about Solar sailing
Introduction:
Solar sailing (also called light sails or photon sails) is a revolutionary way of propelling a spacecraft through space. Large reflective sails on a solar sail spacecraft capture the momentum of light from the Sun and use it to propel the spacecraft forward. Since the 1980s, a number of spaceflight missions to test solar propulsion and navigation have been proposed.
?How does solar sailing work
Photons are the particles that make up light. Photons have no mass, but they do have momentum as they travel through space. When light strikes a solar sail with a bright, mirror-like surface, photons keep bouncing off the sail (i.e. they reflect off it, just like a mirror). When photons interact with the sail, their momentum is transferred to it, giving it a slight push.
IKAROS space-probe with solar sail in flight (artist's depiction) showing a typical square sail configuration
How does a solar sail control its direction?
When a solar sail is directly facing the Sun, photons propel the spacecraft forward and away from the Sun. A solar sail, on the other hand, can move in different directions by tacking like a sailboat, changing the angle of the sail relative to the Sun. It's even possible to change the spacecraft's orbit around the Sun by angling the sail so that solar photons push against it. Solar sails can also change their direction by changing their centre of mass or by using tip vanes.
What is the advantage of solar sailing?
When spacecraft are launched from Earth, they gain the amount of their momentum, and then most increase their speed or change course using chemical rockets that burn fuel that the spacecraft carries on board. However, more rocket fuel means greater weight, limiting how much may be carried. Most spacecraft attain their maximum speed before coasting through space or relying on gravity assistance from other planets to reach their destinations. A spacecraft covered with solar sails can continue to accelerate as long as there is light pushing on it. Sunlight can continually force on the sail within a solar system, speeding the spaceship throughout its entire journey. This means that solar sail-powered spacecraft can reach speeds that chemical rockets would find difficult to match.
How large must a solar sail be?
There is no theoretical minimum size for a solar sail, but larger sails will capture more sunlight and accelerate the spacecraft faster for the same mass spacecraft. In the 1970s, a NASA team led by Planetary Society co-founder Louis Friedman proposed a 600,000 square metre (6.5 million square foot) solar sail that would be used to send a spacecraft to rendezvous with Halley's comet. This is equivalent to a square 800 metres (half-mile) by 800 metres – the size of ten New York City square blocks! Of course, the feasibility of building and deploying such a massive sail is debatable. However, if such a sail could be developed successfully, incredible destinations could be reached.
What is the top speed of a solar sail?
The speed of a solar sail is determined by its size and mass. For the same mass, a larger sail captures more sunlight, gaining more momentum and accelerating faster. A lower mass spacecraft will accelerate faster for a given sail size. The acceleration is also affected by the distance between the object and the light source, as well as the intensity of the light source. The amount of sunlight available to a solar sail spacecraft decreases as it moves away from the Sun, causing it to accelerate less quickly. In theory, powerful lasers could be directed at a distant solar sail, providing additional acceleration as the spacecraft moves away from the Sun.
To give an example of solar sail speed, LightSail 2's 32-square-meter sails accelerate it at 0.058 mm/s2. The spacecraft's speed would increase by 549 kilometres per hour in one month of constant sunlight, roughly the speed of a jet airliner at cruising speed.
How much does solar sailing cost?
Building a solar sail, particularly a large one, is a difficult task that requires further development. And research and development can be costly. However, once solar sails are better tested and understood, they could be a low-cost means of propulsion. Because sunlight is free and limitless, a solar sail-powered spacecraft could travel longer distances without needing to carry more fuel, and it could potentially use smaller, cheaper launch rockets for the same trip. So, solar sailing could be a cost-effective way to travel at high speeds to distant destinations or to provide long-term stability in an otherwise unstable orbit. The Planetary Society's LightSail spacecraft was entirely funded by Society members, private donors, and backers of a Kickstarter crowdfunding campaign. These tens of thousands of people banded together to build and launch solar sailing spacecraft. This is a historic example of what is possible when people come together in support of space exploration.
