There are millions of hefty space rocks in the solar system, mostly concentrated within the main asteroid belt between Mars and Jupiter. But only some ever fly close to Earth, and even fewer pose a risk to our planet.
NASA classifies asteroids that orbit within 30 million miles (50 million kilometers) of Earth as near-Earth objects (NEOs). But within this population of space rocks is a sub-group of particularly worrisome objects that are so large, and orbit so closely to Earth, that they could pose a real threat to our planet if a direct collision were to occur.
