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Remembering Starwisp

Mention beamed propulsion and people invariably think you’re talking about lasers. The idea seems obvious once you’ve gotten used to solar sail principles — if photons from the Sun can impart momentum to push a sail, then why not use a laser beam to push a sail much farther, into the outer Solar System and beyond? These are regions where sunlight is no longer effective, but a laser infrastructure of the kind envisioned by Robert Forward could produce a tightly collimated beam that could drive the sail to an appreciable fraction of the speed of light.

But are lasers the best way to proceed? Although he would sketch out a range of missions with targets like Alpha Centauri and, the most audacious of all, Epsilon Eridani (this for a manned crew, with return capability!), Forward himself quickly turned away from lasers and began exploring microwave propulsion. I’m fairly certain the turn to microwaves came at Freeman Dyson’s suggestion, and when I asked Dyson about it in an interview some years back, his response all but confirmed the fact. “It doesn’t matter who came up with it,” Dyson said, “the question is whether it would work. It’s problematic but a good system to look at.”

Problematic indeed. But also a system with serious advantages over lasers. Forward wanted to reduce the weight of his unmanned probe as much as possible, so he conceived of making it out of nothing more than a wire mesh a solid kilometer in diameter, one that weighed a mere sixteen grams and included microchips at each intersection in the mesh. The name Starwisp seemed a natural for this spider’s web of a starprobe, a mission so lightweight that it would actually be invisible to the eye.

Forward intended to accelerate Starwisp at 115 g’s using a 10 billion watt microwave beam that would take it to one-fifth of the speed of light within days. It’s probably the speed of Starwisp as much as anything else that catches the imagination. In a time when we speak of thousand year sail missions to the Centauri stars as the fastest conceivable using near-term technologies (and even that is quite a stretch), Forward was talking about putting a probe with data return capability into Centauri space within twenty one years. It would be a fast flyby, to be sure, but all those microchips embedded in the vehicle would use microwave power to return imagery as Starwisp ripped through the Centauri system.

And here’s where the advantages come in. A laser beam mission requires a sail, but if you want to reduce the size and weight of your vehicle, microwaves can operate with something much more like a grid. It’s a function of the wavelengths involved. Recently I discussed these matters with microwave specialist James Benford, president of Microwave Sciences in Lafayette, CA. We’ll be talking about the beamed propulsion experiments that Benford and his brother Gregory performed at the Jet Propulsion Laboratory in coming weeks. But for now, Benford was musing about Forward’s thinking as it moved into the microwave realm:

Bob could see that one of the advantages of microwaves is that the wavelength is comparable to the human hand. These are dimensions you can see as opposed to lasers, which operate at invisible, minute wavelengths. With microwaves, you could push a grid that you could see right through. It would therefore be much lighter in mass and yet still rigid, because the grid has only to be spaced more than some fraction of a wavelength. Ordinary window screen would be just fine; in fact, it would be more than you need. The whole point is that microwaves are stopped completely by a conducting surface as long as the gaps are smaller than a wavelength or so.

All of which makes for powerful advantages. Then we can throw in the cost factor. Benford again:

Microwaves are a whole lot cheaper than lasers, typically by two orders of magnitude in terms of the cost of the optic that you use to broadcast, or the power efficiency of the laser. Optical surfaces for good telescopes that are used in lasers cost on the order of a million dollars per square meter. Whereas a good microwave surface is somewhere south of ten thousand dollars per square meter and in fact, at the kind of wavelengths Bob was talking about, which were down in the lower microwave region, that’s where commercial satellite antennas are available on the order of ten dollars a square meter. So the differences in economy are enormous.

Why isn’t Starwisp at the forefront of interstellar mission thinking? Alas, Geoffrey Landis went to work on the concept and discovered that the effect of the intense microwave beam on the materials Forward was working with would be disastrous. The lighter the wires the better for propulsion purposes, but wires as light as Starwisp’s would absorb rather than reflect the microwaves, destroying the craft within microseconds. Moreover, those long microwave wavelengths (compared to visible light) make for enormous beaming systems — Forward wrote about a lens 50,000 kilometers in diameter in his original Starwisp paper.

A lens considerably larger than the diameter of Earth? Clearly, something has to give. But the advantages of microwaves are unmistakable both within the Solar System and beyond. We’ll be talking more about microwaves soon, with more from my interview with Jim Benford and thoughts on how the microwave concept, already well established in the laboratory, can be applied to practical space technologies.

Until then, if you’re interested in the original Starwisp paper, it’s Forward, “Starwisp: An Ultra-Light Interstellar Probe,” Journal of Spacecraft 22 (1985b), pp. 345–50. And see Geoffrey Landis’ significant follow up, “Advanced Solar- and Laser-Pushed Lightsail Concepts,” Final Report for NASA Institute for Advanced Concepts, May 31, 1999 (downloadable from the still available archives at the NIAC site).

Comments on this entry are closed.

  • James M. Essig October 21, 2008, 14:13

    Hi Paul;

    It occurred to me that if it were possible to produce beam sails hundreds of thousands if not millions of kilometers accoss that were of a grid or net like construction, then perhaps such sails could be used to reflect ultra-low frequency coherent electromagnetic waves provided that the nets where composed of high temperature superconducting material with extremely high current density criticality thresholds and extremely high magnetic field criticality thresholds.

    If such nets had mechanisms to bleed off excess current or electrical power transmission within, such as might be used to divert power for other electrodynamic propulsion elements such as ion or electron rockets, photon rockets, and the various forms of proposed electrodynamic-plasma-hydrodynamic propulsion mechanisms, then perhaps highly focused electromagnetic radiation beams with frequencies as low as 1 Hertz or less could be used to push the sail and attached craft forward. The caveat is producing a coherent beam of ultralow frequency EM radiation that has high enough electric and magnetic field component amplitudes commensurate with a beamed power level capable of accellerating a large interstellar manned space craft to perhaps high gamma factors.

    The use of superconducting gridlike net sails with large spacings between the grid lines might permit relatively very low mass high capture area sails for such ultralow frequency EM radiation while reflecting the EM radiation with maximal or near maximal effeciency.



  • dad2059 October 21, 2008, 14:26

    Definitely lightweight materials are the way to go, given mass to delta v ratios.

    Carbon nanotube bucky paper perhaps?


  • Adam October 22, 2008, 6:08

    Hi dad2059

    Bucky-tube quarter-wave sails would definitely be a big step forward for star-sailing – enabling high-speed probes propelled by sunlight alone. The tricky bit will be deploying them so close to the Sun.

  • djlactin October 22, 2008, 10:18

    As I was reading this, I was thinking “Metal? Microwave?” We all know the rules about household microwave ovens… So I was glad to see that the problem had been noticed and quantified.

    As for somebody’s Dad’s idea, I suggest a related one: a geodesic sphere of carbon nanotubes. (The biggest Buckminster Fuller design ever?) I haven’t had time to play with the numbers, but some of you may remember my idea of a carbon nanotube space elevator (that would weigh less that 0.24 g for the 40000 km length).

  • ad koppen October 25, 2008, 8:13

    Is it possible to built a propulsionlaser in space that uses sunlight ?, kind of big lenses or other type of collectors in space that combine that light from the sun in one and make a laserbeam out of it, there is plenty of energy out there near the sun. It sounds to me as reasonable and technically within our reach. With such a laserbeam men can propel tiny minituaristed space explorers into the solar system.