NASA Demonstrates Space Sails (In The Lab)

caelumcrux writes: "Once again, technology repeats itself. NASA's jet propulsion laboratory released information on their latest testing: the use of lightweight sails and beamed-energy propulsion for space travel. It's only a matter of time before kids are flying photon kites." The laser sails have been covered here before but the microwave experiments seem like a greater success. Thankfully, this has a few more details than many NASA releases, and even features a link to a (not very good, artists-conception type) picture of the thing.

Comments

Re:I don't get it...

[mcelrath]

Momentum is only mass*speed for massive particles. The full expression is E^2=m^2*c^4 + p^2*c^2, where for a photon E=hbar*frequency, m is mass, p is momentum, and c is the speed of light. (Man, where is MathML when you need it?) If you solve for p, and make some assumptions about the energy of a massive particle, you get p=m*v. But for a photon, m=0, and thus p=E/c=hbar*frequency/c. Thus the momentum transferred to the craft is just proportional the energy of the photon, or the frequency of the photon.

Since the photons are reflected, the total impulse is delta-p=2*p, so you get twice the momentum of the photon. (A previous poster incorrectly assumed the photons were absorbed. This would burn up the sails, and impart less impulse to the craft)

To answer another question I saw, why aren't ion engines better: Ion engines require a power source on board the craft, and produce little thrust (~10^-4 Newtons). Solar sails can produce large thrusts (~10 Newtons). The article mentions that they acheive several times g accelerations. The ion engine also requires fuel (though not very much).

There is also the "plasma bottle" engine being developed (at JPL, I think). It uses a confined plasma to create a huge pancake shaped magnetic field, which interacts with the solar wind and gets a push from it. This also requires a largeish power source on board to sustain the plasma. It is also a "solar sail" though it uses the solar wind (energetic particles from the sun), not the sun's photons.

Solar Sails realy are a Good Idea. Not only can they use the sun, which is an always-shining abundant power source, but they can also use ground-based power sources. It's a hell of a lot cheaper to build a ground-based power source and a laser than it is to put that same power source in orbit. (At $10,000 a pound to launch stuff) Also by turning your sail WRT the sun you can get thrusts in different directions. This will allow you to use the solar sail to come back from Mars, for instance, just by turning the sail in the right direction. Orbital mechanics is a little counter-intuitive -- pushing "outward" doesn't necessarily get you farther away since both you and the planet are going in a circle, around the sun. You really want to push in the direction of motion around the sun that you're traveling (or against it) to increase/decrease the size of your orbit, and thus move you closer or further from the sun. Pushing directly away from (or towards) the sun will just get you in an elliptic orbit.

--Bob