Spacecrafts' solar panels can serve double-duty as sails

MANOEUVRING A SATELLITE in orbit usually requires thrusters. Sometimes the thrust is provided by a fuel-burning rocket motor. Sometimes it comes from electrically heated gas. Both methods, though, add weight in the form of propellant, thus reducing launch payload. They also involve parts that may fail. And eventually they run out of juice. Moreover, satellites carrying an energetic fuel like hydrazine must undergo special tests to be certified as safe for inclusion in a launch. Other ways of man...