Comets are dark chunks of dirty ice, gas and rock, and most of them are just a few kilometres in diameter. Though black, the comet is snowy white underneath, hence the term “dirty snowballs”. They comprise a loose aggregate of ice and dust left over from the birth of the solar system.

Comet Ison
Comet ISON shines brightly in this image taken on the morning of 19th November 2013. [Image credit: NASA/MSFC/MEO/Cameron McCarty]
Of almost 7 000 known comets, nearly 900 have been catalogued and their orbits calculated. They typically follow more eccentric and more-inclined orbits than other bodies in the solar system. Almost 300 of them are “periodic comets”; that is, their orbital paths have been proven following multiple returns to the inner solar system. These in turn are divided into short period comets, having orbital periods of less than 20 years; and, long period comets, having orbital periods of more than 200 years.

The Kuiper Belt is analogous to the Asteroid Belt and is composed of ice-rich bodies that never had enough time to form into a larger planet.

Short period comets

Short period comets come from the Kuiper Belt and orbit in the same direction as the planets, and many of them have orbital periods of less than ten years. They have an aphelion no further than Jupiter. Those that stay on this path eventually decay and disappear under the action of solar radiation as they repeatedly orbit past the sun. The giant planets continually pull and nudge at the Kuiper Belt objects, sending new cometary projectiles on their way through the solar system.

The short-period comets are further split into two groups: the Jupiter-family of comets with periods shorter than about 20 years and the Halley-type comets with periods of longer than 20 years but shorter than 200 years. Jupiter-family comets are also known as “ecliptic comets,” and typically have orbits that are modestly inclined to the ecliptic up to about 35°. Halley-type comets can have much higher inclinations, including retrograde orbits (ie they go around the Sun in the opposite direction).

The inclinations of the cometary orbits provide important clues to their origin. The great concentration of Jupiter-family comets orbit close to the ecliptic and can only originate from a “flattened source”, namely the Kuiper Belt, but also from the so-called Scattered Disc. At around 4.8 billion kilometres from the sun these two features merge, the Kuiper Belt extending to a further 14.9 billion kilometres and the Scattered Disc to much further, reaching 150 billion kilometres range, taking it into the Oort Cloud.

Long period comets

The long-period comets have much longer orbits and originate, not from the Kuiper Belt but from the much more remote Oort Cloud. Their orbital planes are inclined at random so they can approach the planetary system from any direction. This was one clue as to the theory that the Oort Cloud was a sphere, not a disc. Long-period comets are also known as “nearly isotropic comets.”

The Oort cloud—still a theoretical feature—surrounds the solar system and stretches out to interstellar distances.

The general explanation for the formation of comets in the Oort cloud is that they are icy planetesimals that escaped from the interplanetary regions during the formation of the giant planets. This might account for about 5% of the ejected comets. It is hypothesised that most of the rest were ejected into interstellar space to assume extreme hyperbolic orbits.

Neptune can gravitationally dislodge comets from the scattered disc region inward to become Jupiter-family comets or outward to join the Oort cloud.

Cometary material

The black materials in comets are silicate rock in the form of small dust particles. Underneath is white snow. When the comet gets closer to the sun—somewhere about the outer edge of the asteroid belt—frozen carbon dioxide and carbon monoxide in the nucleus starts to sublime (turn from a solid directly into a gas). Once inside the orbit of Mars it becomes hot enough for water to join in the activity. The nucleus of the comet then quickly surrounds itself with an expanding spherical cloud of gas and dust called the coma, which can become as large as 100 000 kilometres across, and bright enough to see. As the comet continues through the inner solar system it will lose a layer of about two metres thickness from its surface. Eventually this action will destroy them. In any event, comets are structurally very weak and can shatter at random. Comets continue to whizz through the solar system only because the Kuiper Belt replenishes the stocks.

Nearer the sun the comet will be surrounded by solar radiation that will stretch its coma into a curved dust tail pointing away from the sun. Action of the solar wind also generates a tail, but in this case a straight line made up of ionised gas that similarly points away from the sun. This straight tail can extend many tens of millions of kilometres into space.

The source of the Halley-type comets with their intermediate inclinations and eccentricities is still a matter of debate. Both the scattered disc and the Oort cloud have been suggested as sources. It may be that the explanation lies with a combination of the two cometary reservoirs.

It is thought that the meteor that struck the earth and wiped out 75% of animal and plant species across the planet, including dinosaurs, was in fact a giant comet, about 15 kilometres long.

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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar

Last updated: Wednesday, 1st April 2020