The launch of an exploration of Earth’s defences

THE DARTDART, on October 16th, if all goes well, is not the only forthcoming mission with asteroids as its destination. On November 24th should follow. The Double Asteroid Redirection Test, though, has a more practical purpose than . It will assess the feasibility of changing an asteroid’s path, should one be discovered that threatens to collide with Earth., a probe weighing 600kg, is intended to crash, in September 2022, into Dimorphos, a tiny asteroid in orbit around a larger one, Didymos, at a velocity of 6.2km per second. The intention is to alter the speed of Dimorphos’s orbit by about half a millimetre a second, thus shortening its orbital period, now 11.9 hours, by about ten minutes.