What is #5: An Asteroid?

What is an asteroid? That's the question in the fifth post in a long, on-going series of 'What is' questions in astronomy. An asteroid is a smallish, rocky/metallic object orbiting the Sun in the solar system.

Ceres - NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / Justin Cowart

Above is an image of Ceres, the largest asteroid in the solar system, prior to its designation as a dwarf planet. Ceres has a diameter of 600 miles, but most asteroids are much smaller, typically only a few tens of meters in size. Asteroids are the rocky/metallic material leftover in the solar system that never accumulated as part of a planet or moon.

Asteroids can be found in various places in the solar system. There are many located in the asteroid belt, about 2-3 astronomical units from the Sun. Another set of asteroids, known as the Trojan asteroids, are located in Jupiter's orbit about the Sun. There is a set leading Jupiter in its orbit around the Sun and another set trailing Jupiter in its orbit around the Sun.

The most concerning set of asteroids to Earth are those known as Near Earth Asteroids (NEAs). These are asteroids that have orbits that bring them very close to Earth's orbit, meaning a collision could be possible at some point in the future. As of 2016, there were over 15,000 known NEAs with many more found each week. Don't worry though as none of these are known to have an orbit that will collide with Earth in the next century. Although Earth has been hit in the past and will likely be hit again in the future, the odds of this happening in any given year, or even decade, are very, very, very low. 

Asteroid Ida with its small moon - NASA/JPL Photojournal of Galileo mission

Some asteroids even have moons. Above is asteroid Ida (35 miles in size) with a small moon orbiting it (1 mile in size). Each asteroid is different from all of the others and without them, our solar system would be a much more bland place to live!