What are Asteroids, Meteors and Comets?

Asteroids, Meteors and Comets are visitors from the far reaches of space. They are rock and ice from deep space but what is the difference?

Asteroids, Meteors and Comets are commonly grouped together as they are basically the same thing - rock and ice from deep space. Despite being grouped they are very different and are easily distinguishable from each other.

What are Asteroids?

Asteroids are rocky objects which range from small rocks 10 metres in size up to Ceres which is 933km (580 miles) across. Most asteroids orbit the Sun between Mars and Jupiter in a region called the Asteroid Belt. There are a few asteroids called the Greeks and Trojans which share the same orbit as Jupiter, moving slightly ahead and slightly behind the great planet in what is known as Lagrange points. They orbit at the same speed so they are never caught by Jupiter's gravity.

There is also an asteroid belt closer to Earth, a group called Near-Earth Asteroids. They are much closer to the Earth and occasionally may cross our orbit.

There are different types of asteroids. Pictured below is asteroid Itokawa which is believed to be a 'rubble pile' formed from fragments that have cohered over time. Other asteroids such as Gaspra are monolithic (one massive chunk of rock). One asteroid called Ida even has a smaller satellite orbiting it (Dactyl)

What are Meteors, Meteoroids and Meteorites?

Meteors are probably better known as "shooting stars". They are small pieces of space debris and dust or fragments from comets. They range in size from a grain of sand to about the size of a cricket ball. When they hit the Earth's atmosphere they heat up and glow leaving a streak across the sky. Some of the larger meteors explode or burst into flames and become a fireball as they travel through the atmosphere.

A meteor shower occurs when the Earth passes through the trail of a comet which left a lot of dust behind. These showers can be predicted and come every year or so as the Earth passes once more through the trail. Meteor showers are named after the constellation that the meteors appear to be coming from (radiate), for example, Orionids from Orion, Leonids from Leo and so on.

A meteoroid is the name given to the space dust and small objects as they exist in space. Once they enter the Earth's atmosphere they become meteors. If the meteor is sufficiently large or made of strong material, it may impact the surface and is called a meteorite.

Scientists estimate that 1,000 tonnes to more than 10,000 tonnes of meteoritic material fall on the Earth each day. However, most of this material is very tiny - in the form of micrometeoroids or dust-like grains a few micrometres in size.

What are Comets?

Comets are the "dirty snowballs" of the solar system. They consist of ice and rock in a tightly packed nucleus ranging from 1km to 20km. They travel on an elliptical orbit around the Sun, which takes them out as far as the Kuiper Belt or, for a small minority, the Oort Cloud.

As the comet moves closer to the Sun Solar energy causes the nucleus to heat up, and the ice vaporises, taking dust and small pieces of debris with it. As the comet is moving through space, this dusty vapour forms the characteristic tail. The name "comet" comes from the Greek meaning "long-haired" because of their tails. When the Earth passes through the debris left behind the comet we see a meteor shower.