On the night of 18 February, engineers at the Space Telescope Science Institute pointed the James Webb Space Telescope at an almost empty patch of sky and waited. They were looking for a rock roughly 60 metres across, hurtling through the void many millions of kilometres from Earth. At that distance, in Webb’s narrow field of view, it would appear as something barely brighter than background noise. Finding it at all would require the kind of precision that makes astronomers quietly...