Although NASA hasn't calculated an average, asteroids range in size from Ceres, measuring 590 miles in diameter, to smaller ones that measure less than a half-mile across. The last near-Earth asteroid count was performed in 2013. NASA found 861 asteroids measuring more than 0.6 miles across.
There are more than a half-million known asteroids, most of which are found in the asteroid belt between Mars and Jupiter. NASA has determined that the total mass of asteroids in the asteroid belt is less than the mass of the Earth's Moon. Scientists have also been keeping a close eye on near-Earth asteroids — those that cross the Earth's orbital path around the Sun.