Comet Composition

Dirty snowballs, stars with hair, and omens of doom are all ways that comets have been described in the past, yet recent scientific discoveries have sparked debate over comet composition, a subject that many thought well covered in the distant past.

It was Aristotle that gave comets their name when he described them as “stars with hair.” Due to the long tail that streaks behind a comet having the appearance of strands of hair, this thought came to the ancient Greek observer, and thus the root word of comet is kome, which means “hair of the head.” But the tail of the comet is just one part of these heavenly travellers; there is also the coma and the nucleus.

A Look at Comet Composition

The comet is made up of three parts. The nucleus is where the heart of the comet lies, the physical mass that is travelling through space. The first clues to what the nucleus might be made up from came from the other two parts, the coma and the tail. Early observers thought that perhaps the halo (coma) surrounding the comet and the tail were gasses and debris streaming off the nucleus, water vapour and rubble. The water vapour they imagined came from ice on the nucleus burning off as the comet approached the sun. So it was thought that the nucleus of the comet was mostly ice and rock, which lead to their nickname of “dirty snowballs.”

Comet Size

Comets are not very large in comparison to the other objects they share space with. Estimates of most comets are less than 50km across—yet their coma can be larger than the sun itself, and the tail can stretch as far as one astronomical unit or more, roughly the distance from the Earth to the Sun, or 150 million km.

Not many comets can be seen with the naked eye- those that can are called “Great Comets”, such as Comet Hale-Bopp or Haley’s Comet. The distinctive feature of these great comets is often the tail. Not only does the many-stranded tail stretch to incredible distances behind the comet, but it actually splits in two directions. This is because there are actually two different tails, each responding to pressures from the sun in different ways.

The trail of dust particles often follows the trajectory of the comet, and is curved. The trail of gasses coming off the comet, however, behaves unusually. This gas tail always points directly away from the sun, as it is more susceptible to pressure from solar winds.

Debate Over Comet Composition

Thanks to advancing technology, Earth scientists have been able to acquire close up imagery of comets, and even collect material from the tail of some. This has lead to a new debate over comet composition, a subject many thought well explored when it was determined that comets were mainly balls of rubble and ice. New evindence has revealed that many comets are solid, dry, arid surfaces, leaving many astronomers scratching their heads and asking “where is the ice?” Some thing that water ice is still involved, but under cover of the solid rock they see in the imagery.

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