According to most historical views of comets, the common belief was that they were not actual celestial bodies. As a matter of fact, in most older Western cultures, comets were thought to have some sort of religious significance; this came in the form of the comet being a harbinger of evil, or some kind of messenger from heaven of doom to come. Even the English poet John Milton once stated that Satan must be a comet that "from its horrid hair shakes pestilence and war." This view of comets and the evil times that they predicted persisted until more detailed studies began to be made of them.

In 1577, the Dutch astronomer Tycho Brahe began making the first real observational studies of comets by observing a bright comet that appeared in Earth's skies in the year. According to his studies, he concluded that the comet had to exist outside of the Earth's atmosphere due to the fact that the position of the comet in the night sky appeared not to change in relation to an observer's location on the Earth. This information suggested that the comet had to be very far away, beyond the atmosphere.

Sir Isaac Newton became curious about comets and began to apply his laws of gravitational motion to these celestial wanderers. His conclusions stated that comets were accelerated by gravity towards the Sun and that they move in orbits about the Sun that are either parabolic or highly elliptical in nature. We now know today that this is relatively accurate; cometary orbits are periodic in nature, with periods as long as millions of years or as short as that of Comet Encke, which has an orbital period of 3.3 years.

Figure Two: Comparison of Planetary and Comet Orbits

Edmund Halley (1656 - 1742) extended Newton's study of comets and cometary motion. He took particular interest in a series of very bright comets that appeared in 1531, 1607, and 1682; Halley noted that the orbits of these comets were very similar (according to the records) and that all three could actually be the same comet with an orbital period of 76 years. His prediction proved correct when he accurately predicted the return of the comet in 1758. In his honor, the comet was named Halley's Comet, and it is one of the more famous comets known. The last appearance of Halley's Comet took place in 1986, and it is not scheduled for a return visit until the year 2062.

The exploration of comets has, in recent decades, taken stellar leaps. Rather than be content with simply sitting around on Earth waiting for the comets to pass into the observatory's view, scientists have decided to go to the comets. This has been accomplished through the use of interplanetary probes that are launched from the Earth and rendezvous with a comet. The first such encounter took place in 1985 when the U.S. satellite International Sun-Explorer was moved from its set orbit so that it could perform a flyby of the comet Giacobini-Zinner. The spacecraft, now named International Comet Explorer (ICE) passed about 7800 kilometers behind the comet, traveling through the tail on September 11, 1985.

Other craft have visited the more famous Halley's Comet. When Comet Halley passed close to the Earth in 1986, an armada of six probes was set to intercept it and gather information about the comet. Two craft, the U.S. ICE probe and a Japanese craft called Sakagaki observed the comet from a distance of many million kilometers while a second Japanese probe, Suisei, passed to within 1 million kilometers of the comet. The other three spacecraft were targeted to gather data on the nucleus of the comet. Two Soviet probes, VEGA 1 & 2, encountered the cometary nucleus on March 6 and 9, 1986, respectively; they both passed into the comet's atmosphere and passed to within 8000 kilometers of the nucleus. The final spacecraft, a European probe named Giotto, made an even closer rendezvous with the nucleus on March 14, 1986, passing to within 605 kilometers of the nucleus. At this close proximity, Giotto was actually able to beam back images of Halleys nucleus...

Figure Three: The Nucleus of Halley's Comet

There are three primary parts of a comet: the nucleus, the head or coma, and the tail. The nucleus of the comet is the true comet; it is formed of a solid lump of material that is usually a few kilometers across. The head, or coma, is the large diffuse cloud-like feature that surrounds the nucleus, making the comet much more visible than it normally would be. Cometary tails are the glowing streams of wispy matter that seem to originate at the head of the comet.

Figure Four: The Anatomy of a Comet

Nucleus:

This is the actual comet itself, being made up of a solid lump of matter a few kilometers across. The existence of cometary nuclei was a concept that seemed very reasonable, but comet nuclei were not actually observed and measured until 1970. Since then, comet nuclei have been observed by many means, telescopic observation and up-close encounters with unmanned spacecraft being included. So what do these "hearts" of comets look like? What are the materials that make them up?

From observational evidence, it would seem that comet nuclei could take almost any conceivable shape... anything from almost spherical to potato-shaped, like Halley's Comet (see figure of Halley's Comet's nucleus). Were we to see a comet nucleus far away from the Sun, it would seem like a nondescript lump of matter, cold and uninteresting. The most famous view of the material makeup of a comet belongs to Whipple, and he calls it the "dirty snowball" model. In this model, Whipple suggests that most comets are a mixture of large amounts of water ice combined with grains of various silicates and dust. Upon later observation, Whipple's opinions were determined to be mostly correct. Current spectrographic data suggests that cometary nuclei consist of large quantities of water ice that is mixed with other ices (CO₂, CO, NH₃, and CH₄) in addition to "dirt", bits of dark hydrocarbons and silicates.

Upon approaching the Sun, the comet's surface temperature will increase causing outgassing, or a jetting of material from the comet into the vacuum of space, as cometary ices on the surface melt away. In the outgassing process, dust and other particles are released with the melted ices and both flow away into space (since they are being forced away by the jetting of outgassing and because the gravitational pull of the comet is so little).

Head or Atmosphere:

When we see a comet, we dont actually see the nucleus, at least not from our vantage point on the Earth. One of the main parts of the comet that we see is the comet's head. The cometary head is that portion of the comet that forms around the nucleus due to the outgassing process described in the previous section. Materials that are outgassed are usually released in all directions, so the head of the comet surrounds the entire nucleus. The speed of ejection of this matter is typically on the order of 1 kilometer per second; ejection speeds of this magnitude usually lead to comet heads, or comas, that are very large, roughly 1 million kilometers across.

The following table shows the chemical composition of the coma of Halley's Comet:

Parent Molecules	Percent Composition
H2O	79.0
H2CO2	4.5
H2CO	4.0
CO2	3.5
CO	1.5
CH4	2.0
C2H2	1.5
C3H2	0.2
N2H4	0.8
HCN	1.0
N2	0.5
H4C5N4	0.5
NH3	1.0
S2	0.2
H2S	0.2
CS2	0.2

The parent molecules are the original molecules that are assumed to have broken apart to form the actual materials that were measured by observing the comet.

Tails:

Perhaps the most spectacular part of a comet that we notice is the cometary tail, which is simply an extension of the comet's atmosphere or coma. Beginning in the 16th century, comet observers had begun to notice that the tail of a comet tended to point away from the Sun rather than backwards along the path of the comet's orbit. Isaac Newton attributed this strange behavior of comet tails to some sort of repulsive force that originated from sunlight.

Newton came pretty close to the mark; the primary component that shapes comet tails is that of the solar wind, which is a stream of ionized particles that emanates from the Sun. This "wind" of plasma interacts with the ions in the comet's atmosphere, and the result is a tail of ionic material that is forced away from the comet on the solar wind. This type of tail is called the plasma tail, and it tends to be the most prominent type of comet tail. Plasma tails usually consist of ions such as CO⁺, N₂⁺, and H₂O⁺. The second type of tail that is common to many comets is that kind called a dust tail. Dust tails are the result of radiation pressure acting upon the comet and knocking loose bits of material from the atmosphere. This happens because each individual photon of light that originates from the Sun has a certain amount of (small) momentum associated with it. When enough of these photons impact the comet atmosphere, the pressure builds up to the point to where matter is ripped free of the head.

Cometary tails get longer and more spectacular as the comet approaches the Sun. This happens because the intensity of the solar wind and the radiation pressure increases in closer proximity to the star. Some comet tails have been recorded to have been as long as 150 million kilometers, which is the same as the distance from here to the Sun!

Figure Five: The Plasma and Dust Tail of Comet Mrkos

So where do comets come from? We know that most comets are members of our own solar system, rather than being intruders from interstellar space. Astronomers believe that comets that originate within our solar system come from one of two places: the Kuiper Belt or the Oort Cloud.

The Kuiper Belt is a disk of comets that exists from 30 to 100+ AU (remember that one AU = 150,000,000 kilometers = 93,000,000 miles) from the Sun orbiting within the plane of the ecliptic. From observing the orbital characteristics of various short period comets, the Dutch-American astronomer Gerard Kuiper proposed the existence of this band of 100's of millions of comets. Kuiper's suspicions were confirmed in 1992 when it was actually observed for the first time. It is thought that gravitational interaction with the nearby planets of Uranus and Neptune is what causes Kuiper Belt comets to become dislodged from their circular orbits within the disk to follow more elliptical orbits that take them further in toward the Sun.

For numerous longer period comets, the value of the aphelion, or the furthest point in the comets orbit from the center of the solar system, is typically around 50,000 AU (remember that one AU = 150,000,000 kilometers). These measurements were first made and noted by a Dutch astronomer named Jans Oort. In the year 1950, Oort suggested that many of these comets originated from a large, roughly spherical grouping of matter that orbited the Sun at roughly 50,000 AU (for scale, the orbit of Pluto is at about 40 AU, which is about 1/1000 of the distance). He called this the Oort comet cloud, and the existence of the Oort cloud is widely held in astronomical circles today; however, it should be noted that, unlike the Kuiper Belt, the Oort Cloud has not yet been directly observed. According to Oort, the comet cloud is relatively stable, except when the gravitational attraction of some passing celestial body (star, asteroid, another comet, etc.) disturbed the cloud, then a shower of comets would be shaken loose to be pulled towards the inner solar system by the Sun's gravitational influence. Oort calculated that there could be as many as 10¹¹ comets occupying this cloud.

Exploration of the Universe, Abel, Morrison, and Wolff. 6th edition. Saunders College Publishing, 1991.