Want to stay on top of all the space news? Follow @universetoday on TwitterAsteroid orbits are often analogous. A bunch of asteroids will have similar orbits and fall into a certain group. There are several groups. There is the main belt, the Trojans, the Apollos, the Atens, and the Amors. There is also the Kuiper belt to be considered. What this article will do is give you a brief overview of the orbits of each group so that you have an idea where each is located in space and which groups pose a threat of impact to the Earth.
The main belt is between Jupiter and Mars. The asteroid orbits there continue to be appreciably perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, a Kirkwood gap occurs as they are swept into other orbits. While the orbits stay between Mars and Jupiter, they can be effectively changed by the gravity of Jupiter, causing them to impact each other on a fairly regular basis.
The Trojans orbit sixty degrees ahead and behind a planet in two of the Lagrangian points that exist between two massive bodies. Jupiter has many and Mars has one known Trojan asteroid. Neptune is now known to have four Trojan asteroids. Trojan asteroid orbits always are on the L4 or L5 Lagrangian points of a planet. Those on the L4 side are known as Greeks and the Trojans are on the L5 point. Each individual asteroid is named after a hero of the Greek and Trojan war.
The Apollos, Atens, and Amors are all near Earth asteroids. They are Earth crosser asteroids, meaning that the asteroid orbits cross that of the Earth. The near Earth asteroids orbit all have an orbital semi-major axes greater than that of the Earth (> 1 AU) and a perihelion distance (q) < 1.017 AU and a asteroids have their aphelion greater than one AU. Their proximity to Earth and their susceptibility to gravitational keyholes make them a small threat for Earth impact. A very small threat. The are observed from time to time by astronomers, but not generally worried about.
The Kuiper belt is beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20–200 times as massive. Most of the asteroid orbits are consistent, but occasionally impact one another and can come shooting into the inner solar system as a comet.
Each set of asteroid orbits are very similar, although there are occasional exceptions where an asteroid is collided with an changes its orbit or comes completely free. If an asteroid slips too close to the gravity of a planet it will change its orbit, also. In general, asteroid orbits are stable and predictable. That is how astronomers have learned so much about them over the years.