Unfortunately, the limits of modern engineering and materials science will limit the use of solar sails (see Chapter 13) to small robotic spacecraft and cargo until some sort of breakthrough occurs, or until we learn how to construct ultra-thin, large sails in space from in situ resources. Recall that solar sails derive their propulsion by reflecting sunlight. Lots of sunlight, falling on a very large and ultra-lightweight structure is required to achieve even the most modest of robotic science missions. (A 150-meter square solar sail with an areal density of less than 15 grams per square meter is required to propel a 400-kilogram spacecraft!) Typical human-support sized craft will weigh 40,000 kilograms or more. Clearly, much larger and lighter weight sailcraft will be needed to accelerate these ships. (Don't forget, Newton's law clearly applies—for a given force, the higher the mass, the lower the acceleration. As the spacecraft mass goes up, the acceleration provided by the propulsion system, in this case a solar sail, goes down.)
Robert Frisbee of NASA's Jet Propulsion Laboratory has looked at the problem and estimated the sizes of sails needed to support human exploration missions to Mars. A Martian solar-sail cargo vehicle capable of sending metric tons of supplies to astronauts exploring the planet Mars would be 2 kilometers on a side (assuming a square sail configuration) and require approximately 4 to 5 years to make the one-way journey. This assumes a sail using first-generation technologies to achieve an areal density of approximately 13 grams per square meter. While the overall mass density of the sail is comparable to those that will be fielded in the next several years, the overall size of the sailcraft (hence the issues that will be encountered in simply manufacturing the sail itself) places this application at least 15—20 years into the future.
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