10 Facts About Asteroids and Comets

Asteroids and comets are both celestial bodies that orbit our Sun. They may have irregular orbits and are made of materials which were left over from the formation of our Solar System some 4.5 billion years ago. The main difference between the two is what they are composed of. Asteroids are mostly metal and rock, while comets are made from ice, dust, rock and organic compounds.

Here are 10 more interesting facts about asteroids and comets.

1. The only asteroid that is normally visible to the naked eye is Vesta

Surface of Vesta.

This is the second most massive asteroid and was visited in 2011 by the spacecraft ‘Dawn’.

2. Trojan asteroids lie in the same orbit as a planet or moon

Trojan Asteroids and the asteroid belt.

They are found either 60o ahead or 60o behind the main orbiting body. Earth has only one known trojan.

3. An asteroid impact was responsible for the mass extinction of life at the end of the Cretaceous era

Chicxulub Crater, Yucatan peninsula.

An asteroid of about 10km diameter (6 miles) impacted the Earth at the tip of what is now the Yucatan peninsula.

4. The first asteroid to be discovered was Ceres in 1801

Structure of asteroid Ceres.

We now know Ceres to be a ‘differentiated’ asteroid — it has a crust, a water ice mantle and a rocky core.

5. Many of the meteorites that have landed on Earth have come from asteroids

Meteorite that has come from the asteroid Vesta.

These fragments of asteroids have been knocked out of the asteroid belt between Mars and Jupiter by collisions or by the gravitational forces of the Sun and Jupiter.

6. Halley’s Comet was the first comet to be visited by a spacecraft (Giotto)

1986 apparition of Hallet's comet.

It orbits the Sun every 75 – 76 years so for most people, it is a once in a lifetime event. It was named by French astronomer Nicolas de Lacaille in honour of the English Astronomer Royal, Edmund Halley, who correctly predicted its return.

7. In the past, comets were thought to signify that a ruler or other royalty was going to die

Halley's comet on the Bayeux tapestry.

In Shakespeare’s play Julius Caesar, his wife Calpurnia says “When beggars die there are no comets seen; The heavens themselves blaze forth the death of princes.”

8. The brightest comet of the 20th century was Comet Ikeya-Seki

Brightest comet of 20th century.

This was so bright that it could be seen easily in daylight. It is one of a group of comets known as the ‘Kreutz Sungrazers’.

9. The solid nucleus of a comet consists of ice and dust and is often coated with dark organic material

Comet nucleus 67P.

American astronomer Fred Whipple came up with the description of the ‘dirty snowball’ during the early part of the 1950s.

10. The tail of a comet always points away from the Sun.


The tail is created from dust and gases that are ejected from the comet as it is heated by the sun. The radiation and solar wind mean that the tail is always on the opposite side of the comet to the Sun.

Taught science for 16 years at a secondary school in the East Midlands.