

    However, I imagine that sizeable quantities of antimatter could be produced utilizing an electromagnetic field and a black hole. In the space near the black hole virtual particle/antiparticle pairs are created, but one falls into the black hole and the other radiates away, becoming real and carrying away some of the hole’s mass-energy–Hawking radiation. With an electromagnetic field we might be able to force the antiparticle to become real.

    Unfortunently, there are no black holes handy.

  • Administrator January 15, 2007, 15:01

    The actual amount of antimatter needed wouldn’t be so onerous. Robert Forward ran the numbers for a one-ton robotic probe to Alpha Centauri at 10 percent of light speed. He came up with forty pounds of antimatter and four tons of liquid hydrogen or other propellant. A two-stage mission with deceleration at the Centauri stars called for 770 pounds of antimatter. Huge numbers, of course, compared to current antimatter production, but much better than billions of kilograms. And the possibility of using minute amounts of antimatter in concepts like Howe’s antimatter sail means that some form of antimatter use in propulsion needn’t be that far off.

  • hiro February 1, 2007, 15:24

    It won’t take million years to produce that amount of antimatter. If we can build a 1 PeV or a 10 PeV collider in the next hundred/thousand years, then we’ll have enough antimatter to do everthing. The only problem is that the politicians won’t allow it happens like the SSC story back in the 90s. I think the cost should be around 500 billion dollars, so it’s possible to build one if we have to do in some special situations. I think antimatter fusion is our future in the next thousand years. After that, I have no idea.

  • Timothy J Mayes March 6, 2007, 11:37

    The answer may be to use a nuclear electric propulsion interstellar ramjet
    that collects ionized hydrogen gas from interstellar space with an electric field
    and then uses it as reaction mass in electric ion or plasma rocket thrusters.
    The onboard energy sources would be fusion reactors, or breeder type fission reactors.

  • Timothy J Mayes March 6, 2007, 11:53

    A simple workable interstellar ramjet can be made in the following
    way. We could use a 20,000 kilogram design which has two ion rocket
    engines, and one ion ramjet engine. Mass ratio may be 2/1 , so Velocity achieved at burn out of the rocket part of the vehicle is eqaul too
    Ve * Ln 2/1 . At rocket burn out the mass is down to 10,000 kg ,
    and velocity may be 100-150 km/second with the ion rocket specific
    impulse in the range 10,000 to 20,000 seconds.Then you turn on the ion ramjet engine to continue the acceleration of the vehicle indefinitely.
    The ion ramjet engine in its simple form is a hollow cylinder containing
    three electricaly charged grids in a line . Grid number 1 generates a -1000
    volt electric field to electrostaticly attract + hydrogen ions , and draw
    them inside the ion ramjet engine. Grid 2 may have a charge 0f -10,000 volts
    on it and grid 3 may have a charge of of -1000,000 volts on it. The ions collected by grid 1 will be accelerated n the electric field between grid 2 and grid 3 . Then they will be expelled to generate thrust and will be neutralized
    by capturing electrons from the electron emitters. The Acceleration A = F/M
    m/sec^2 for the ion ramjet maintained indefinetely for time T