The Atira class of Earth approaching asteroids are ones whose orbit is entirely within that of the Earth's path about the Sun. Less than 3 dozen of the approximately 24,000 known near Earth asteroids fall into this category of near Sun objects. The largest one, Atira itself, is actually two asteroids orbiting each other once every 15.5 hours. One is about 3 miles in diameter and the other is perhaps 1/6th its size.