How many NEOs exist?
There are many more small NEOs than large ones. Astronomers estimate that there are approximately 1100 Near Earth Asteroids (NEAs) larger than 1 km in diameter, and more than a million larger than 40 m in diameter (the approximate threshold for penetration through the Earth’s atmosphere). The largest NEAs are less than 25 km in diameter. There are probably many more comets than NEAs, but they spend almost all of their lifetimes at great distances from the Sun and Earth, so that they contribute only about 10% to the census of larger objects that strike the Earth, and probably less than 1% of NEOs less than 1 km in diameter.
