Many people confuse comets with meteors. When you see a quick streak of light in the sky what you are seeing is a meteor, not a comet. A typical comet hangs for weeks or months in the sky, night after night, moving slowly among the stars. But the distinction isn't quite that simple. It turns out that most meteors (commonly called shooting stars) are caused by the entry of tiny specks of dust into the earth's upper atmosphere. They move very fast and the air friction burns them up before they hit then ground. But where do the specks of dust come from? Comets.
But let's start from the beginning, shall we? Our solar system consists of 9 planets which orbit our star, the sun. In terms of mass and size, the sun is really what the solar system is all about. The planets are like insignificant afterthoughts. Along with the planets are thousands of small pieces of rock which also orbit the sun. By small I mean from the size of a house to say, the state of New Jersey. These are the asteroids (or minor planets).
One curious thing about the planets and asteroids is that they all orbit around the sun in the same direction and more or less in the same plane. The reason for this is that the planets and asteroids formed out of a flat, rotating disk of hot gases that once orbited the sun. The gases precipitated to form tiny grains and those grains came together to form big chunks of rock. Most of the big chunks of rock smashed together to eventually form the planets. In the end the gas in the disk was either made into rocks or swept away when the sun "turned on" and began to shine like we see it today. So in the place of this once mighty disk of gas we find a few isolated planets and some left over chunks of rock, the asteroids.
But that's only half of the story, and this is where comets come in. The hot disk of gas that once orbited the sun at the time it was forming was hotter near the center--near the sun--than it was near the outer edges. This affected the kinds of grains that precipitated out of the gas. In close to the sun it was too hot to form grains of water ice. It was only much farther out that these grains were able to form. When the ice grains came together, much the like rock grains, they built larger icy bodies rather than rocky ones--like great orbiting icebergs. These icebergs also smashed together to form planets, but only in the outer parts of the disk.
This is why the solar systems breaks down into two basic types of planets; the four inner rocky ones, and the four outer "gas giants." In the outer regions there was more material to make planets from because of all the icebergs floating around, so the planets got much larger. Not only that, but the larger planets have stronger gravitational pulls, so they were able to hold onto the lighter gases from the original disk. The result was really large planets that are mostly made of gas such as Jupiter.
Just like there are left over chunks of rock from the formation of the solar system there are also left over icebergs. The icebergs are the comets. Or more accurately, the iceberg is the nucleus of the comet.
It is likely that there came a time just after the planets formed when there were still thousands of comets running around the solar system. Even though the comets were all out where it's cold, the gravity of the newly formed gas giants, particularly the massive Jupiter, probably sent large numbers of comets flying into the inner solar system where the earth is. It is likely that many of these comets hit the earth at that time, and may be the source of at least some of our water. The farthest of these icebergs, which were formed far beyond the farthest planet, are still there in a very large disk of comets, called the Kuiper Belt.
Earlier I said that there were nine planets in the solar system. But that may be a matter of semantics, as one planet, Pluto, doesn't really seem to fit in with the rest. Pluto, accompanied by its three little moons, the largest is which is Charon, is the farthest planet from the sun, and has the most inclined and elongated orbit. As a physical body it looks more like one of the icy moons of the gas giants than a planet. So what is Pluto? It is probably more like a giant comet--a leftover from the formation of the giant planets, which found it's way into a stable orbit such that it remains with us today.
The Oort Cloud
But there's more! In the very beginning of the solar system, long before the disk of gas that orbited the sun, the sun itself was forming out of a giant cloud of gas. The cloud of gas was once much larger than the solar system we know today and roughly ball shaped rather than a disk. Apparently, as it collapsed inward ice grains precipitated out and these ice grains came together to form comets. This happened far away from then sun which was forming at the center.
Today, we're reasonably certain that these original, first generation, comets still orbit far away from the sun, and unlike the planets, they orbit in any direction and are found all around the sun. This "cloud" of far away comets that orbit the sun is called the Oort Cloud after the astronomer who first suggested it.
No telescope has ever detected a comet in the Oort cloud. They are just too small and too far away. So how do we know it's there? We'll get to that in a minute.
Throughout recorded history there are accounts of great comets in the sky. They appear in the sky for weeks or months, getting larger and brighter, and then slowly fade away, often just after they pass close to the sun. What we are seeing is one of those great icebergs coming in from cold storage, far away from the heat of the sun. Out there the iceberg is merely a big chunk of ice. But as it moves closer to the sun the ice begins to melt from the heat. Well, technically, in space ice can't melt. To melt is to pass from solid form into a liquid, and liquids can't exist in the vacuum of space. So when the ice "melts" it skips the liquid form and goes right to a gas--water vapor; steam. So the heat of the sun makes comets steam. The steam forms a really big steam cloud around the iceberg. This steam cloud is called the coma of the comet. In addition to water vapor, carbon dioxide is also released.
Left alone the steam (and carbon dioxide) cloud would simply hang there about the iceberg, orbiting right along with it. But the sun does more than just melt the iceberg. The sunlight breaks the water vapor up into it's parts. We all know that water is H2O. That means it is made of hydrogen (H) and oxygen (O). The sunlight separates the water vapor into hydrogen and oxygen, and the oxygen ends up with an electric charge.
But that's not all the sun does to the steam cloud. The sun has a wind. The solar wind is a stream of charged particles that flow outward from the sun in all directions. The charged particles flowing by the steam cloud carry away the charged oxygen and carbon dioxide, making a long streaming tail. Since the solar wind always blows away from the sun, the tail always points away from the sun, regardless of the direction that the comet is moving.
But there's more! The iceberg isn't clean. It's actually rather dirty, like a snowball that got mixed with dirt when you scraped it off the ground. These "rocky" grains get carried away from the dirty iceberg with the steam and end up floating in the steam cloud. The solar wind carries away these dirt, or "dust" particles just like it does the oxygen, except that they move much more slowly. So as the dust grains are swept slowly away into a "dust tail" the comet has time to move, sort of leaving the dust tail behind.
This is why comets often have two tails. Comet Hale-Bopp was a prime example of this. The sun blows the charged gas tail quickly away from the comet, making a narrow tail that always points directly away from the sun. The oxygen and carbon dioxide ends up glowing mostly blue as it absorbs and re-emits solar energy, so this tail looks slightly bluish. The charged particles in this tail are called ions, so we call it the ion tail. The blue can be hard to see to the eye, but photographs show it well.
The more slowly moving dust tail lags behind the comet. In the picture here of Hale-Bopp, the comet is moving from right to left; the bright tail curving off to the right is the dust tail. Hale-Bopp has a lot of dust in it so this was actually the brightest tail. It simply reflects sunlight.
All that dust eventually ends up spread around the orbit of the comet, and some of it may eventually end up striking the earth. The tiny dust grains burn up in the atmosphere making a quick bright streak of light in the sky--a meteor. Obviously, the dust will be concentrated in certain places along the orbit of the comet, and if the earth passes through one of these concentrations we will see many meteors all at the same time. This is a meteor shower, and the earth regularly passes through the orbits of several comets so meteor showers occur at the same time every year.
It is unlikely that anyone has ever seen the nucleus of a comet through a telescope. It is quite small and by the time we could see it because it is close to us it is already surrounded by a cloud of steam. What you see through a telescope is the steam cloud, or coma. We do have some picture, however. The one on the right was taken of the nucleus of comet Halley by the European Giotto spacecraft. It shows a dark body with steam venting on the sunlit side.
The Existence of the Oort Cloud
So we're left with one unresolved question: if we have never seen a comet in the Oort cloud, how do we know it exists? The answer comes from studying the orbits of comets that we do see. These comets come from far away, pass close to the sun, and then head back out to great distances. A good first guess would be that these comets are simply floating between the stars and our solar system eventually runs into them. But the orbits of these comets always seem to end at about the same distance from the sun. In fact, this is about the distance at which any object would be so far from the sun as to be only just barely held in an orbit at all. At this distance, it would be very easy to push an object into a different orbit--one that would bring it falling in toward the sun and the warmth of the "inner" solar system.
So it looks like these comets are coming from a cloud around the sun, with perhaps billions, or even trillions of icy comets.
In the above scenario comets come in unannounced from the far reaches of the outer solar system, put on a show, and then head back out, perhaps never to be seen again. But as usual, it's not that simple. As minor bodies comets are easily pushed around by the gravity of the planets, particularly those big gas giants like Jupiter. It's fairly likely that one or more of these planets will permanently change the orbit of the comet as it passes through the inner solar system. It's even possible for a comet to hit a planet, as we saw when Shoemaker-Levy 9 smacked into Jupiter. The "new and improved" orbit for the comet may no longer carry it back into deep space. It might end up in an elongated orbit that brings it back near the sun every 76 years like comet Halley. Or, perhaps, it could end up in a more circular orbit out among the asteroid "belt" where most asteroids are found (between the orbits of mars and Jupiter).
These are the periodic comets, so called because they come back near the sun "periodically." There are a few hundred of these known, and we can easily predict when they will be visible from earth. The periodic comets are given a "periodic number" by the IAU, which is usually followed by a "P". An example is comet 2P/Encke, periodic comet number 2. Encke takes a little over 3 years to orbit once around the sun.
Eventually a periodic comet is going to pass close to the sun so many times that it will literally run out of steam. Does it dissolve away completely, or does it leave behind a rocky body that otherwise looks like an asteroid? We're not sure yet.
Interest to Science
Comets are of interest scientifically because they appear to be unaltered leftovers from a time very early in the history of our solar system. Studying them can reveal clues about how the planets formed.. They also tell us something about the original cloud of material from which then sun formed--a cloud of gas and stardust. The stardust was formed deep within stars long ago.
It would be nice to have a sample of comet
dust to study in the lab, and even though comet dust constantly rains down
from the sky it burns up before it reaches the ground. That's why the exciting
Stardust mission was flown. This spacecraft rendezvoused with comet Wild 2,
took samples from the come and tail, and return them to earth.
Nothing like this had ever been done before. Scientists will spend years
studying the material bright back. To find out more about Stardust,
see the JPL Stardust
Skyhound's Guide to Finding Comets
Astronomical Headlines (IAU)