COMETS




Make up of a Comet


Comets, those little fireball-like objects seen flying across the sky, are actually small bodies that orbit the Sun. Basically, comets are dirty snowballs that go directly from a solid to a gas state (sublimation) when they are near the Sun. There are several parts that make up a comet, and they include the nucleus, coma, hydrogen cloud, dust tail, and the ion tail. The nucleus, or head of the comet, is where the ice, gas, dust, and rock can be found. This some what stable and solid piece of matter is usually 1 to 10 kilometers across. The vapor that sublimes off the comet is called the coma. This dense vapor cloud contains carbon dioxide, water, and other neutral gases. There is also a hydrogen cloud made of neutral hydrogen gas that can be millons of kilometers in diameter. Although people may only be able to see one tail on a comet as it nears the Sun, there are actually two. When seen as one, these tails can strenched for 150 million km in length. The two types of tails are dust tails and gas (ion) tails. Comets don't develop their tails until they are within 3 astronomical units from the Sun. More often, comets have gas tails, but those tails are harder for people to view. Gas tails, usually a bluish color, contain plasma and are nodded and twisted. The yellow or white dust tails have no significant features and are composed of smoke-like particles. Just so that you know, the only reason that comets can be seen from Earth is the fact that the comet reflects the light of the Sun.
Picture courtesy of NASA

The Orbit of a Comet


Comets are said the have extreme elliptical orbits, and ellipses are shaped by two focus points. Coming from outside the solar system, the main foci of their orbit is the Sun. The comet travels at a faster speed when it is near the Sun because of the pull of the Sun's gravity. As the distance between the comet and the Sun decrease, the cold head of the comet goes through a process called sublination in which the ice changes directly into a vapor. The solar wind, created by the comet's speed, forces the resulting coma (or atmosphere) to flow off the comet head and away from the Sun in a tail-like structure. This tail structure elongates the closer the comet comes to the Sun. Everytime a comet has an encounter with the Sun, a part of its head melts off. Through this process, small comets will eventually evaporate into nothing after about 1,000 orbits. The orbits of comets are periodic, but they can be drastically altered if the comet passes too close to the Planet Jupiter.

Classification of Comets

When new comets are discovered, they are recorded by using two types of classification methods. Prelimanary classification takes place in the order in which an astronmer discovers a comet.They are named by giving the year they were discoverd with a lowercase letter in alphabetical order following it (i.e. 1987a). After some observation, the comet is later classified according to their chronological order of passage at the perihelon (the closest distance to the Sun) with the year discovered and a Roman numeral (i.e. 1987 I).

Comets can also be classififed according to their orbital periods. Short orbital periods are anywhere from 3 to 25 years. An example of the shortest period of a comet is known as Encke's Comets which orbits within a period of 3.3 years. A comets that has an orbital period between 25 to 200 years is known as an intermediate-period comet. An example of this is Halley's comet whose orbiting period aruond 76 to 79 years. A long-period comet orbits up to 10,000 astronomical units from the sun. Comet Mrkos orbit is one example with a HUGE orbit of under 13,000 years. (WOW!!!)


Comet History


The title bestowed upon the dirty ball of ice that floats through space, "comet", comes from the Latin word cometa which means "long-haired". First seen by Chinese civilizations in 1059 B.C., Aristotle thought that comets were earthly exhaltions that were ignited and driven by heat into the upper atmosphere. He was later proven wrong by a man named Tycho Brahe who had critical evidence that comets were heavenly bodies. Brahe acomplished this task by deducing that a comet of 1577 had a distance from the Earth four times that of the distance to the moon. While Johannes Kepler believed, in 1619, that comets travelled in a straight line, Isaac Newton had developed the concept that comets orbitted in either a circle, ellipse, parabola, or hyperbola. To test this theory, Edmand Halley used the idea of a parabola shaped orbit and compared it to the orbit of 24 comets. The parabola orbit worked for 3 comet orbits which he later proved to actually be one comet (the Halley comet) by predicting the next apperance of that comet. By studying comets, we can learn more about the materials and process that created our solar system.
Picture courtesy of NASA

Theories of Origination


Where do comets come from? Some kids may say that they were the snowballs that were thrown a bit too high during a snowball fight. Others, mainly scientists, have at least two theories dealing with the origin of comets. By 1950, an astronomer by the name of Jan Oort had devised the first known theory. He believed that there was a cloud 100,000 astronomical units in diameter encircling the solar system. In that cloud, called the Oort Cloud, comets are created and dragged towards the center of the solar system by the gravitational pull of passing stars. The second theory involves the Sun's ability to form comets. It explains that while the Sun is orbiting, it will pass throuhg clouds of dust and gas. As the Sun travels into these clouds, the gravitational force from it pulls the dust and gas particles together to form the comet's nucleus.

Comets Don't Let Comets Orbit Drunk

It is true that comets don't drink in space, but that hasn't prevented them from crashing into other objects. The first example to prove this statement is that of the famous comet, 6 to 9 miles across, that killed off those large grusome creatures. No, not your mother-in-law, but those other large and grusome creatures otherwise known as dinosaurs. The Tunguska event is a good example of a hit and run driver. In June, 1908, a 100,000 to 1,000,000 ton comet struck central Siberia traveling at 100,000 km/hr. Talk about going over the speed limit. This comet after flattening 2,000 square kilometers of pine forest, burning everything there, and creating winds that threw people and shook buildings, had the nerve to vaporize into thin air without leaving a crater. The last example refers to the most recent crash of the comet P/Shoemaker-Levy 9 with the planet Jupiter. This was the first collision in which there were human witnesses. The comet, now totalled, was broken into 21 pieces by Jupiter's gravitational pull. So, during July 16-22, 1994, the accident occured on the night side of the planet. Although these cases were devastating, average comets disintergrate before they strike a planet with an atmosphere.

Did You Take a Shower Today?

Have you taken a shower today? That's a good question isn't it? Well you wouldn't mind being asked this question if you knew what type of "shower" the question was reffering to. During the sublimation process of a comet, rocks are released along with the gas and dust particles. These rocks stay in the orbit with the comet, and these orbiting rocks create what is known as a meteroid stream. If a planet passses through this stream, a meteor shower results. When the Earth orbits through comet Swif-Tuttle's meteroid stream in October, we exeperience the Orionid Shower. The Eta Aquarid meteor shower travels with the Halley Comet, and the Perseid shower can be seen every year between the 9th and 13th of August.


Some Known Comets

Halley


Witnessed for over 2,000 years, the first comet to prove that some comets are a part of our solar system, was first observed by Chinese civilizations in 240 B.C. Edmond Halley predicted the 1758 apperance of the Halley comet by comparing the dates of three prior sightings. The oblong nucleus of this comet is said to be black and contain carbon and oxygen due to the results recorded 1985-1986. Geyser-like eruptions expel the gas and dust from the comet's head.

Chiron


The Chiron comet, named after the wisest of the Centaurs in Greek mythology, has not been traveling in its current orbit for long. The group that this comet has been placed into was given the Centaurs title to highlight their comet/ asteriod chacteristics(A Centaurs is a half man half horse creature). Although the Chiron comet has an obvious coma, its volume is 50,000 times larger than that of a standard comet. This feature would be more typical of a large asteriod. The Chiron was discovered by a man named Charles Kowal Nov. 1, 1977. This 50.7 year orbiting comet, and others like it are believed to have come from the Kuiper belt.

Hale-Bopp


The brightest star that has been witnessed in our solar system, with a magnitude of -1, is the Hale-Bopp comet. This comet, discovered by Alan Hale and Thomas Bopp July 23, 1995, will "burn out" its volitile gases during the next year. The average size of a comet's nucleus is about 3 miles in diameter, but this comet is said to be 19-25 miles across. While observing the passage of this comet, a clear separation of debris from the comet can be seen for the first time.

If you wish to observe more information on comets or other astronomy topics, I suggest that you visit the following links:

My astonomy teacher's web page contains more astronomy links as well as useful information on certain topics.

You may also visit my friends' web pages.
Aditya Burkule: Galaxies
Richard Bowen: Space Technology
Katrina Fobbs: Blackholes and Quasars

This page was writen by Claire Elliott in the astronomy class of BCC/ Broward County July 1998