Comets, meteors and asteroids

What are comets, meteors, shooting stars, meteorites and asteroids made of? And what is the difference between them all? 

Comets, meteors, meteorites and asteroids include some of the smallest interplanetary objects astronomy is concerned with but they are among the most spectacular and the only ones we are likely to come into contact with before space travel becomes common!

Whether you are wishing on a ‘shooting star’ or wondering about the likelihood of an asteroid ending life on earth, keep your eyes on the skies for these objects.

What’s a comet?

Generally speaking, a comet is a frozen ball (of water, carbon dioxide, ammonia and other organic carbon compound ices) hurtling through space. As these substances stream off the comet they form a spectacular gas and dust cloud of enormous length that can often be seen from earth with the naked eye.

Meteors and meteorites

A meteor is a piece of space debris that burns up as it enters the earth’s atmosphere creating a ‘shooting star’. Certain times of the year are known for spectacular displays and these are generally associated with comets that have passed by after spewing fragments in their wake.

Occasionally, a larger fragment will fail to burn up completely and when it hits the ground it is called a meteorite.

Eta Aquariid Meteor Shower in Mount Bromo by Justin Ng
Eta Aquariid Meteor Shower in Mount Bromo by Justin Ng

Asteroids and minor planets

Asteroids range greatly in size with some approaching the size of small planets. The largest asteroid is Ceres, it is 1003 km in diameter. Pallas and Vesta (the only asteroid at all visible to the naked eye) have diameters of about 500 km and 30 more asteroids have diameters greater than 200 km. Most asteroids, however, are small objects only a few kilometres across.

The compositions of asteroids are very similar to those of meteorites and this has led to the idea that meteorites originated in the asteroid belt.

The comet 46P/Wirtanen was visible in our skies over December 2018, so we ran a live broadcast from the Annie Maunder Astrographic Telescope at the Royal Observatory.

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