Parts of a Comet

Parts of a Comet

 

The Parts of a Comet

Aristotle compared the physical appearance of what he believed to be rare emissions from Earth to hair flowing from a human head.   He called them kometes, which literally meant “long haired”.  Although Aristotle didn’t understand the true nature of comets, his name obviously stuck.  So what was Aristotle really seeing in the night sky? 

 


Contents:

 

Nucleus of a Comet

Comet Tempel-1 Nucleus

Nucleus of Comet Tempel 1 (9P/Tempel)

Credit: NASA / Deep Impact Spacecraft

The first part of a comet is the nucleus. Comet nuclei were formed in the earliest stages of our solar system billions of years ago.  They are made of rock, iron (troilite), dust and a variety of frozen chemicals including water and gases like carbon dioxide, nitrogen, carbon monoxide, ammonia and methane.  Each comet nucleus, like each planet in our solar system, contains different chemicals in varying amounts.


Astronomers have observed irregularly shaped comet nuclei as small as 328 feet (100 m) and as large as the 37.3 mile (60 km) wide Hale-Bopp Comet. A comet nucleus can remain in a solid, pristine state, unobserved by even the world’s most powerful telescopes until it approaches a Sun.

 

 

 

 

 


 

Coma of a Comet

The cold, solid nucleus warms as it approaches the Sun and the comet begins to transform.  Expelled water, dust, gas and debris form a fuzzy haze around the nucleus called the coma. In January 2009, a U.S. satellite named Explorer observed Comet Lulin’s (C/2007 N3) nucleus expelling 800 gallons (3,000 liters) of water per second.

 

A comet’s coma can span a few miles in diameter to hundreds of thousands. 

 

Our Sun can cause ice trapped beneath the nucleus’ surface to heat up, vaporize and break the nucleus apart entirely.  This is called fragmentation.  Fragmentation can also occur when  the nucleus has a close encounter with a planet, moon or the Sun as was the case, in 1992, when Shoemaker-Levy 9 passed too close to Jupiter and separated into twenty-one pieces.  When fragmentation occurs, a comet can get much brighter from Earth’s perspective as new surface area is exposed to the Sun.

 

 

Dust Tail of a Comet

As the nucleus and coma move closer to our Sun, the comet continues its transformation into one of the most breathtaking objects our solar system. Heavier particles and debris are released by vaporizing ices.  They form another part of the comet called the dust tail.  The dust tail is primarily affected by the comet’s orbital path, which means this tail curves behind the nucleus as it orbits the Sun.  There can even be more than one dust tail. One of the most stunning recent examples of a comet with mulitple dust tail was Comet McNaught (C/2006 P1), which could be seen in the Southern Hemisphere in January of 2007.

 

 

Ion Tail of a Comet

Hale-Bopp Comet
Hale-Bopp Comet on April 4, 1997.

© 1997 by H. Mikuz & B. Kambic.

Some ionized gases are pushed directly away from the Sun by the Sun’s high speed solar wind.  These ionized gases form the comet’s blue, ion tail. Comet Hale-Bopp (image left) is a good example of a comet with a very distinct ion tail and dust tail.


Both the ion and dust tail(s) can be hundreds of thousands or even millions of miles long.

 

Aristotle was right, in one respect.  Some comets look like hair, but, like hairstyles, each is unique.  The tails of a comet can be white, yellowish, blue, straight or curved depending on the comet’s chemical composition, vaporization, number of fragments, orbital path and movement in space relative to the Sun and Earth.


Each time a comet orbits the Sun it sheds material.  Eventually, it will have nothing left to expel and be classified as an asteroid with a nucleus incapable of shedding material to form a coma or tail. That's the principal difference between comets and asteroids. Asteroids do not develop tails since they have no icy materials left to shed.

 

 

Comet Trail (Dust Trail)

Every comet leaves behind a dust trail, consisting of dust and chunks of debris that mark the path the comet travels through the inner solar system.  Earth returns to the same relative position in space once every 365.242 days.  If a comet passed close to this position in space, then our planet passes through the dust and debris left by this comet once every year on the same day or days.  This results in a meteor shower.