The asteroid Eros. Photo by the NEAR Shoemaker spacecraft. Source: NASA/JPL/JHUAPL
What is an asteroid?
An asteroid is a chunk of rock and metal in outer space that is in orbit around the Sun. Asteroids vary in size from just a few feet across to hundreds of miles in diameter.
Most asteroids are not round, but are lumpy and shaped like a potato. As they orbit the Sun, they tumble and spin.
Types of Asteroids
There are three main types of asteroids based on what type of elements make up the asteroid. The main types include carbon, stony, and metallic.
Carbon - Carbon asteroids are also called carbonaceous asteroids. They are made up mostly of rocks rich in the element carbon. They are very dark in color. Around 75% of all asteroids are the carbon type.
Stony - Stony asteroids are also called silicaceous asteroids. They are made up of mostly rock and some metal.
Metallic - Metallic asteroids are made up of mostly metals, primarily iron and nickel. They often have some small amounts of stone mixed in.
Asteroid Belt
The majority of asteroids orbit the Sun in a ring called the asteroid belt. The asteroid belt is located between the planets Mars and Jupiter. You can think of it as a belt between the rocky planets and the gas planets. There are millions and millions of asteroids in the asteroid belt.
Largest Asteroids
Some asteroids are so large that they are considered minor planets. The four largest asteroids are Ceres, Vesta, Pallas, and Hygiea.
Ceres - Ceres is by far the largest asteroid. It is so big that it is categorized as a dwarf planet. Ceres is 597 miles in diameter and contains around one third of the total mass of the asteroid belt. It is named after the Roman goddess of the harvest.
Vesta - Vesta has a diameter of 329 miles and is considered a minor planet. Vesta is more massive than Pallas, but slightly smaller in size. It is the brightest asteroid when viewed from Earth and was named after the Roman goddess of the home.
Pallas - Pallas was the second asteroid to be discovered after Ceres. It is the largest body in the Solar System which is not round. It is named after the Greek goddess Pallas Athena.
Hygiea - Hygiea is the largest of the carbon type asteroids. It is named after the Greek goddess of health. It is roughly 220 miles wide by 310 miles long.
Several asteroids compared by size including Ceres (the largest asteroid) and Vesta Source: NASA, ESA, STScI
Trojan Asteroids
There are other groups of asteroids outside the asteroid belt. One major group is the Trojan asteroids. Trojan asteroids share an orbit with a planet or a moon. However, they don't collide with the planet. The majority of the Trojan asteroids orbit the sun with Jupiter. Some scientists think there may be as many Trojan asteroids as there are asteroids in the belt.
Could an asteroid hit Earth?
Yes, not only could an asteroid hit Earth, but many asteroids have already struck Earth. These asteroids are called Near-Earth asteroids and they have orbits that cause them to pass close to the Earth. It is estimated that an asteroid larger than 10 feet across strikes the Earth around once a year. These asteroids usually explode when they hit the Earth's atmosphere and cause little damage on the Earth's surface.
Interesting Facts about Asteroids
Italian astronomer Giuseppe Piazzi discovered the first asteroid, Ceres, in 1801.
The word asteroid comes from a Greek word meaning "star shaped."
Scientists estimate that there are over one million asteroids larger than 1km in diameter within the asteroid belt.
The five largest asteroids make up more than 50% of the total mass of the asteroid belt.
Some scientists have theorized that the extinction of the dinosaurs was caused by a large asteroid colliding with the Earth.
