Solar sails may evoke images of large sailing vessels of old, such as clipper ships, or modern America's Cup racing yachts. However, the principles, construction and operation of solar sails are quite different from sailboats.
What is a Solar Sail?
A solar sail is a very large mirror that reflects sunlight. As the photons of sunlight strike the sail and bounce off, they gently push the sail along by transferring momentum to the sail. Because there are so many photons from sunlight, and because they are constantly hitting the sail, there is a constant pressure (force per unit area) exerted on the sail that produces a constant acceleration of the spacecraft. Although the force on a solar-sail spacecraft is less than a conventional chemical rocket, such as the space shuttle, the solar-sail spacecraft constantly accelerates over time and achieves a greater velocity. It's like comparing the effects of a gust of wind versus a steady, gentle breeze on a dandelion seed floating in the air. Although the gust of wind (rocket engine) initially pushes the seed with greater force, it dies quickly and the seed coasts only so far. In contrast, the breeze weakly pushes the seed during a longer period of time, and the seed travels farther. Solar sails enable spacecraft to move within the solar system and between stars without bulky rocket engines and enormous amounts of fuel.
What is a Solar Sail Made of?
For a solar sail to be a practical way of propelling a spacecraft, it must have the following characteristics:
- Large area - It must collect as much sunlight as possible, because the larger the area, the greater the force of sunlight.
- Light weight - The sail must be thin and have a minimal mass, because the more mass, the less acceleration that sunlight imparts to the sail.
- Durable and temperature resistant - It must withstand the temperature changes, charged particles and micrometeoroid hazards of outer space.
To meet these characteristics, most solar sails are made of thin, metal-coated, durable plastics such as Mylar or Kapton. For example, the solar sail of Cosmos-1 is made of aluminum-coated Mylar, has a thickness of 0.0002 inches or 5 microns (ordinary Saran Wrap is about 0.001 inches or 25 microns thick) and an area of 6,415 square feet (600 square meters).
Solar sails come in three major designs:
- Square sail - requires booms to support the sail material
- Heliogyro sail - bladed like a helicopter, the sail must be rotated for stability
- Disc sail - circular sail that must be controlled by moving the center of mass relative to the center of pressure
Cosmos-1 has a solar sail that is a cross between a square sail and a heliogyro sail. It is a rounded solar sail that is divided into eight triangular blades with inflatable booms for support. The sail does not have to be rotated for stability.