During the Age of Sail, ships circled the globe on voyages of discovery and trade. That era ended in the 1800s, when coal-fired steam engines began to replace wind power. Now we may be entering a new age of sail—but this time in space. Reversing history, engines and fuel could be replaced by sails on some spacecraft, pushed not by wind but by sunlight.
The idea is still in development, but we know it works. Just a few weeks ago, NASA hoisted sail on a new test craft, a satellite called the Advanced Composite Solar Sail System (ACS3). It has a square sail 9 meters wide that allows it to
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