In order for sunlight to provide sufficient pressure to propel a spacecraft forward, a
solar sail must capture as much Sunlight as possible. This means that the surface of the sail must be big – very big.
Cosmos 1 is a small solar sail intended only for a short mission. Nevertheless, once it spreads its sails even this small
spacecraft will be 10 stories tall, as high as the rocket that will launch it. Its eight triangular blades are 15 meters (49
feet) in length, and have a total surface area of 600 square meters (6500 square feet). This is about one and a half times
the size of a basketball court.

For a true exploration mission the requirements are far greater: when a NASA team in the
1970s, headed by Louis Friedman, suggested using a solar sail spacecraft for a rendezvous with Halley’s comet, they
proposed a sail with a surface area of 600,000 square meters (6.5 million square feet). This is equivalent to a square of
800 meters (half-mile) by 800 meter – the size of 10 square blocks in New York City!

Even with such a gigantic surface, a solar sail spacecraft will accelerate very slowly
when compared to a conventional rocket. Under optimal conditions, a solar sail on an interplanetary mission would gain only
1 millimeter per second in speed every second it is pushed along by Solar radiation. The Mars Exploration Rovers, by comparison,
accelerated by as much as 59 meters (192 feet) per second every second during their launch by conventional Delta II rockets.
This acceleration is 59,000 times greater than that of a solar sail!

But the incomparable advantage of a solar sail is that it accelerates CONSTANTLY. A rocket
only burns for a few minutes, before releasing its payload and letting it cruise at a constant speed the rest of the way.
A solar sail, in contrast, keeps on accelerating, and can ultimately reach speeds much greater than those of a rocket-launched
craft. At an acceleration rate of 1 millimeter per second per second (20 times greater than the expected acceleration for
Cosmos 1), a solar sail would increase its speed by approximately 310 kilometers per hour (195 mph) after one day, moving
7500 kilometers (4700 miles) in the process. After 12 days it will have increased its speed 3700 kilometers per hour (2300
mph).

While these speeds and distances are already substantial for interplanetary travel, they
are insignificant when compared to the requirements of a journey to the stars. Given time, however, with small but constant
acceleration, a solar sail spacecraft can reach any desired speed. If the acceleration diminshes due to an increasing distance
from the Sun, some scientists have proposed pointing powerful laser beams at the spacecraft to propel it forward. Although
such a strategy is not practicable with current technology and resources, solar sailing is nevertheless the only known technology
that could someday be used for interstellar travel. http://www.planetary.org/solarsail/whatis.html