NASA to Attempt Historic Solar Sail Deployment

June 27, 2008,

"Hold your hands out to the sun. What do you feel? Heat, of course. But there's pressure as well – though you've never noticed it, because it's so tiny. Over the area of your hands, it only comes to about a millionth of an ounce. But out in space, even a pressure as small as that can be important – for it's acting all the time, hour after hour, day after day. Unlike rocket fuel, it's free and unlimited. If we want to, we can use it; we can build sails to catch the radiation blowing from the sun."

These words were spoken not by a NASA scientist but by a fictional character – John Merton – in Arthur C. Clarke's short story The Wind from the Sun. If all goes well, Merton's prophetic words are about to become fact.

NASA researchers, thinking "out of the box" (or maybe "out of the rocket") have long dreamed of the possibility of sailing among the planets with sails propelled by sunlight instead of by wind. Except in works of fiction, though, no one has yet successfully deployed such a sail anywhere beyond Earth.

"There's a first time for everything," says Edward "Sandy" Montgomery of NASA's Marshall Space Flight Center.

Montgomery's team and a team from Ames Research Center (led by Elwood Agasid) hope to make history this summer by deploying a solar sail called NanoSail-D. It will travel to space onboard a SpaceX Falcon 1 rocket, scheduled for launch from Omelek Island in the Pacific Ocean during a window extending from July 29th to August 6th (a back-up extends from August 29th to September 5th).

"NanoSail-D will be the first fully deployed solar sail in space, and the first spacecraft to use solar pressure as a primary means of attitude control or orbital maneuvering," says Montgomery, who is NanoSail-D's payload manager.

"We are always on the lookout for opportunities. Ames owns a slot on the Falcon 1 launch and asked us if we wanted to go along. We said, 'Yes!' We'll use the Poly Picosatellite Orbital Deployer, or P-POD, developed by the University of California Polytechnic Institute to deploy our sail."

A few years ago, the Planetary Society attempted a mission like NanoSail-D called Cosmos I, but the launch vehicle failed and destroyed the undeployed spacecraft. Montgomery and team believe that NanoSail-D, however, will unfurl four gossamer wings from its pod in the blackness of space like a butterfly from a cocoon: movie.

"The structure is made of aluminum and space-age plastic," says Montgomery. "The whole spacecraft weighs less than ten pounds. We carry it around in a special suitcase -- airplane carry-on luggage size." Fully opened, the kite-shaped sail spreads out to about 100 square feet of light-catching surface.
"A success would be huge for the future of space exploration," Montgomery believes.

Why so important? Solar sails could extend our reach as far as our dreams. Because there's no friction in space, once a solar sail starts moving, it can go on forever. Indeed, long after a rocket would run out of gas and begin to coast, a solar sailship could still be accelerating, achieving speeds much faster and covering distances far greater than any rocket. No rocket in existence could carry enough fuel to reach the outer solar system in as short a time. And like a marine sail, a solar sail could also bring you home. You could use the solar sail to tack your vessel, making it travel "against the wind," back to Earth.

"It's not so much about how far a sail will go compared to a rocket; the key is how fast," says Montgomery. "The Voyagers have escaped the solar system, and they were sent by rockets, but it's taken more than three decades to do it. A sail launched today would probably catch up with them in a single decade. Sails are slower to get started though. So, for example, between the Earth and the moon, rockets might be preferred for missions with a short timeline. It's a trip of days for rockets, but months for a solar sail. The rule of thumb, therefore, would be to use rockets for short hops and solar sails for the long hauls."

All of this may sound like speculation, but NanoSail-D could show that solar sails are truly feasible. And there's an added bonus to this technology demo:

"Currently, micro-satellites in orbit above a few hundred kilometers can stay in orbit for decades after completing their mission," explains Montgomery. "This creates an orbital debris collision risk for other spacecraft. NanoSail-D will demonstrate the feasibility of using a drag sail to decrease the time satellites clutter up Earth's orbit. Although our sail looks like a kite, it will act like a parachute (or like a drag sail) in the very thin upper atmosphere around Earth. It will slow the spacecraft and make it lose altitude, re-enter the Earth's atmosphere and burn off in a relatively short period of time. A drag sail is a lighter alternative to carrying a propulsion system to de-orbit a satellite."

And finally, the question everyone wants answered: What does D stand for?

"We chose the 'D' in the name, not because it came after models A, B, and C, but because it can stand for demonstrate, deploy, drag, and/or de-orbit," says Montgomery.

Soon, 'D' may stand for something new: "DID IT!"

Source: NASA

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3.3 / 5 (3) Jun 27, 2008
>No rocket in existence could carry enough fuel to reach the outer solar system in as short a time. And like a marine sail, a solar sail could also bring you home. You could use the solar sail to tack your vessel, making it travel "against the wind,"

Huh? Its possible with marine sail cause the ship has keel in the water. How the hell do they want to use solar sail to travel from the outer Solar System back to Earth??

>And finally, the question everyone wants answered: What does D stand for?

Screw D, I want to know why a giant sail has to jump on the "Nano" bandwagon.
1.8 / 5 (4) Jun 27, 2008
Sadly, this technology falls short of any revolutionary deep space exploration technology.

It gives me about as much excitement as a swizzle stick.

3 / 5 (1) Jun 28, 2008
About tacking and whatnot: I believe what you do is reflect the solar wind tangent to your orbit around the sun, or some shenanigans like that. I may be wrong though... I've never tried doing it myself :P
3.5 / 5 (2) Jun 28, 2008
I don't see how tacking could be possible in a frictionless environment. I wonder if Montgomery has even ever sailed!
4 / 5 (1) Jun 28, 2008
"Tacking" back towards the Sun involves using the Sun's gravity.

Angle the sail to reduce the craft's transverse orbital speed (solar orbit) and gravity will reel it back in.
5 / 5 (2) Jun 28, 2008
What some comments so far seem to be forgetting is that calling this sailing is just an analogy, and no analogy is 100%. As such, any "tacking" a solar sail-craft will be very different from what a sea-sailing ship would do.

Also, it seems to me that people making the very common mistake of thinking that one goes in a straight line from the Earth to wherever. You don't. whether it's to the Moon, to Mars or to Saturn you go out in a spiral (geodesic).

So imagine we've build a huge sail-craft in the L5 point behind the Moon int its orbit. We un furl the sail and keep it always pointed at the Sun. Not only will we start to move out & away from the Sun but we will still have the angular momentum from orbiting with the Earth. That's why we spiral outwards.

However, pointing the sail directly at Sun is not the only thing we can do.
5 / 5 (1) Jun 28, 2008
The width of the sail is perpendicular to the direction of the sun, and if you remember public school science the angle of reflection equals the angle of incidence and our "push" from the Sun since us exactly opposite to the reflection. In this case that is directly away from the Sun. This results, as I said before, in a spiral path going out from the Sun.

But what happens if we change the orientation of the sail so that it is no longer facing the sun but on an angle?

Now, because our solar ush is now at an angle it accomplishes 2 thing because any vector can be broken down into two smaller vectors at 90 degrees to each other. So now we have in effect two pushes - 1 that is pushing us away from the Sun and a second that is adding to our orbital velocity.

Even though we have a smaller push away from the Sun, increasing your orbital velocity has the effect of "throwing" you farther away from the Sun. Add that "throw" to the decreased "push" and you have a better than even trade off, up to a point.

Now this is where tacking comes in. What if we tilted the sail backwards? Well, when we break down the vector of our push into it's perpendicular components we still have the push away from the Sun but now we have a push that is *subtracting* from our orbital velocity. For a rocket-craft, without a sail, a reduction in orbital speed would cause it to be "pulled" back toward the Sun by gravity. For us with a solar sail, however, obviously that "pull" has to be greater than the "push" before we'd start to drop in toward the Sun. Otherwise we would just slow down (but not stop) the rate at which recede from the Sun.

Now back to that "up to a point" bit I mentioned up when we were tilting the sail forwards to increase out orbital velocity.

The amount of push we get from the Sun on our is not a constant because it is based on the cross-sectional area or sail shows the sun. If we tilt the sail 45 degrees away from the Sun we have reduced the cross-section by half and now we only receive half the push we were getting before. So if we were trying to add to our orbital velocity, at first the decrease in push is insignificant because the physics of a faster orbital velocity means the "throw" we get is larger than both the over-all decrease in thrust and lessening outward push. At some point, however, you come to a break even point, after which extra "throw" is less than the amount of push lost by your decreasing cross section facing the Sun.

I hope that helps explain thingg for some people.
1.5 / 5 (2) Jun 28, 2008
Tacking won't work (or at least not well) because the energy required to tack will have to be present already on the ship. E.g. a rocket or nuclear reactor or something else with a good energy density that can keep applying the torque required to the sail in order to hold it in a non-neutral position. Otherwise your attempt to tack will simply wind up in a spinning craft that on average is facing the Sun and therefore not tacking. But if you add the weight of this energy source to your solar sail vehicle, it will take a very very long time to get moving. Also since the energy required to tack will be the same energy you are attempting to apply against the pressure of the Sun, you might as well haul in your sail and fire your rocket.
1.5 / 5 (2) Jun 28, 2008
One solution might be to build a gigantenourmous radiometer as the core of you vessel and get it spun up to a very high rate of speed. This would give you an intertial against which you could effectively tack for a while. But then the slightest micrometorite damage would probably cause it to tear itself apart.
4 / 5 (1) Jun 30, 2008
Thanks for explanation CoryCa, yeah such "tucking" could indeed work although it seems a bit far fetched as there are obvious problems:

1. Very small cross-section due to large angle needed,
2. Huge distance from the Sun in the Outer Solar system and therefore low photon flux,
3. Always present radian acceleration away from the Sun

The sail would gain much more orbital momentum on the way to the Outer Solar System then it would be able to give off using such "tacking" unless it was decelerating almost the entire way to the outer Solar System, but that would greatly prolong the time needed to get there.

There wont be any rotating force acting on the sail even if its at an angle to the Sun. In the case of wind sail such rotation is the result of air dynamics.

All in all the comparison with sailing has more to do with the romantic appeal of sailing then with similarity of underlying physical principles.
not rated yet Jul 02, 2008
why do you need torque? I mean cant you just use a solar powered flywheel to move the sail? Plus why would the spacecraft begin spinning in the first place? Isn't the force from the sun essentially constant across the sail?

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