Tailless comets could threaten Earth

mission was an impressive success. On September 26th, using a probe weighing 600kg, it hit Dimorphos, an asteroid 160 metres across that was, at the time, 11m kilometres away from Earth. The purpose was to determine whether asteroids which might otherwise strike Earth could have their paths altered by what is, in effect, a game of cosmic billiards. Though the results are not yet in, it seems likely that they could.But that depends on seeing them early enough. Asteroids have predictable orbits, so cataloguing them is feasible, and is currently being done. This does not generally apply to comets. Though some (like Halley’s) have known orbits, most sweep in from the farthest reaches of the solar system, with little notice of their arrival. And that lack of notice applies, , to a newly discovered class of them that do not flag up their appearance in the conventional way, by developing a tail.