The actual nature and inner workings of comets do justice to their reputation; they are fundamental bodies that are a Rosetta stone to the origins of the solar system and the structure of the galaxy beyond it. They also are unique laboratories for the study of chemistry and the environment between the planets. Comets may have even played an important role in the early development of life on Earth, and, as movies like 'Deep Impact' and 'Armageddon' imply, continue to affect our environment in the form of periodic massive impacts..

What are comets anyway?

Most of us are aware of the classical features of a comet including the sweeping tails and the diffuse spherical coma. What many people are not aware of is that almost no one has ever actually seen a comet. Comets themselves are small and dark objects typically no more than 10 miles across, which is about the size of Madison. While 10 miles seems like a lot to us sitting in this room, from our typical vantage point a comet of that size would appear about as big as a dime would, if it were on top of the capital building and you were looking at it.....from downtown Milwaukee. That doesn't sound too impressive, does it? In fact, the only comet nucleus that has ever been seen with any precision is Comet Halley (Halley picture).

So, if we can't see the nucleus of a comet, what is going on?

Comet nuclei are mostly made of what we call volatile compounds, which is to say, things that can melt or evaporate (How they formed and why they continue to survive in the solar system will be addressed later). The most abundant volatile in a comet is water, which is why they are often referred to as 'Dirty Snowballs'. As a comet nears the Sun, the volatile compounds begin to melt off in increasingly larger amounts. This evaporation can occur generally over the surface of its nucleus or from active sectors that produce jets (Hyakutake Jets picture). A typical active comet can lose 5000 gallons of water, enough to fill a small backyard pool, every second (Inner tube picture). Along with that water are all manner of other nasty sounding compounds, things like carbon monoxide, ammonia, cyanide, alcohol and other hydrocarbons, and soot. Indeed, the environment near a comet bears a more than passing resemblance to the exhaust pipe of a car that's burning oil.

As unappetizing as this recipe sounds, it is the evaporation of these volatiles that makes a comet visible to us. The outgassed material expands away from the comet at about ½ mile/second, forming a structure called the coma, the first of three major elements of what we think of as a comet. The coma is visible because the dust (soot) scatters light and various gasses in it fluoresce in different colors when sunlight hits them. For instance atomic Oxygen is red while Cyanogen (CN) is green. Putting all of the different compounds in the coma together is like reassembling a rainbow, and the end result is the whitish glow we are used to seeing. The apparent size of the coma depends on what we are looking at (Hyakutake & Other picture). Some volatile species are destroyed by sunlight very quickly, but others, like atomic hydrogen can extend out to more than 10 million km from the nucleus (SOHO picture), a volume more than a billion billion times larger than that of the nucleus! It's easy to see why the coma stands out.

As the gasses move away from the nucleus, they are exposed to ultraviolet solar radiation, which can break apart molecules or strip away electrons from some of them to form ions. These ions will in turn interact with a stream of charged particles produced by the Sun called the solar wind. It is this same solar wind that impinges on the Earth's magnetic field and is responsible for the Northern lights. It moves very fast, up to 400 miles per second, and it 'picks up' the comet ions and drags them along with as it flows past the comet. The ions are accelerated by this process and form a long stream of material extending directly back along the line between the Sun and comet. The two primary ions we see are CO+ (which is blue) and H2O+ (which is red). Together they form the second element of a comet, the ion tail (Mrkos picture).

The final piece of the puzzle involves the debris of evaporation. While much of the dust liberated from a comet is in the form of very small grains of material on the order of 1 micron in size, larger pieces are produced as well. Some larger particles are simply blown off the surface, while others are produced by fracture events caused by day/night transitions or structural failure on the nucleus as the underlying ices evaporate away. This larger debris tends to spread out behind the nucleus along the orbit of the comet. These particles will continue to break apart, creating a vast spray of material that extends behind the comet. We call this aggregate of dust, pebbles, and boulders the dust tail, and it is the final visible element of a comet (Hale-Bopp picture).

(Aside) Because of the way a comet orbits the Sun, the direction of a comet's orbit and the line between the Sun and comet are often parallel. This means that the ion and dust tails overlap. Because this happens frequently, many people don't realize that a comet has 2 distinct tails that have very little in common physically.

The amount of material in the dust tail is function of the structure of the nucleus. Some comets are more cohesive than others. In an extreme case, the nucleus itself can break apart and the entire comet will become an extension of the dust tail. Two recent examples of this were comet LINEAR/S4 and Comet Shoemaker-Levy/9 (SL9 picture).

Where do comets come from?

If one thing is clear, it is that any object that is made of ice can not be a normal occupant of the inner solar system where the terrestrial planets (Mercury, Venus, Earth, and Mars) were formed. It simply won't last (Hale-Bopp Desert picture). Indeed, from a cursory examination of the moons and small bodies found in the outer solar system it is clear that comets must have begun their journey from at or beyond the orbit of Jupiter. As it turns out we can break down the home of comets into two distinct regions, the Kuiper Belt and the Oort cloud (Scales Picture) The locations, properties and origins of these associations are tied up in the history of how our solar system formed and evolved.

In the modern model of solar system formation, the Sun and all the planets formed from a gigantic cloud of interstellar gas and dust (eagle nebula picture). Following some external perturbation, this cloud began to collapse inward onto itself. In the center, a large mass of gas began to accumulate: this would become the Sun. Around this there formed an orbiting collection of material called the protoplanetary disk (Beta Pic disk picture). In recent years we have discovered similar protoplanetary disks around nearby young stars. Within the disk dust and gas larger objects begin to accumulate, but it was a slow process that built over time. In a manner analogous to rolling a snowball, collections of dust and ice began to form into ever larger objects, some as small as boulders, others up to 1000 km or more in diameter. We refer to the smaller of these objects as planetesimals. Near the orbit of the Earth, where temperatures were high enough to prevent ice from forming, the planetesimals were made up only of rock, but in the outer solar system ices and gasses became trapped in the planetesimals, and dominated their structure. Today, we refer to these planetesimal groups by their more well known names, asteroids and comets..

The asteroids and comets are believed to be the building blocks of the planets. As they added mass from the disk, they also collided with one another, sometimes merging into a larger body. Eventually these new "protoplanets" became large enough to form their own disks of accreting material, and began to emerge as the planets we know today. Once this happened, they also began to perturb the orbits of the remaining planetesimals. Those that were not in dynamically stable orbits were lost to subsequent accretion by the planets, ejection from the solar system, deflection into the hot inner solar system, or thrown out of the solar system into far more distant, but stable orbits. The Asteroid belt, Kuiper belt and Oort cloud represent the three outcomes that preserved a tiny fraction of the planetesimals to the modern solar system. The Kuiper belt objects, which have only been discovered in the last few years (Kuiper Picture) range in size from boulders up to near planetary dimensions, were in stable orbits from the very beginning (scale picture). The newly formed planets did not disrupt them. The Oort cloud on the other hand, was formed from objects that were flung to the edges of the solar system by the planets. Ironically, this means that the comets in the Oort cloud were actually formed closer to the Sun than those in the Kuiper belt, despite having a much more distant modern orbit. The total number of comets in both regions is unknown, but it is believed to be immense with up to several hundred of billion in the Oort cloud, with several billion more in the Kupier belt. The Kuiper belt also has many more large members, including (to some peoples way of thinking) Pluto. Statistically, it is estimated that up 100,000 objects 60 miles or more in diameter exist there.

You might well ask "If the Oort cloud and Kuiper belt are stable, then why do we occasionally see comets in the inner solar system?" . It's a good question. Early in the history of the solar system, it was literally teeming with comets as their orbits were being disrupted by the forming planets (mainly Jupiter). Perhaps as many as 1000 Hale-Bopp's may have been present in the inner solar system at any given time. The Earth and other terrestrial planets were pelted by these comets and asteroids at a rate of about 1 every century (Impact Picture). Eventually things settled down to the stable arrangement we have now. By comparison, things are much calmer. The Earth is hit by large objects about 100,000 times less frequently (fortunately!) than it was. Still, we see new comets about 1-2 times each year. The answer to why appears to be interactions between the inhabitants of the stable zones. They are widely spaced, but there are hundreds of billions of comets. Just like the lottery, occasionally some overcome the odds and pass close to each other. When this happens, their orbits are disrupted. One goes in toward the Sun, while the other is flung out of the solar system.

Here's a question for you. If so many comets were ejected from the solar system by orbit disruption, then how do we know when a new comet enters the solar system whether it came from the Oort cloud or was ejected from some other planetary system?
The orbit of a deflected comet is very different from the nearly circular paths the planets follow. Comets initially come from very far away, and their path reflects this (Orbit Picture). They come careening in from a great distance, fling themselves by the Sun, and go back out again. It is the nature of these highly elliptical orbits that objects in them spend most of their time at their greatest distance from the Sun. Comet Hale-Bopp is close to the Sun for about 1 year out of a 3000 year orbit. This nature of comet orbits is what allows them to survive many passages by the Sun. This isn't the end of the story though. As they pass through the solar system, comet orbits can become further perturbed by the planets, mainly Jupiter, and their orbits will change. Thus, while a comet may initially have come from the Oort cloud or Kuiper Belt, their orbits may not reflect this anymore (Orbit Modification Picture). This perturbation can fling the comet out of the solar system (are you getting the idea that interstellar space is full of these things?), or it can rein in the comet, forcing it into a smaller orbit.

Any comet we see more than once has been perturbed into a smaller orbit by the planets. Periodic comet orbits can be very short (Encke is only 3.3 years) or moderately long (Halley is 76 years). Eventually, these comets will pass by the Sun SO often that they either evaporate away or become covered in a think mantle of insulating dust that prevents further evaporation. We no longer see these 'dead' objects, but the debris left from them continues to orbit the Sun. Occasionally the Earth will pass through the plane of a comet orbit (dead or alive), and we will be treated to a shower of tiny grains of dust left over from some long ago passage (Leonids picture).

Why do we study comets?

Ok. We now know what we are looking at when we see a comet, and we know where they come from. So why do scientists still like to study them? As it turns out, comets are actually very important objects on several levels that make them extremely interesting to study.

1) Comets are Time Machines. As remnants of the final stages of the formation of the solar system, comets are clues to the conditions that prevailed in the solar nebula. They are essentially unmodified markers of the composition, temperature, and density of the outer regions of the protoplanetary disk. Except for the asteroids (which are the primordial remnants of conditions in the inner solar system), the other inhabitants of the modern solar system are all substantially modified from their original composition. From comets we are able to provide a basis for developing a model for the formation of the planets.

2) Comets are Astrophysical Probes: As we said above, the solar system formed out of a vast cloud of gas and dust in the galaxy. Substantial amounts of this material exists today, both in the Milky Way and other galaxies. We call this material the interstellar medium (ISM), and it is an important because it is the source of new stars and substantial fraction of the galaxy's mass. Like all astrophysical phenomena, we can only look at the ISM remotely, and this limits our perspective on it. As we discussed earlier, comets are essentially clumps of the dense disk of material out of a collapsing section of the ISM, and are thus representative of its content. Missions like Stardust seek to bring the ISM home in the form of dust from a comet, while remote observers look at the composition of gasses in a comet for clues about the nature of the ISM.

3) Comets are Probes of the Sun and Solar Wind: Two of the major elements of a visible comet, the coma and ion tail, offer important clues as to the nature of the Solar radiation field and to the charged particles the Sun emits. The invisible (UV and EUV) spectrum of the Sun (SOHO picture) is important to us in that it drives chemistry in the Earth's upper atmosphere and is harmful to living things (especially where O3 is depleted). The chemical evolution of different gasses in a comet results mainly from the absorption of solar radiation. By looking at the coma and its spatial distribution, we gain insight into the nature of the solar spectrum and the physics of the various gasses that make it up. In many ways comets are an ideal chemistry laboratory that is better than any we could build on Earth.

As for the solar wind (SOHO Wind Picture), well it does far more than just cause the aurora;

• it heats and expands the upper atmosphere, which can then begin to drag on satellites
  
• it creates particle storms in the space around the Earth which is a hazard to satellites and astronauts.
  
• it creates electromagnetic noise in the atmosphere that can disrupt communications equipment.

We study the solar wind by looking at the Sun and through remote space probes, but comets are able to play an important role here, because they also interact with the solar wind (Hyakutake Tail Picture). Through observations of the ion tail we can learn about the density and velocity of the solar wind. Moreover, because comets move so quickly through the solar system, we get a perspective on many different regions of the solar wind in a short period of time.

4) Comets Tell us about Ourselves: Comets can't survive today in the inner solar system, and they didn't form here. However, they are a part of us all the same. The early terrestrial planets formed in a hot, volatile free environment. So where did the oceans and atmospheres come from? One possible answer is....you guessed it....comets! The very collisions we make movies about stopping today, may well have been essential to the development of life on Earth. Over the course of the first billion years of the solar system, the Earth was hit millions of times with comets and other debris. Added up, this makes for a substantial amount of air and water. Without comets, there is a very good chance that no life would exist on the Earth today. This idea of comets as a progenitor to life is carried even further in some corners of the scientific community. Through a theoretical process called "Panspermia", it has been postulated that life itself formed first in comets, and was then delivered to Earth by impacts.

5) Comets are a threat: As we hear about in classes on the dinosaurs and in popular movies, comets and asteroids have played an important role in the history of life on Earth, and will continue to do so into the future. Rare, but infrequent impact driven extinction events have basically erased the blackboard of life several times and forced it to go in new directions. As much as we fear an impact today, we almost certainly owe our existence to one that occurred 65 million years ago (Impactor picture). It opened the door for mammals to become the dominant large land animals. Until very recently, we lacked even the knowledge that these things could even happen. Within the next century or two we will learn how to prevent them altogether. It's yet another way in which humans are affecting the development of life on Earth. We shouldn't be too proud of ourselves though. We won't exactly be preventing an extinction holocaust, even if we stop a rogue comet someday, because one will have already happened. Its an interesting and chilling perspective. For all the damage we envision to our environment from a massive comet impact, human activity, by itself will drive more species into extinction by the end of this century than did the comet impact that destroyed the dinosaurs. Disasters can take many forms.......

Comets in the Public Conciousness:

I want to conclude with a bit of levity about the important role that comets have played in our psyche over time, that exists in addition to their immense scientific contributions.
There is no doubt that comets have always been held in a special category of astronomical phenomena; right up there with a solar eclipse. This is perhaps because, in all of astronomy, there are few phenomena that can inspire the kind of emotion and outright terror that comets have throughout our history. They do, after all, have all the trappings of a divine message or an apocalyptic precursor; just the sort of sudden sign that the gods would send us just before (or during, or after) sending a big flood, destroying a harvest, or bringing a victory in battle. Even the scientific term for a comet passage, an 'apparition', suggests something paranormal. History seems to bear this (mistaken) conclusion of ours out. Comets do always seem to appear at or near some significant historical event. The flaw in this reasoning, of course, is that there is almost always something, somewhere that has just happened, is about to happen, or is going on! Civilization is a busy place. Comets have been ascribed to everything from the eruption of Mt. Vesuvius to being a sign that Napoleon should invade Russia. A comet was prominent in the skies over London as the black plague was ravaging the city.

You should wait a minute though, before getting too smug about our modern scientific sensibilities. Our discoveries about the true nature of comets hasn't done much to quell the hysteria; we just find new ways to make fools of ourselves whenever they appear. During the 1910 comet Halley apparition, snake oil salesmen used a report that the Earth would pass through the comet's tail to sell "comet pills" and gas masks to hysterical consumers. (Scientists had just discovered HCN (Cyanide Gas) in comets). Less amusing were the "Trailing UFO" rumors about comet Hale_Bopp that culminated in the Heaven's Gate cult's mass suicide in 1997. But by far the biggest modern hysteria associated with comets is the idea that one will hit us. This is at least rooted in fact. Unless we are here to stop it, another impact will happen. It's inevitable. However, the chances of it are VERY small; there is less than a 1 in 100,000 chance that a large impact will occur in the next century. We have better things to worry about than this.