These are the ones that worry astronomers because they could collide with our planet. The chances of one of these hitting Earth in the next century are about one in 10. Fortunately, it now seems that most will buzz Earth several times before impact and, in these circumstances, even the small ones should be easy to spot. Although comets are sometimes described as giant dirty snowballs they can pack a punch if they hit Earth. They usually come from the outer reaches of the solar system.
Hazardous comets are more difficult to detect than asteroids because of their speed, and distance.