Comets

comet
Kohoutek Comet - Image from NASA

If you see a bright light in the night sky and you aren't sure whether you are seeing a comet or a meteor, you can always tell the difference between them because a comet will remain in the sky much longer. Comets can remain visible for days or weeks. In contrast, a meteor is just a brief flash of light that disappears very quickly. Comets also appear to move more slowly than meteors.

Meteors are much more common than comets. Comets only appear about once a year, but meteors occur frequently (Maran, 2005).

Hale-bopp comet
Hale-Bopp Comet - Image from NASA

The most famous comet is probably Halley's comet. Halley's comet was last visible from Earth in 1986. It won't come around again until 2061.

Another famous comet is the Hale-Bopp, which was visible from Earth back in 1997. It was discovered by Alan Hale and Thomas Bopp. Unlike meteors, comets are named after the people who discover them. The Hale-Bopp comet isn't expected to come around again for about another 4,000 years.

There are many other comets that aren't as well known as these two. Some of the lesser known comets are even brighter than the more famous ones.

Some comets are so bright they are visible during the daytime.

If you spot a new comet (or any new space object), to report it go to the Central Bureau for Astronomical Telegrams (CBAT) website. On this site you'll find the guidelines for reporting space objects as well as an e-mail address to report your object.

What Is A Comet?

close up of comet

Comets may look pretty as they travel across the night sky, however, they are really just chunks of ice and rock that are in an elliptical orbit around our Sun. They consist of three parts: the nucleus, coma, and tails.

What astronomers call the nucleus of the comet is the irregularly shaped ball of ice, dust, and rock. The ice is mostly frozen water, but also consists of other frozen gases, and even some carbon containing compounds like methane. The nucleus emits no light of its own.

The closeup of the comet Wild 2 was taken by NASA's Stardust spacecraft in 2004. The image shows the comet's nucleus, which is about 5 km (3.1 miles) in diameter.

From Earth, the visible parts of a comet are the coma and the tails, which reflect sunlight. The coma is made up of a cloud of gas and dust that surrounds the nucleus. One tail is made up of dust particles that reflect yellow light and the other tail consists of gas that usually reflects a blue color (Maran, 2005).

Comets sometimes come close to planets and moons and occasionally run into them during their orbit. For example, in 1994 the Shoemaker-Levy 9 comet collided with the planet Jupiter. Before the impact, the comet had already broken up into several pieces due to a previous orbit when the comet came too close to Jupiter's gravitational field, which busted the comet into smaller pieces (Chaisson and McMillan, 2005).

Comets also lose ice and dust as they travel through space. These cometary fragments remain in space and become meteors when Earth's orbit takes us through them (Yeomans, 2005).

Sometimes when comets get too close to the Sun during their orbit they completely break apart.

Origin of Comets

Scientists think that comets formed 4.6 billion years ago at the same time as the rest of the planets formed. Astronomers also believe that some of the water and carbon compounds necessary for life may have been brought to Earth by comets (Yeomans, 2005). Because comets travel so far out into space, and sometimes collide with other planets, this has implications for the possibility of finding the presence of water and carbon compounds capable of supporting life on other planets as well.

If comets were able to bring water and carbon containing compounds to Earth, then it is very likely they were able to bring these molecules to other planets and moons.

REFERENCES

Chaisson, E. and McMillan, S. (2005). Astronomy Today. Pearson Prentice Hall: Upper Saddle River, NJ.

Maran, S. P. (2005). Astronomy for Dummies. Wiley: Indianapolis, IN.

Yeomans, Donald K. "Comet." World Book Online Reference Center. 2005. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar125580.

Images of the comets Kohoutek and Hale-Bopp: Courtesy of NASA Johnson Space Center (NASA-JSC).
Image of Wild 2 comet: Courtesy of NASA/JPL-Caltech.