This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Short Period Comets and Comets in General

by Dr Jamie Love Creative Commons Licence 1997 - 2011

Most short period comets reside at about 40 AUs from the Sun. [You'll recall that an AU is an Astronomical Unit and one AU is the distance between the Earth and the Sun, so 40 AUs are 40 times the distance between the Earth and the Sun.] That's about the orbit of Pluto or slightly beyond. Because these comets are closer to the Sun their orbital periods are measured in hundreds of years not millions of years.

Short period comets have a different origin and that influences their orbital properties.

Think back to the early formation of the Solar System. The gravitational pull of the large, outer planets caused most of the debris to either be dragged into them or ejected from our neighborhood. Exactly how this ejection occurred is beyond the scope of our lesson (it involves complex orbital mechanics) but you might think of it this way. Imagine a chunk of leftover is pulled towards Saturn by Saturn's strong gravity. That chunk is on a collision course with Saturn but Jupiter just happens to swing by, in its natural orbit. Now Jupiter's gravity gets a hold of that chunk and tugs it away from Saturn. But, instead of pulling the debris into itself, Jupiter's tug is just enough to pull the debris off course and fling it into the outer reaches of the Solar System.
Some of this ejected material was probably hurled completely out of orbit and lost in deep space. But not all of it. Instead, some debris ended up in fairly stable but distant orbits. So the complex patterns of tugging among the four giant outer planets (Jupiter, Saturn, Uranus and Neptune) flung debris into far off orbits.

Note that the accretion plane played a part here. The accretion disk formed the planets and caused their orbital inclinations to be close to the ecliptic. Also, the material I am talking about (here) was once part of the accretion disk. These two factors acting together caused this group of material to take up orbits fairly close to the orbital plane of our Solar System - close to the ecliptic. So, unlike the population of long period comets whose orbits form a kind of sphere around our Solar System (because their orbital inclination can be anything), the short period comets are more likely to be found near the ecliptic.
Also, it is perfectly feasible that the most distant part of the accretion disk simply condensed into small objects that we call comets.
Regardless, these comets are all near the ecliptic, just very far away from the Sun.

A year after Oort published his theory about the origin of long period comets, a Dutch-American astronomer named Gerard Kuiper argued that Oort's Cloud (if it existed) could not be the origin of short period comets - comets that take less than 200 years to complete an orbit. His argument was based upon complex orbital mechanics. Kuiper proposed that another, closer group of comets orbited the Sun at a distance slightly beyond Pluto.
Comets in the Kuiper Belt (as it has come to be known) are ejected from their stable orbits by the gravitational influence of the large outer planets or by collisions with other comets in the belt. Because they start off from orbits closer to the Sun than those from the Oort Cloud, comets from the Kuiper Belt have much shorter orbital periods.

In 1992 David Jewitt and Jane Luu, using sensitive imaging equipment and the largest telescope in the world, detected an object nearly 300 kilometers in size at a distance of 44 AUs. Careful measurements and calculations showed it will take 291 years to complete one orbit. The following year they discovered three more - one about the same size and orbit as the first and the other two smaller (100 kilometers) and with slightly smaller orbits (averaging about 40 AUs from the Sun). Since then dozens of Kuiper Comets have been identified. The Kuiper Belt definitely exists and it is the source of our short period comets.

Comets in the Kuiper Belt may have orbital periods of more than 200 years but when they are ejected from the belt they take on periods of less than 200 years and thus are considered short period comets.

The shortest period comet I know of is Encke's Comet with an orbital period of only 3.3 years. At perihelion it passes within the orbit of Mercury and at aphelion it does not go as far as Jupiter's orbit. With a good telescope this comet can be followed throughout its entire orbit (except for when the Sun gets in the way). This is a fine, if somewhat extreme, example of a short period comet. It once "lived" in the Kupier Belt where it orbited the Sun, taking a couple centuries to complete its orbit (which had an inclination close to the ecliptic, like all Kupier objects). One day, long ago, something perturbed it from its stable almost circular orbit and it headed towards the inner Solar System. The laws of gravity (actually Kepler's third law of orbital motion) dictate that an object moving closer to the Sun will complete its orbital period more quickly, so now we have a comet in a highly eccentric orbit that it completes in 3.3 years.

So, short period comets come from the Kuiper Belt and long period comets from the Oort Cloud.

Right, and those "homes" are fairly stable, nearly circular orbits. But when a comet is perturbed and it falls towards the Sun its orbit becomes much more eccentric and shorter. When at "home" short period comets (in the Kuiper Belt) have shorter orbits than long period comets (in the Oort Cloud). When either of these types of comets are perturbed into eccentric orbits, their orbital periods are made much shorter. However, these perturbed short period comets will still have shorter periods than the perturbed long period comets! [Phew!]
You should know, however, that it's possible for a long period comet to be influenced by other bodies (such as Jupiter) and thus tugged into a much shorter period.

But wouldn't such a "short period comet" still have an orbit at an extreme inclination from the ecliptic? (Assuming it had an extreme inclination while it was in the Oort Cloud.) That would give away its true home.

By Jove, that's right! Any short period comet with an orbit that is highly inclined to the ecliptic probably originated in the Oort Cloud not the Kuiper Belt. However, a short period comet whose orbit is near the ecliptic could be from the Kuiper Belt or it could simply be an Oort Cloud comet whose inclination just happened to be near the ecliptic (by chance) and whose orbit has been "tamed" (made smaller) by the gravitational pull of the planets (especially Jupiter).

These scenarios can get complicated and are best left to the professionals to sort out, using arguments based on detailed analysis of each comet, but I hope you will agree that there is some order to comet orbits.

What exactly is a comet made of? And why is it shaped the way it is?

Comets represent the early nebula from which the Sun and planets were formed. Just as biologists learn about our origins from fossils, astronomers learn about the origins of our Solar System from studying the composition of comets.

Basically a comet is a "dirty snowball" - frozen water and gases along with chunks of minerals. The exact chemistry of comets is due to the chemical reactions that occurred long ago as the nebula condensed and "aged". Water and simple gasses (like methane, carbon dioxide and ammonia) were formed by slow chemical reactions forming covalent (shared electron) bonds. Minerals containing iron, nickel, and other metals were also created by slow chemical reactions occurring deep in space. As long as a comet remains in its distant, stable orbit (in the Oort Cloud or Kuiper Belt) it remains frozen. But when it is ejected into a highly eccentric orbit that brings it close to the Sun, it has a chance to melt and become the comet with which we are familiar.
As a matter of fact, some folks don't consider these objects to be "comets" until they start to melt. It's only a different way to look at it. They argue that a comet must have the structures I am about to describe. I belong to the "camp" of astronomy teachers who think it is better to call these objects "comets" even before they melt. I'm not trying to confuse you but to warn you that other teachers might say something like, "the comet forms when the ice starts to melt". Frankly, I like my way of looking at comets (big surprise, huh? ) and I hope you do too.

The frozen portion of a comet is called the nucleus and it's the only part of a comet that exists when it is far off and away from the Sun. The objects discovered by Jewitt and Luu are the nuclei (the plural of nucleus) of particularly large comets. Most comets have much smaller nuclei but I am sure you can appreciate the fact that Jewitt and Luu (and others using similar methods) are more likely to find the biggest comets first. Most comet nuclei are less than 100 kilometers in diameter.

A comet ejected into a highly eccentric orbit starts to melt as it approaches perihelion. As it is warmed by the Sun, water and gases are released from weakest areas in the crust of the nucleus. This material, often with bits of dust or rocks mixed in with the gas and water, rockets away from the nucleus in the form of tiny geysers. These tiny geysers are called jets. The closer the comet comes to the Sun the more jets are produced and the more water, gas and dust are released from the nucleus.

The jets are usually not powerful enough to hurl the material away from the gravitational pull of the nucleus so the material tags along forming a shell called a coma. Generally speaking, the larger a comet's nucleus and the closer it gets to the Sun, the larger will be its coma.
As the comet moves closer to the Sun, roughly within the orbit of Mars, the Sun's powerful rays and solar wind push the coma away from the nucleus producing a comet's tail.

A comet's tail always points away from the Sun because the Sun is the source of the solar wind that pushes on the coma (causing it to streak back away from the Sun). Sometimes a comet will have two tails!

How?

It has to do with the way the coma materials interact with the solar wind. This wind is produced by atoms (mostly ions, actually) constantly hurled away from the Sun as a natural part of its activity. (You learned about the solar wind back in May when I taught you about the aurora.) The Sun's rays and solar wind cause some of the gases in the coma to ionize and these are pushed away as a glowing tail of emitted light. This tail is white but often has a bluish tinge to it and it's easily pushed back by the solar wind because it's made of nothing but gas. Therefore, a comet's gas-ion tail will point directly away from the Sun and has a rather ragged appearance. Both its shape and direction are due to the fact that the gas-ion tail is dominated by the solar wind.
The dust tail, on the other hand, is made of heavier materials - mainly dust and rocks of silicates. This material reflects the sunlight so we see a dust tail as a white or yellow-white glow. Because the dust tail is made of materials with a denser composition than the gas-ion tail, it is not swept into a straight line behind the comet. Instead, the dust tail has a gentle curve to it because the dust tail is produced by the chunks that are following behind the comet along its orbit. The solar wind isn't powerful enough to push the dust right back (like it can to the gas materials) but the wind eventually does influence the dust and ends up pushing the dust tail into a feathery curve.
Not all comets display two tails and most only show the gas tail - like the image of Comet Hyakutake that I showed you earlier.

Each perihelion of a comet's orbit causes it to lose some materials so each orbit causes the comet's nucleus to become smaller and smaller. Long period comets (like Hyakutake) are usually very bright and produce very long tails because they still have a large nucleus. On the other hand, a short period comet has been through so many perihelions that its nucleus has been reduced so it produces a less spectacular comet. To put it another way, long period comets age more slowly and are usually more dazzling than short period comets. However, we can only predict the arrival of the short period comets because the long period comets have such long orbital periods that we never know when the next one will come along!

The most famous short period comet is Halley's Comet. Interestingly, Edmund Halley didn't really discover this comet - it was named after him. This comet had been known for a long time. As a matter of fact, the ancient Chinese recorded Halley's Comet (although they didn't know what it was) as early as 1059 BC. Halley observed this comet in 1682 and after calculating its orbit he discovered that it was very similar to comets reported in 1607 and 1531. He figured they were observations of the same comet made at intervals of 76 years so he predicted that the same comet would return in 1758. Most astronomers thought Halley was nuts because they believed that comets traveled in straight lines and simply passed through the Solar System only once. Halley died long before the return of the comet but in December 1758 a German amateur astronomer named Palitzsch was the first to see the return of this comet. It reached perihelion in March of 1759 and changed the way we think about comets!

Halley's discovery, that some comets are periodic and therefore return regularly, set off a series of observations and predictions about comets. Indeed, most of the "historic" comets reported in the past have now been properly identified by simply doing as Halley did - finding records of comets with similar orbital properties and discovering which ones fit a regular pattern. This isn't always as obvious as you might think because not all approaches are observed (because of the position of the Sun, etc.) or recorded.
For example, J F Encke decided that the comets of 1786, 1795, 1805 and 1818 were the same comet but if you look at those years you will see that there isn't a clear, regular pattern. Encke calculated the orbit of these comets and decided that one comet, with a period of 3.3 years, could explain each of these observations. He went on to predict that the comet would return in 1822. The comet did return and in the predicted position too so, like Halley, this comet was named after Encke.

Halley's comet has an orbital period of 76 years and was last at perihelion in 1986.

On the night of March 13-14 (1986) the European space probe Giotto flew though Halley's coma and produced this (color enhanced) picture of the nucleus. The dark part is the potato-shaped nucleus estimated to be about 15 kilometers across. One particularly powerful jet on the left and several smaller ones nearby are producing the coma material that makes up most of the orange colors. Space scientists estimate that about 250 million tons of material are lost from Halley during each perihelion and that there is enough material in the nucleus to last for at least 150,000 years.

We know that Halley will return again and again every 76 years for a long time. Like clockwork. But maybe not. Comets are often tugged around by the gravity of the giant planets and it is possible that one day Halley's Comet may be dragged into a planet or flung completely out of the Solar System.

Even short period comets can be unpredictable and that is why they are so fascinating.

For example, in 1886 Comet Brooks 2 came so close to Jupiter (within the orbit of Io) that several small pieces were torn away from the main nucleus and its orbit was altered from a period of 29 years to its current period of 7 years. Lexell's Comet of 1770 came very close to Jupiter and its orbit was so completely altered that we now have no idea where it went! The small size of comets, compared to planets like Jupiter, means that they can get pushed around. Of course, we can use computers to predict some of these events but small changes or errors in calculations mean that it is hard to make accurate predictions far into the future. [This has to do with an area of mathematics called "chaos theory".] And, as you may know, comets can even crash into a planet - but they are more likely to crash into the Sun.

Another thing that makes short period comets unpredictable is their composition. A comet is not a uniform ball of "ice and dirt". Instead, it is a complex mix with a unique structure that we cannot see clearly. This structure affects the way the coma is produced and the size of its tail. For example, it is a fair guess that a comet will be at its brightest (absolute magnitude) when it is closest to the Sun because that's when we expect the comet to be heated to its maximum. But sometimes a particularly strong jet will release a huge amount of materials when the comet is still pretty far away from the Sun. This increases the comet's magnitude and we see the comet has experienced an unexpected "flare up". Conversely, a comet with a good reputation for being very bright and beautiful could be a real dud at its next perihelion because the nucleus just happened to have used up its best jets in its previous perihelion. I saw Halley's comet in 1986 and was very disappointed by it! (But I'm still glad I saw it! )

We don't know when Halley's Comet first left the Kuiper Belt but we know that with each return it will become smaller and smaller, eventually disappearing entirely. The gas and water will be blown away with the tails but most of the dust and rocks will remain. With each orbital passage these cometary materials will be spread out along the comet's path until eventually they will form a ring of dust and rock tracing out the comet's entire eccentric orbit. (More on that later.)

Are there any comets to see this month?

Yes, and no.

With a very powerful telescope you would be able to find comets in the Kuiper Belt, but they would be very dull because they are so far away that they have no coma or tail(s). You would have to be a professional astronomer with professional equipment to find them. Closer to home are comets like Encke whose orbits are so small that they can be followed throughout their path using a good telescope. But I don't think that's what you have in mind either.

You can always hope that another comet like Comet Hyakutake is on its way. That's the exciting thing about comets. You never know exactly how bright they might be or when the best ones (the long period ones) will come along. Because comets can be strongly influenced by their composition (which affects their magnitude and the size of the coma and tails) and by other planets (like Jupiter) it is important to appreciate that you have to stay informed to really get the most out of comets.

Indeed, the most spectacular comet event of the 20th century seemed to have come out of nowhere!

In March of 1993 the comet-hunting team of Eugene Shoemaker, Carolyn Shoemaker and David Levy co-discovered their 9th comet so it was named Comet Shoemaker-Levy 9. Calculations showed that this comet had been orbiting Jupiter (yes, Jupiter) for about 20 years but on July 2nd 1992 it had skimmed so close to the cloud tops of Jupiter (at its "perijove") that its nucleus had been torn apart and its orbit drastically altered. The twenty fragments (lettered A to W) were put on a collision course for Jupiter! In mid-July of 1994 the world watched, mostly through Hubble images, as these fragments slammed into Jupiter. [Actually, these collisions occurred on the dark edge of Jupiter so we could not see the impacts directly.] Each fragment was only a few kilometers in size but they were traveling at tremendous speeds and the "scars" they left in the Jovain atmosphere lasted for months.

What would happen if a comet hit the Earth?

The next lesson about meteors and asteroids will tell you.




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.