A European spacecraft has begun close inspections of a harmless asteroid rammed by NASA four years ago in a planetary defense test. The European Space Agency said Wednesday that the Hera spacecraft is studying Didymos and its moonlet from just 15 miles to 18 miles away. Launched in 2024, Hera will spend months flying around the pair. NASA’s Dart spacecraft deliberately slammed into little Dimorphos in 2022 to see whether such a move might help save Earth one day from a killer space rock.