JPL Accomplishes Laser Sail First

Keith Gabryelski writes, "space.com has an article on how, in late December 1999, engineers at JPL used a laser beam to move extremely lightweight material using only the pressure of light." For those of you who haven't been keeping up with your science fiction, the idea is that with a tremendously large and incredibly thin sail, you could launch a spacecraft that would be propelled away from the solar system by the miniscule impact force from the light of the sun/gigantic lasers/mirror-focused light striking the sail. The theory is simple enough, but the execution is, shall we say, non-trivial.

Comments

Take care

[Kaufmann]

While I do understand that this is an exciting feat of engineering with extensive implications, I think it's a scientist's duty to keep his or her head in its place when accessing the results of his or her own research - and these guys seem to be gushing excessively. Feynman wisely warned us to be wary of any scientific paper which doesn't offer any questioning of its own conclusions.

Uhh, actually that's something different

[tesserae]

Your links point to a different technique for using lasers as part of a propulsion system: the laser heats the air that's inside the "combustion chamber" (quotes, because nothing actually burns), which then expands through a nozzle as with any rocket. The laser is providing the energy, instead of chemical reactions. They pulse the laser, so air flows back into the chamber between expansions. (Note that with this method, you must provide some source of gas -- an ablative lining, or somesuch -- at great altitudes where the air is too thin for this trick to work).

The news article describes a second method of laser propulsion, in which the momentum of the laser-generated photons are used directly: you bounce the photons off the "sail," and gain a tiny amount of momentum with each photon. This is very much like using a spinnaker sail, which is where the term "light sail" comes from. And because each photon has such a small momentum compared to the sail, it takes huge fluxes to cause a measurable accleration. This is why they talk about the elevated temperatures: they are throwing so much light at the sail that they're seriously heating it. (I glossed over the fact that you can make it work by absorbing the photon instead of reflecting it, which is actually what they appear to do -- with a loss in efficiency. So don't flame me, okay?)

Accelerations with proposed light sail vehicles are generally very small; they are effective because they are continuous, and the velocity builds up over time. This isn't completely novel, though -- we routinely account for the perturbing effects of sunlight on the orbits of spacecraft...

While I'm at it, I might as well point out that there are at least two other propulsion concepts which use lasers: laser-induced fusion concepts, where the laser is just the "trigger" and you use the heat from fusion to heat a working fluid (like more gas) or even directly exhaust the fusion products (definitely high-tech); and the ultimate propulsion system, where you "exhaust" nothing but light itself, substituting photons for atoms in your rocket (not the sort of thing you'd like to be behind, while it was working!).

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