Apollo Asteroids

A simple schematic of the inner solar system, the yellow star in the middle is the Sun, the gray circle is Mercury, the grayish-yellow circle is Venus, the blue circle is Earth, the red circle is Mars, and the orange circle is Jupiter. The green band between Mars and Jupiter is the asteroid belt. The brown band covering the area around the Earth is the location of Apollo asteroids.

The third and final class of near-Earth asteroids are the Apollo asteroids. The first asteroid to be determined to be an Apollo asteroid is, surprisingly, 1862 Apollo. Okay, maybe not that surprising. Apollo asteroids are defined by three things:

1. They all have semi-major axes of greater than one AU
2. Their perihelions are all less than 1.017 AU (the distance from the Sun to the Earth at aphelion).
3. They have the potential to collide with the Earth, i.e. they are all Earth-crossing asteroids.

There are only 5766 known Apollo asteroids, though more are being found. Of those 5766, 832 are numbered. Remember that a numbered asteroid has a known orbit and can have its position predicted at different times.

The largest known Apollo asteroid is 1866 Sisyphus which has an average diameter of 8.5 km and a mass of approximately 7.7e18 kg (7.7 followed by 17 zeroes for non-math people), 773,900 times smaller than the Earth. Even though it is much smaller, if it even did hit Earth, everyone on Earth would have a really bad day. Luckily, it has been predicted that its closest approach will be on November 24, 2071 (set your calendars) and will only be 0.11581 AU away from Earth (still farther than the Earth-Moon distance). The Chicxulub asteroid which is believed to have killed off the dinosaurs 65 million years ago was comparable in size.

The Chelyabinsk meteor on February 13, 2013 was also a Apollo asteroid. It was estimated to only be 20 meters in diameter with a mass of 13,000 metric tons (13 million kilograms), again much smaller than 1866 Sisyphus.

Though Aten asteroids have the potential to cross Earth's orbit, Apollo asteroids are the ones with which we should be the concerned. Every single Apollo asteroid has the potential to collide with Earth as all of them cross the Earth's orbit. Apollo asteroids, by far, make up the majority of known near-Earth orbits (which is called an observational bias - comment if you have a question on this). As technology improves, possibly hundreds if not thousands more near-Earth asteroids, and by default, Apollo asteroids, will probably be discovered.