Comet

What is a comet?

A comet is a large piece of rocks and ice that occasionally gets close to the Sun. When it gets very close to the Sun, it "grows" a tail that can sometimes be seen at night without a telescope.

Facts about Comet

• Comets are often described as a giant "dirty snowball", because they are mostly made up of ice.
• Comets have two "tails", one made up of rocks and dust, the other made of gas.
• Comet tails always point away from the Sun, wheither they are approaching to or receeding from the Sun.

What is comet's surface like?

The surface of a comet is mainly chunks of ice mixed with rocks, usually about the size of a grain of sand or even smaller. When the comet is far away from the Sun it is cold and airless, and you would detect almost no gravity. If you were an astronaut standing on the surface of a comet, you could literally jump off the comet and fly into orbit around the sun using just the energy from your own muscles.

When a comet gets close to the Sun, however, things become a bit different. The ice begins to thaw and geysers form, which start to send out gasses, water vapor, and dust. These gasses can't be held by the gravity of the comet, so it drifts away from the comet. This is what causes the tail of a comet to form.

How big is a comet?

As far as objects in our solar system are concerned, they are actually quite small, from a few kilometers in diameter to about 100 kilometers across. The tail of a comet, however, can be millions of kilometers long, sometimes stretching from the orbit of Mars to the orbit of the Earth.

Can I see a comet in the sky?

Not all comets are visible without a telescope. Comets that produce a spectacular tail are called a "great comet" and are quite unusual. Being able to see one of these comets only happens about once every century, which is why seeing a comet like that is considered a once in a lifetime experience. When a comet does come close to the Earth, it can take in a very large portion of the sky and is different from anything else you might see in the sky.

Other comets can be seen with a telescope, and these are more common. Sometimes a comet's orbit will change due to passing very close to some planets like Jupiter or the Earth, and then all of the ice can "boil off" by repeatedly coming close to the Sun. When this happens, a comet is considered to be "dead", although it is still an interesting thing to study. Often before this happens, a comet will break apart into hundreds of pieces or more.

When a comet breaks apart, it creates a trail of smaller rocks and pebbles in space that drift along in approximately the same orbit that the original comet traveled around the sun. These are the recurring meteor showers that happen from time to time, and most major meteor showers have now been identified either with an existing comet or the remains of a comet that was observed eariler, usually in previous centuries. When the Earth travels through this "swarm" of rocks left behind, you can see shooting stars or meteorites at night.

How many comets are there?

The exact number is hard to come up with, but there are thousands of comets in our solar system. Many of these comets simply burn up by falling into the Sun, or have orbits that cause them to leave the solar system. The spacecraft SOHO has been used to identify almost 1000 comets in just a few years. That is particularly significant because SOHO was not even designed to study comets, but instead was supposed to be strictly a tool to study activity on the Sun.

Of comets that orbit the sun and come close to the Earth on a regular basis, the number discovered so far is about 160 comets.

How is a comet named?

A comet is usually named after the astronomer who first discovered it. When several people are involved in its discovery, sometimes you will see multiple names on a comet, like Comet Hale-Bopp, or Comet Shoemaker-Levy. It is generally considered to be a great honor to have a comet named after you.

Comets that were observed in the past weren't always named like this. Instead, they had names like "The Great September Comet of 1882", where only the year that it was observed was mentioned in the name of the comet. This changed, however, when Edmond Halley was able to determine that comets which appeared in 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759. This has become known as Halley's Comet.

What are some famous comets in history?

• Halley's Comet - Perhaps the most famous of all comets, and this was the first comet to be identified as a recurring comet.
• Comet Encke - The second comet to be identified as a recurring comet.
• Comet Shoemaker-Levy - This was the first comet to have been observed hitting another body in the solar system. In this case, it did a direct hit into the planet Jupiter, perhaps the most studied astronomical event in history.

Where do comets come from?

There are many theories about how comets form, but the current theory most widely accepted by astronomers is that they come from the Oort Cloud, a region of the solar system that is about as far as you can travel from the Sun but still be primarily affected by our sun and not other stars.

Do Comets bring bad luck?

In ancient times people didn't have a very good understanding of what Comets really were or where they came from. They were seen as very unusual objects in the sky, and very temporary in nature as well. In some societies it was often a sign of bad events in the future when a comet arrived, associated with the death of a king or a significant military defeat. In other countries Comets were considered to bring good luck, with increased fertility and more food. The ancient Chinese astronomers seem to have done the best job of actually recording when comets appeared in the sky, and left detailed descriptions of what they looked like and approximately where in the sky that each comet was seen.

Even as recently as the 1910 appearance of Halley's Comet there was widespread panic when it was discovered that the Earth might pass through the tail of that comet. The panic was over having the gasses from the comet flood the atmosphere of the Earth with poison. The reality was that there is so little gas in a comet tail that there is no measurable effect in the content of the Earth's atmosphere when an event like this occurs.

More recently popular media culture has produced news reports and motion pictures with the sensational idea of a comet striking the Earth and causing widespread destruction. This does have some basis of fact when historical and geological evidence suggests that in the past comet fragments or even whole comets may have struck the Earth. An explosion in a remote part of Siberia (eastern Russia) occurred in 1908 with effects very similar to a nuclear explosion in terms of damage to the environment. A substantially larger explosion called the K/T Event is thought to have caused the extinction of the dinosaurs. More recently astronomers were a witness to the impact of Comet Shoemaker-Levy into the planet Jupiter. Had that comet hit the Earth instead it would have destroyed almost all of the cities on the Earth.