What are comets and why do they have tails?

Since the beginning of recorded history, people have been in awe and in fear of comets, star-like objects with "long hair" that appeared in the sky unannounced and unpredictably. We`ve discovered that these star-like objects are dirty-ice formed when our solar system began around 4.6 billion years ago. They are the least-changed objects in our solar system and, may give us essential clues about how our solar system was formed. We can predict the orbits of many of them, but not all.

New discoveries all the time

Around a dozen of these objects are discovered each year. Short-period comets are more predictable because they take less than 200 years to orbit the Sun. Most come from a region of icy bodies beyond the orbit of Neptune. These icy objects are variously called Kuiper Belt Objects, Edgeworth-Kuiper Belt Objects, or trans-Neptunian Objects. Long-period comets are less predictable, many of them come from a distant region called the Oort cloud about 100,000 astronomical units (that is, 100,000 times the average distance between Earth and the Sun) from the Sun. These bodies can take as long as 30 million years to complete one trip around the Sun. And you thought a year was a long time! As many as a trillion comets may be in the Oort cloud, orbiting the Sun near the edge of the Sun's gravitational influence. Now that`s a lot of comets!

What are these things made of?

Each of them has only a tiny solid part, called a nucleus, it`s no bigger than a few kilometers across. The nucleus contains icy chunks and frozen gases with bits of embedded rock and dust. In the center, the nucleus may have a tiny, rocky core.

As one of these objects nears the Sun, it begins to warm up. It gets bright enough to see from Earth while its atmosphere, called the coma, gets bigger. The Sun's heat causes ice on its surface to change to gases, these gases look kind of like a neon sign. "Vents" on the Sun-warmed side may release fountains of dust and gas for tens of thousands of kilometers. The escaping material forms a coma that may be hundreds of thousands of kilometers in diameter.

The pressure of sunlight and the flow of electrically charged particles, called the solar wind, blow the coma materials away from the Sun, forming the objects long, bright tails, which are often seen separately as straight tails of electrically charged ions and an arching tail of dust. The tails of a comet always point away from the Sun.

Safe travels

Most of these star-like objects travel a safe distance from the Sun itself. Comet Halley comes no closer than 89 million kilometers from the Sun, which is closer to the Sun than Earth is. However, some of them, called sun-grazers, crash straight into the Sun or get so close that they break up and vaporize.

Impacts from these objects played a major role in the evolution of the Earth, primarily during its early history billions of years ago. Some believe that they brought water and a variety of organic molecules to Earth.

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More comet resources

Halley's Comet