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).