Are comets dangerous to Earth?
Comets can hit Earth, and a large one would be very dangerous, but such impacts are extremely rare. Comets are also harder to warn about than asteroids because many arrive unexpectedly.
Comets, like asteroids, can collide with planets. The impact of Comet Shoemaker–Levy 9 on Jupiter in 1994 showed how powerful such collisions can be.
Risk. Comets make up only a small fraction of the impact risk to Earth, because they are far less common near Earth than asteroids. However, long-period comets arriving from the outer Solar System can travel very fast, often more than 50 kilometres per second, so an impact by a large comet would release enormous energy.
Warning time. Asteroids that could hit Earth can often be found decades in advance. Long-period comets, however, may be discovered only months to a couple of years before passing through the inner Solar System. This gives less time to prepare. Studies of deflection options for comets suggest the response would need to be rapid.
Past impacts? Some scientists have suggested that the Tunguska event of 1908 could have been caused by a comet fragment, though many now favour a stony asteroid. Comet impacts may have occurred in Earth's distant past, and some researchers suggest comets delivered water and organic compounds to the early Earth.
Meteor showers. The dust from comets causes meteor showers, which are harmless.
Monitoring. Sky surveys discover comets as well as asteroids. Missions such as Europe's planned Comet Interceptor will study pristine comets.
Overall, the chance of a large comet striking Earth in any given century is extremely low, but it is one of the scenarios planetary defence experts consider.