We have lost more than 900 near-Earth asteroids. We’d seen each of these potential near-Earth asteroids once, but we didn’t continue tracking them, so we don’t know where they are or if they’re on a crash course with Earth.

Between 2013 and 2016, 17,030 potential near-Earth asteroid (NEA) candidates were added to a list maintained by the International Astronomical Union’s Minor Planet Center. Of those, about 11 per cent were categorised as “initially unconfirmed”. This means that the few observations we had were …