For millennium, one of the most fascinating objects observable in the night sky, and sometimes even in the daytime, have been comets. They have been believed to be the harbingers of disasters and signs of good luck. Regardless of how they’ve been perceived, they have been around far longer than man has been around to observe them.
Comets still have a lot of mystery surrounding them, but the knowledge about them steadily grows. We do know that they orbit the sun, usually in eccentric ellipses. Some of them pass well closer to the sun than the orbit of Mercury, and many do not follow the line of the ecliptic, or the general plane on which the planets orbit the sun.
As comets get closer to the sun, they produce tails from out gassing of frozen gas and dust. The tails can extend millions of miles from the core of the comet, called the Coma. An interesting to note fact is that because of the solar wind, or highly energized particles produced by the sun, that flow into space, the tail of a comet always points away from the sun. This means that after it swings around the sun, the comet actually follows it’s tail.
Comets can be short period or long period. Short period comets are those that make a complete orbit around the sun in a very short period of time. The shortest period comet known is Enke, which takes only about three and a third years to orbit from its position near Jupiter, around the sun, and back again.
Haley’s Comet is an example of a long-term comet. It takes approximately 79 years to orbit the sun from its position well beyond the orbit of Pluto. Some comets may take thousands of years to make the orbit, and still others sit very far outside the orbit of Pluto, awaiting a nudge from another body to cause it to start making an orbit. Scientists estimate that there are many hundreds of thousands of comets, all together.
At the center of a comet is a mass that can be one to three miles in size, and made of both rock and frozen gases. The frozen gas is the main contributor to the comet’s tail.
Spectrographs have shown that comets consist of mostly carbon, hydrogen, nitrogen, and oxygen, both in elemental form and as molecules. Sodium, magnesium, iron, and nickel have also been found, as well as amino acids (building blocks of life).
Comets are fascinating, and most are actually found by amateur astronomers. But they are also signposts that give us insight into what the early solar system was like. The majority of comets have never been closer to the sun than the orbit of Pluto, so they are left virtually unchanged since our solar system was first formed.
Watch the night sky. If you are lucky enough to sight a comet, report it. It will be named after you if you are the first to see it, and your name will be immortalized. At the very least, you can enjoy the spectacle.