More than 1.3 million chunks of ice and rock have been discovered in the asteroid belt – a wide zone between the orbits of Mars and Jupiter. But asteroids don’t always stay there. They can be kicked out of the belt by collisions, or by the gravity of Jupiter. Some of these outcasts come close to Earth’s orbit, so they’re called near-Earth asteroids.

Astronomers group these objects based on their orbits. The largest group is the Apollos. They account for more than half of all near-Earth asteroids – more than 20,000 have been discovered so far. Their orbits cut across Earth’s orbit.

Apollos are named for the first member of the group ever discovered, which was first seen in 1932. Apollo probably is about a mile in diameter, and it appears to have a moon that’s about the length of a football field.

The largest Apollo is called Sisyphus. It’s perhaps five miles in diameter. That’s about the size of the asteroid that wiped out the dinosaurs when it hit Earth 66 million years ago.

Many other Apollos have also hit us. An asteroid just 60 feet in diameter exploded above Russia in 2013, for example. It shattered windows across a wide area, injuring about 1500 people. And most of the meteorites found on Earth are fragments of Apollo asteroids. So astronomers are keeping their eyes on these potentially dangerous outcasts from the asteroid belt.

Script by Damond Benningfield