The general principle of solar sailing is relatively simple and elegant when compared with traditional methods of propulsion.  While traditional methods depend on a complex engine and the ability to carry extra fuel for the spaceship, the solar sail requires no propellant since it depends only on the suns’ light.  The sail works as follows: as sunlight hits the solar sail, and the photons of light are reflected back, their momentum is exchanged with the solar sail system.  As a result of this, the solar sail must be very large and made from a very thin and light material, with a high reflectivity.  Solar sails typically have very small accelerations, but what is important to remember is that the acceleration is almost constantly applied.  Even a small acceleration of 1 mm/s^2 will build up into large velocities as the seconds turn into minutes, the minutes to hours, and the hours to days.

The true beauty of the solar sail is in terms of its reusability.  Conventional spacecraft are severely limited for interplanetary missions since they must carry their own propellant making return trips to Earth improbable.  The solar sail in theory would be perfect for interplanetary trips as it would not be required to carry any propellant, which leads to the interesting question of how the solar sail is to be steered.  Solar sails can be directed by orienting the surface of the sail with respect to the incoming light, changing the direction of the applied force.

There are three main types of solar sails: square, heliogyro, and disk.  Disk sails spin and are shaped as their name would indicate, as a disk.  While offering the best possible performance of the three, are for all intensive purposes almost impossible to control.  The heliogyro offers the next best performance and can be thought of as slowly rotating helicopter blades.  The heliogyro is also difficult to control, but could possibly be controlled by orienting the individual blades with respect to the incoming light.  The third variety, the square sail is reputed to be the easiest to control, and it is also the most natural intuitively in that it resembles the sails that date our history book.  The largest drawback to the square sail, however, is that it offers the poorest performance of the three.

The solar sail as a relatively new technology does have some problems, most notably including deployment, electrical charging, rips in the solar sail, and structure stability depending on the sail type.  As was mentioned above, a large solar sail has not yet been deployed in outer space, and thus this area could obviously use some work.  Structural stability is also of major concern, since as the weight of the support structure goes up, the acceleration of the sail goes down.  However, if the structure is not stable it can lead to dynamic loads on the sail creating ‘flapping’ of the sail.