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 manoeuvring spacecraft would thus be welcome. And two, in particular, are now being developed.The first takes advantage of errant air molecules that have wandered into space from Earth’s atmosphere. In orbits near to Earth, where these molecules are most abundant, the resistance they provide is such that a satellite with a small forward-facing surface area will slowly gain on another launched at the same speed with a larger such area. For this effect to be useful, engineers have calculated that a satellite needs to be able to enlarge or shrink its forward-facing area on demand by a factor of about nine. If it can do that, then the method of “differential drag” becomes a practical way of manoeuvring satellites relative to one another. And serendipitously, that factor of nine has proved reasonably easy to arrange.