Comets

Overview and A Bit of History

Comets have been around since the beginning of the solar system, and they have been recorded since written history began. Originally, comets were known as "hairy stars," and were unexplained, unpredictable (although it was thought that they predicted many things), and brought death and destruction. Very early ideas of what comets were included clouds of fire in the Earth's atmosphere, or extra bodies moving though the heavens.

As science progressed, people were better-able to explain comets. In 1577, Tycho Brahe and Mastlin used parallax - the measure of the change of location in the sky of a body over the course of six months - to show that comets were not in the atmosphere, but rather they were very far away, even further than the moon.

The public and scientists had to wait until 1704 for Edmund Halley to actually fully explain what comets are. He used Newton's newly developed Laws in order to calculate the orbits of comets based upon their motion in the sky. He calculated orbits for many comets, but he also found patterns in the orbits - some comets seemed to actually have the same orbit as others! He correctly hypothesized that these were the same comet; it had documented appearances in 1456, 1531, 1607, and 1682. He predicted that this same comet had an orbital period of 76 years, and would return in 1758. It did, and now bears his name: Halley's Comet. Unfortunately, he did not live long enough to see it reappear.

Anatomy

The center of a comet, the nucleus, is very small compared to the rest of the comet, and it is usually only a few kilometers in diameter. It is the part of the comet that is always there, at least as long as the comet exists. The nucleus is composed of rocks and ices. As comets age, they lose their ices, and a comet is considered "dead" when it no longer has any ice, for it can no longer sport any feature other than its nucleus. The picture to the left (below) was taken in 1986 by the spacecraft Giotto of comet Halley's nucleus.

The other parts of a comet are only in existence when the comet approaches the sun - usually once it is closer to the sun than Jupiter (5.2 A.U.). The sun's heat melts some of the ice in the nucleus to form a huge glowing "head," the coma. The coma can grow to immense proportions, sometimes becoming over 10,000 km (6,700 miles) across (that's larger than the Earth). Because the ices of the comet are made of various molecules (such as carbon, nitrogen, oxygen, and water), and the different molecules weigh different amounts, some can escape the nucleus' weak gravity more easily. Therefore, another minor feature of the coma is the Hydrogen Cloud, which forms the outer part of the coma.

Finally, the part of a comet that is most well-known is the tail. Most comets have two tails, and usually one is much brighter than the other. The tails form as a result of the sun's solar wind - the stream of charged particles that emanate from the sun. The solar wind dislodges gas and dust from the comet and forces the material into very narrow (relative to their length) tails. The tails always point away from the sun, which is sometimes counter-intuitive. This is because even when the comet is traveling away from the sun, the tail faces away, so the comet is, in effect, following its tail.

The gas and dust form separate tails due to the charge of their constituents. The dust is not highly charged, and so forms a bent tail that slightly lags behind pointing directly away from the sun; its color is yellow because it reflects the light of the yellow sun. Grains that are smaller than about 0.06 mm will actually be pushed away from the sun by radiation pressure - the grains absorb the photons and the momentum from the photons is imparted to the dust. Grains much smaller than this won't be able to absorb the photons well enough to be pushed in this manner, but will be pushed by collisions with solar wind particles. Anything larger than the 0.06 mm will be left behind.

The gas tail is much more highly charged, made of CO⁺, N₂⁺, and CO₂⁺ ions. The ions - electrically charged particles - interact with the sun's solar wind, causing a comet magnetotail that points away from the sun. The ions travel along the magnetic field lines, so the ion tail points away from the sun. CO⁺ absorbs sunlight and fluoresces, emitting energy at a wavelength of 4200 Å, which is blue light.

The tails can be several million kilometers long each, and the longest have been observed to be over one A.U. (over 93 million miles long). Due to the nature of how the tails form - material being blown off of the comet - the tails are how comets loose the bulk of their mass. The material usually dissipates after several hundred years, but before that happens, the material usually will continue in the orbit of the comet. If the Earth plows through this, we see a meteor shower.

Note that the above comet illustration is not drawn to scale; features have been emphasized for easy viewing.

Impact!

Just like asteroids, comets can hit planets. The most famous impact (and the only one observed in recent history) is that of comet P/Shoemaker-Levy 9. Discovered in 1993, it impacted Jupiter in July of 1994. For more information about this event, see the Jupiter page.

Orbit

Current theory proposes that most comets originate in the Oort Cloud - the vast cloud of comets that surrounds the solar system out to an estimated distance of two light-years - or the Kuiper Belt - the belt of comets and asteroids extending from between 30-50 A.U.. Comets usually stay in the Oort Cloud or Kuiper Belt; however, if a passing object (such as a star) exerts enough gravity, the comet might be nudged free. Sometimes its new path will expel it from the solar system all together, and it is doomed to roam interstellar space. The alternate path takes it into the inner solar system, where it can become a short- or long-period comet.

As it travels towards the sun, the gravitational pull of the planets can change its trajectory. About the time it passes through Jupiter's orbit, the nucleus begins to feel the heat and pressure of the sun, and surface ice starts to melt, forming the extra features that make it recognizable as a comet. The first encounter with the sun usually changes its orbit the most.

If the comet's final orbital path takes less than two hundred years, the comet is classified as a short-period comet. If the orbit takes more than 200 years, then it is called a long-term comet. Halley's comet is a short period with an orbit of seventy-five years; the recent Hale-Bopp with an orbit of 4,200 years is a long-term comet.

Some Famous Comets

* Comet Halley, or Halley's Comet, was actually not discovered by Halley; he was the first person to piece together several comet sightings over the previous several hundred years, and hypothesized that many sightings every 76 years of comets were actually multiple sightings of the same comet. He also successfully predicted the next sighting of it, but was unfortunately not able to live to see it. See above for a more-in-depth history of comets.