What's the Difference Between a Comet and a Meteor?

Question inspired by this tweet by @VirtualAstro.

The best way to answer this question is to look at two slightly different questions:

What is a comet?

A comet is a relatively* small body (we're talking tens of kilometres, rather than thousands) made largely of ice (including water ice as well as frozen forms of substances such as methane, ammonia, carbon monoxide and carbon dioxide) mixed up with rocky material and a variety of organic compounds.

Comet Hale-Bopp, March 29th, 1997.
By Philipp Salzgeber, via Wikimedia Commons
In the outer solar system, comets are often described as 'dirty snowballs': small, dark frozen lumps of stuff that are difficult to detect due to these characteristics. These rocky bodies are known as the 'nucleus' of a comet. When they're on orbits that draw them towards the inner solar system, however, they're much more impressive: radiation from the Sun causes the volatile chemicals to vaporise, forming an atmosphere (called a 'coma') around the comet nucleus.

As parts of the comet vaporise, some of the solid matter (dust and rocky bits) that makes up the comet is carried away too, and some of this is left behind in the comet's trail. As well as this, the Sun's radiation pressure causes some of the vaporised material to form a tail, which is illuminated by the Sun.

Sometimes, if the solar system lines itself up in the right way, a comet comes close enough to the Earth, and is in the right place, for us to see the comet and its tail in the night sky. I remember, as a fifteen-year-old in 1997, being able to view comet Hale-Bopp with my unaided eye for a period of time measured in months - the planet as a whole was able to view Hale-Bopp without the aid of telescopes or binoculars for a little longer than a year and a half.

Comets, especially ones that we can see in all their glory from our vantage point on Earth, are quite rare.

What is a meteor?

A Perseid meteor (the streak to the right of the middle)
By Mila Zinkova (Own work), via Wikimedia Commons
The International Astronomical Union (IAU) defines a meteoroid as "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom"; basically anything from a dust particle up to a lump of rock about 50 metres in diameter.

If the orbit of a meteoroid coincides with that of the Earth, it will enter the Earth's atmosphere and, due to friction, start to burn up. This leaves a trail across the sky. At this point, the meteoroid becomes a meteor.

Meteoroids collide with the Earth's atmosphere (hence becoming meteors) quite often- meteors of about 20 metres in diameter hit around once per century; those with a diameter up to four metres hit around once per year; meteors in the region of 40 centimetres across collide with us about once per day. Meteors of smaller sizes enter the atmosphere over progressively shorter timescales. A meteor's visible journey can last a fraction of a second, and is easy to miss!

What's the difference?

Comets are bigger and (hopefully**) further away. Their bright tales are caused by volatile gases being vaporised due to the comet getting closer to the Sun. If a comet were to enter our atmosphere it would probably be a bad thing. Comets don't turn up all that often (not ones that we can see with our unaided eyes, anyway). When they do turn up, they can remain visible in the sky for days, weeks, or even months.

Meteors are smaller than comets and aren't classed as meteors until they enter the Earth's atmosphere. Their bright tracks are caused by friction with the atmosphere causing their outer layers to burn up. Meteors enter the Earth's atmosphere all the time, and it's not unusual to see one on a clear night, with low light pollution and a bit of luck. When you see one, it's gone in a flash.

* Relative to stars and planets and things.
** If a comet were to enter our atmosphere that would probably be a bad thing. In 1908, something exploded over Tunguska, Siberia. The explosion demolished an area of woodland that was roughly equivalent to the area covered by London within the M25 today. Many astronomers suspect that this event was in fact a small comet entering the Earth's atmosphere and exploding before it hit the ground.


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