World's first solar sail unfurled in orbit | Hacker News Reader

World's first solar sail unfurled in orbit

science.nasa.gov

69 pointsjoshrule22 comments

Not quite. According to the article, the Japanese deployed a solar sail with a Venus probe. This, though, is the world's first solar sail on a probe designed for Earth orbit.

The Planetary Organisation tried to deploy a solar sail back in 2005 ( https://secure.wikimedia.org/wikipedia/en/wiki/Cosmos_1 ) but the rocket failed.

The probe in question: http://en.wikipedia.org/wiki/IKAROS

The article does mention IKAROS, though changing the title from "Solar Sail Stunner" was probably a good move. This satellite is interesting however in that it uses what sounds like a much simpler sail deployment design. IKAROS used centrifugal force to pull the sail out whereas NanoSail-D seems spring loaded.

"World's first unfurling of a solar sail in orbit" would be quite a bit more accurate.

In what way is this a solar sail? It's a drag sail. From the article, it seems to me that the sun doesn't propel it any more than the sun propels my car.

Because it's so close to Earth, the drag force will be much greater than any propulsion force achieved from solar particles, which means its orbit will decay and it will fall out of orbit. However, this doesn't preclude it from being a solar sail, it just means it's not a very efficient sail. Eventually, we'd like to send these into deep space, where there is no drag from the atmosphere, allowing the craft to accelerate indefinitely. But for now, it makes sense to keep it in orbit and study it at close range so we can be sure the "sail" effect works as predicted.

More info: http://www.nasa.gov/mission_pages/smallsats/10-109.html

"The relatively low-deployment altitude means drag from Earth's atmosphere may dominate any propulsive power it gains from the sun, but the project represents a small first step toward eventually deploying solar sails at much higher altitudes... because the sail will deploy relatively close to Earth, researchers will have a difficult time detecting the slight solar effects."

> allowing the craft to accelerate indefinitely.

In theory that's all very nice, in practice the power available for acceleration decreases as the cube of the distance.

Of course a bit of acceleration is better than nothing but this will only work well if you have very large amounts of time, and since all systems will have to survive for all that time I don't see this as a practical means of propulsion. Bussard Ramjets are a lot harder to construct (and shield) but I think if there ever is going to be a practical means of achieving speeds that are a significant fraction of lightspeed then I think that's where it's going to come from, not from solar sails (even though the technology definitely has its charm).

>In theory that's all very nice, in practice the power available for acceleration decreases as the cube of the distance.

This is why any solar sail intended to move very quickly will open up somewhere closer to Mercury.

I don't know that it's very practical either; but the fact remains that it is, in fact, a solar sail, just not a particularly good one.

It would be interesting to see if the magsail or electric sail concepts are developed as well. The simplest design appears to be the electric sail, which only needs radial conductive wires (easy to manufacture and deploy, although it needs to be launched well outside the Earth's magnetosphere).

http://en.wikipedia.org/wiki/Magnetic_sail

http://en.wikipedia.org/wiki/Electric_sail

I see that this is expected to, as one outcome, help reduce further space junk in orbit. I wonder what impact existing space junk - which is predominately small pieces of metal orbiting the earth at speed - would have on the sails?

Without knowing its altitude, even the 70-120 day lifespan makes it a target. I wonder how resistant a 10m^2 piece of thin polymer sheet would be to a lazy 2-inch screw hurtling into it?

> I wonder how resistant a 10m^2 piece of thin polymer sheet would be to a lazy 2-inch screw hurtling into it?

Presumably, not resistant at all... Even the heavy shielding on the ISS can only protect from debris less than 1cm in diameter. The question is, would such a debris hit shred the sail, or would it punch a neat hole, leaving it mostly intact?

Yes, 'not' is how I would have described the resistance. As long as it were a fairly direct hit, I imagine a neat hole would be most likely (though these could add up fairly quickly over time).

"For reasons engineers still don't fully understand, NanoSail-D spontaneously ejected itself." Best line of the article!

I'm going to point people to this article next time some code inexplicably starts working.

Is it a sail or a drag parachute?

It is a proper sail propelled by light pressure instead of wind.

https://secure.wikimedia.org/wikipedia/en/wiki/Solar_sail

"NanoSail-D will remain closer to home. "Our mission is to circle Earth and investigate the possibility of using solar sails as a tool to de-orbit old satellites and space junk," explains Alhorn. "As the sail orbits our planet, it skims the top of our atmosphere and experiences aerodynamic drag. Eventually, this brings it down.""

Pretty clearly not a solar sail...

"They'd also like to measure the pressure of sunlight on the sail, although atmospheric drag could overwhelm that effect."

Dual purpose study.

Excellent question, the article called it a solar sail but only mentioned its drag properties. I kept wondering why you would need a solar sail to keep something in orbit...?

It is a lightweight, inexpensive, spaceship-deployable sheet. Currently it is being used as a drag parachute, but it could just as easily be used as a sail.

I think.

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