Comet deflection mission illustration This comet is an example of a nearEarth object NEO an astronomical object whose orbit intersects with that of the Earth Comets are bodies of ice and dust that enter the inner solar system from the outer solar system As they approach the Sun the heat boils the ice producing a coma around the nucleus along with a tail stretching out behind it Comets passing near the Earth can be a spectacular sight and have been seen as omens of doom They are relatively large bodies and a cometary nucleus can be several kilometres across Large comets impacting the Earth can cause mass loss of lifeDetlev Van RavenswaayScience Photo Library