AFTER a journey of three-and-a-half years , Japan’s Hayabusa 2 spacecraft is sidling up to its destination, a small asteroid called Ryugu. Its mission: to bring some space dust back to Earth.

The approach is tricky, says Elizabeth Tasker of the Japan Aerospace Exploration Agency. Ryugu is a relatively small asteroid , less than a kilometre across, so it is hard to pin down its exact location at any one …