Speaking of Science Column January 19,  2009

Comet a dirty snowball of bits of rock and ice

From the common long “tails” — that resemble hair — of most comets, the name comet comes from Greek, meaning “long-haired.”

Like our moon and planets, comets are made visible by reflection of sunlight and do not glow from their own heat. With telescopes, comets may be first discovered while as far from Earth as our outer planets. They become visible to naked eye or with binoculars only when they come closer to the sun.

Comets travel in long elliptical orbits, mostly from beyond Pluto and around the sun and then back out into space.

Comets are thought to be made of bits of rock and ice, like a dirty snowball. The solid portion in the head of the comet, called the nucleus, may be as small as 2 to 10 miles diameter, but as it nears the sun, the sun’s heat causes gases to vaporize, and the particles spread out to form the “coma.” The coma is like a cloud of dust that forms a bright hazy ball around the nucleus.

The coma may be larger than Earth (10,000 miles in diameter or more) and from this, the tail spreads out and may be longer than the sun’s diameter, a million miles or more.

The tail seems to stream out behind the comet as it moves in toward the sun but, in fact, the “solar wind” is pushing the smaller particles away, and the tail always points away from the sun.

So as it moves away from the sun back into space, the tail leads the way. As the comet moves farther from the sun, most of the tail is pulled back into the nucleus by the gravity of the comet and soon is again too small to see.

Some comets complete their orbits in a period of three to 10 years, but many have periods of 50 to hundreds or even thousands of years. Halley’s comet is one of the most famous.

It has been seen for hundreds of years and returns about every 76 years.

Comets are named for the person who discovers them. Hale-Bopp Comet was discovered in 1996. Two men discovered it at about the same time, so it got the double name. It was first visible to the naked eye or with small binoculars about Jan. 1, 1997. It was very large, bright and beautiful from early March to early May of that year.

A more typical comet was Hyakutake (Hyah-koo-tah-kay), named for the Japanese man who discovered it. It wasn’t nearly so bright as Hale-Bopp and was visible to the naked eye for only about a week, in late March of 1996.

The tail of a comet is mainly dust particles spread out thinly. In addition to the dust tail, which is the brightest and most visible part, some have an “ion” tail composed of charged particles.

Hale-Bopp had a visible blue ion tail appearing to one side of the dust tail.

Pieces of rock from the size of sand to the size of boulders regularly fall from space into Earth’s atmosphere. These objects enter Earth’s atmosphere at speeds of 20,000 to 30,000 mph. Upon hitting the atmosphere at such speed, the collisions with air molecules cause heating to such high temperatures that they glow.

When these fall in the dark of night, the bright visible streak across the sky is called a meteor.

They are commonly called “falling stars.”

The heat causes most meteors to vaporize or break into small pieces and fall to Earth as dust.

Some of these rocks from space, if the size of a baseball or larger, fall to the ground as a solid rock, and these rocks are called meteorites. Some meteorites weighing a hundred pounds or more have been found. These are rather rare and are often placed in museums.

Some meteorites seem to be left scattered along the orbits of comets. Even years after a comet passes, Earth crossing such an orbit may produce an unusually large number of visible meteors. These events are called “meteor showers” and may last from several hours to several days.

A number of meteor showers occur at approximately the same time each year, when Earth’s orbit crosses the orbit of some past comet. One of these is the Leonid meteor shower, which occurs between Nov. 14 and 20.

It is named Leonids because the meteors appear to come from the constellation Leo (the Lion).

Another yearly meteor shower is the Perseid shower, which may last several weeks, but peaks (when it has the most meteors per hour) about Aug. 12 to 13. This shower seems to radiate from the constellation Perseus.

Bert Snyder taught junior and high school math and science in Colorado for 25 years. He also coached a ski team for five years and taught mountain training and winter survival while in the U.S. Army.

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