What is a solar sail?

Jan 20, 2014 by Fraser Cain, Universe Today
What is a solar sail?
IKAROS – solar sail from Japan. Credit: JAXA

I'm Fraser Cain, and I'm a sailor. Well, okay, I've got a sailboat that I take out on the water when its warm and the weather's nice here on Vancouver Island. I think it's one of the reasons I absolutely love the idea of a solar sail.

Here's how they work: Light is made up of photons. Even though they have no mass at rest, they have momentum when they're moving, well, speed. When they reflect off a surface, like a mirror or a shiny piece of metal, they impart some of this momentum to that surface. This effect is negligible here on Earth, but out in space, with forces perfectly in balance, that additional momentum can really add up.

A spacecraft flying to Mars gets pushed off course by several thousand kilometers because of light pressure from the Sun.If mission planners didn't compensate for this drift, their spacecraft would miss the planet, or even worse, crash into it. Even though the total amount of pressure per square meter on a is minuscule, it's constantly streaming from the Sun, and it's totally free….And propulsion that you don't have to carry with you is the best kind there is.

This is more than just an idea. Solar sails have already been launched and deployed in space. The Japanese Ikaros satellite unfurled a 14-meter solar sail back in 2010. NASA launched its own Nanosail-D spacecraft in 2011. An even bigger solar sail, the Sunjammer, is planned for launch in 2014. The Planetary Society is working on a solar sail project as well.

The closer to the Sun you are, the better they work. In fact, a solar sail would be an ideal vehicle to explore the regions of Mercury and Venus, since they receive so much radiation. But you're probably wondering how a solar sail could get down to those planets because light is streaming from the Sun in all directions. It's all about raising and lowering your . If you want to raise your orbit around an object, all you have to do is speed up. And if you want to lower your orbit, you just need to slow down.

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A solar sail launched from Earth would start out with the same orbital velocity around the Sun as the Earth. To get into a higher orbit, it tilts the sail so that the light from the Sun speeds it up. And to get into a lower orbit, it tilts in the opposite direction, and the light from the Sun acts like a brake.

A solar sail might even be the ideal spacecraft to make the journey to another star. An interstellar solar sail could lower its orbit so that it's just above the surface of the Sun. Then, it would unfurl the full sail and capture the most possible photons. A series of powerful laser beams would then target the sail and increase its velocity to a significant fraction of the speed of light.

What is a solar sail?
Solar Sail. Credit: NASA

Of course, you'd need a solar sail thousands of kilometers across, made of a material thinner than a human hair, and lasers putting out more energy than all of humanity. The idea is still intriguing, even though it's well outside our current technology. Once this technology gets better tested, we'll to see many more missions employ solar sails as part of their propulsion system.

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not rated yet Jan 20, 2014
A Mote in Gods Eye uses this concept to launch a crewed mission from an alien race.

This is also the perfect method for moving asteroids. Yes they are Megatons but Square Kilometers of sails can be attached and used to change their orbits enough to move the asteroids, their refined materials or factory ships around as needed. If your sail material can also maintain a magnetic field, you gain additional boost from the solar wind.
5 / 5 (1) Jan 20, 2014
"Even though they have no mass at rest, they have momentum when they're moving, well, light speed. When they reflect off a surface, like a mirror or a shiny piece of metal, they impart some of this momentum to that surface."

Above copied from article, below from my first semester physics textbook;

Momentum= (mass) x (velocity)

5 / 5 (1) Jan 21, 2014
Above copied from article, below from my first semester physics textbook;

Momentum= (mass) x (velocity)

Yeah, they aren't really being fully accurate when they say that photons have momentum. They have energy with a vector, which can be absorbed by an object, which imparts both the energy and its vector to the object that absorbs it (or reflects it). Conservation of energy is at work here, not really momentum. It may start and end as momentum, but they're kinda skipping a couple middle steps.

On another note: Using a solar sail to lower your orbit should be a painfully slow process. Since there's no such thing as a perfect mirror, no matter what angle you tilt the sail, you're still absorbing some of the light, which imparts an outward velocity vector on you. So you still have that outward pressure as you're trying to slow down to decrease your orbit. It's like tacking up a river or against the tide. It'll still work, but not nearly as well as when you're outbound.

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