Short Tour of Comets

1. Their Composition:

Comets are often called: "Dirty snow balls". They are mostly made of about 83% water, 15% dust and silica, 1% carbon gases, 0.5% methane, amoniac and other gases.

A - Short period: - Those that take some 20 years or less

B - Intermediary: - For those whose orbit will take from 20 to 200 years

C - Long Period - Those whose orbit will take more than 200 years to go around the Sun - celles de longue durée - pour celles qui prennent plus de 200 ans à parcourir leur orbite.

We have 2 great sources of comets in our Solar System: the Kuiper Belt ant the Oort Cloud.

The Kuiper Belt is situated beyond the orbit of Neptune. It contains over ten times more objects than the Oort Cloud. Those objects were formed along with the Solar System; in fact, they are left overs from the formation of the Solar System. As for the planets, these objects are found on the same plane as the planets. We may wonder if these are any danger to interplanetary probes? Well, since the belt is not a sphere, the probes can travel on either side of the plane of the belt, contrary to the Oort cloud that is spherical, but then so much sparsely distributed.

The Oort Cloud is a sort of reservoir of thousands of billions of comets! The clouds covers a distance up to some 2 light years, that is just about half the distance to the nearest star: alpha Centaurus! And even though there is an enormous amount of objects forming the Oort cloud, it is practically empty space out there.

The first probe that was ever sent to study comets was ICE (International Cometary Explorer) that was sent to study comet Giacobini-Zinner' tail. It was sent on September 11, 1985. Then in March 1986, the Giotto probe was sent to study comet Halley's core, and also probe Deep Space I, was sent to study comet Borrelly. The on-going Stardust probe will encounter comet P/Wild2 on January 2, 2004. It will take pictures of course, but what will be remarkable, will be the use of a brand new product called 'Aerogel which will collect interstellar matter ans also particles flying off from the comet.

Being so rare, it is quite a feast for amateur astronomers to be able to have a comet, bright enough in the night sky, to be able to see it! When comets get close enough to the Sun, gases get ionized or heated enough to be ejected and become available to the naked eye. We recall comet Hyakutake, Hale-Bopp and Ikeya-Zhang!

When those debris cross Earth's orbit, we get the wonderful meteor showers!

And how about the comet that exploded over Tunguska, in Siberia... the impact of comet SL-9 on Jupiter, and craters left by impacts on Earth...?