Comets

1. Overview

Comets are objects composed of ices and dust particles, and are typically a few kilometres across. They orbit the Sun, and are therefore members of our Solar System.

Comets typically move in very elongated orbits, which is why we normally see a comet easily for only part of its orbit − when it is in the inner part of the Solar System, where Earth is located.

When a comet is near the Sun, some of its material is removed by the Sun’s energy, forming a cloud of material called a coma surrounding the small, icy nucleus.

This material can form a tail as it is ‘blown away’ by a stream of particles from the Sun called the solar wind and also by the Sun’s light; the tail of a comet points away from the Sun regardless of the direction of the comet’s motion.

Comets shine both by reflecting sunlight and by emitting their own light after absorbing ultraviolet light from the Sun.

Image: a comet in the night sky. 

The most famous example of a comet is Comet Halley, which was seen in our part of the Solar System in 1986 but is now too far away to be seen, even with powerful telescopes.

Some comets are on orbits small enough that they are seen regularly, as they repeatedly move through the section of their orbit that brings them close to the Sun and Earth.

These are called periodic comets. Comet Halley returns about every 76 years. The comet with the shortest orbital period is Comet Encke, which moves around the Sun every 3.3 years.

At the other end of the scale, there are comets that are on such large orbits that they are effectively seen only once, taking thousands of years to complete each revolution. Fine examples were Comet Hale-Bopp (seen in 1997), and Comet McNaught (seen in 2007).

They are all ‘new’ discoveries, and because of this, we sometimes have very little warning of their appearance in our night sky.

Bright comets since 1990

(All except Comet Halley were previously unrecorded, because of their very long orbital periods.)

2. The origin of comets

Modern opinion is that comets are remnants of the formation of the Solar System billions of years ago, and that they were formed in the cold outer reaches of the planetary system, in the region of the planet Neptune.

The gravity of the outer planets then caused them to be flung out into more distant orbits beyond Neptune. It is thought that, today, there is a huge ‘reservoir’ of distant comets called the Oort Cloud after the Dutch astronomer Jan Oort.

Comet NEAT (C/2001 Q4), named after the Near-Earth Asteroid Tracking program, which was based in Hawaii. The comet was discovered in 2001 and reached its peak brightness during 2004. This photograph was taken on 9 May 2004 from a location in northern Tasmania. Image: Martin George

Many ‘new’ comets never seen before are thought to come directly from this region and to return there — after a brief passage through the inner Solar System, where they swing rapidly around the Sun.

In recent years, astronomers have confirmed the existence of a closer group of icy objects orbiting in the region beyond Neptune. This is called the Kuiper Belt (after astronomer Gerard Kuiper, who proposed its existence).

Astronomers think that the Oort Cloud is the source of comets of long orbital periods, and that short-period comets come from the Kuiper Belt. Comets with orbits entirely within the Sun’s planetary system have had their orbits shaped by passing close to one or more planets.

3. The composition of comets

Research over the years, including the study of comets by spacecraft, has given astronomers much information about the composition of comets.

In broad terms, a comet can be likened to a big dirty snowball. The nucleus, or relatively solid part, of a comet consists of ices of various compounds of carbon, hydrogen, oxygen and nitrogen; water ice (H2O) and carbon dioxide ice (CO2) are major constituents.

In addition to the ice, there is a good deal of dust in the form of grains containing silicon, magnesium and other heavier elements. This dust, mixed in with the ice, makes the nucleus of a comet dark.

The nucleus of Comet Churyumov-Gerasimenko, imaged by the European Space Agency’s Rosetta spacecraft in 2015. Image: European Space Agency

Photographs of the nucleus of Comet Halley, about 15 kilometres across, showed it to be very dark indeed. Comets’ comas and tails contain both gas and dust, and have offered important clues to the composition of the nucleus itself; 80% of the molecules in the coma of Comet Halley are water molecules, while much of the remainder consist of molecules of carbon monoxide and carbon dioxide.

The dust and gas in comets’ tails are blown away from the comet via different mechanisms. The gas is affected by the solar wind − the stream of electrically charged particles moving outwards from the Sun at a speed of hundreds of kilometres per second.

The dust particles, however, are blown away by interacting with the light of the Sun.

This means that a comet will often display separate gas and dust tails. The gas tail is generally straight, but the dust tail is often seen as a curved line of material.

This is because the dust particles move away from the comet more slowly and retain much of their original motion.

Comets and meteors

The debris from comets is responsible for most of the meteors − often called ‘shooting stars’ − that we see in the night sky. There is a good deal of such debris and, in particular, material remaining in the orbits of comets can produce regular ‘meteor showers’ at predictable times as the Earth moves through it. Larger, rocky objects, some of which reach the ground, are more related to asteroids.

4. Discovering comets

Some comet discoveries are made by amateur astronomers with a thorough knowledge of the night sky, using their own telescopes or large binoculars. This is largely because professionals are normally heavily involved in specific projects, while amateurs are free to choose their activities.

For example, Comet Hyakutake, a very bright comet seen in 1996, was first spotted by Japanese amateur astronomer Yuji Hyakutake using very powerful binoculars. However, more and more comets today are discovered using special professional surveys specifically set up to detect both comets and asteroids. An example is the

Lincoln Laboratory Near-Earth Asteroid Research project (LINEAR) which has resulted in the discoveries of many comets. Comets are generally named after their discoverers, although in some cases two observers have found a new comet independently within a short time and the object has been named after them both. There are even some triple-barrelled names!

Today many are named after the search program, such as LINEAR, that was used to discover them. A numbering and lettering system is also used.

5. Observing comets

Comets do not ‘dash’ across the sky. Like the planets, they appear in slightly different positions each night with respect to the background stars.

A comet passing very close to Earth, however, can appear to move considerably from night to night, as did Comet Hyakutake in March 1996.

Most comets are observable only with telescopes; only occasionally does one become visible to the unaided eye. The brightness of a comet can be quite unpredictable, especially if has never been seen before.

When there is a fine ‘naked-eye’ comet, the best general view is usually obtained with binoculars, not a telescope.

A useful type for general astronomy, including comet observation, is a pair marked ‘7x50’, which has lenses 50 mm across and magnifies 7 times.

It is also very important to be well away from city lights, and to stand outside for some time (at least ten minutes, but preferably longer) to allow the eyes time to get used to the darkness.

Dark-adapted country dwellers with binoculars obtained fine views of Comet Halley in 1986, but many city dwellers, ignoring all of this advice, failed even to notice it at all! It is important to note that time-exposure photography shows much more detail.

Old ideas about comets

Long ago, nobody knew the true nature of comets. The Chinese called them, among other things, ‘broom stars’ or ‘bushy stars’. Comets were often seen to be bad omens, and they were wrongly associated with such events as the deaths of important people. The appearance of Comet Halley in 1066 was thought to be a very significant event relating to the Battle of Hastings, and its presence in the sky was depicted on the Bayeux Tapestry.

Future studies of comets

Much more research needs to be done on comets. Because they contain primitive material, more detailed studies may assist in understanding the early history of the Solar System. Earth-based and spacecraft studies will hopefully continue to expand our knowledge of these strange objects.

Already, there have been several spacecraft studies of comets, including Comet Halley (1986) and Comet Churymov-Gerasimenko (2014-16).