Comets

[Jim Colyer]

http://jimcolyer.com/papers/entry?id=429;fa=3

Comets achieved popularity with the publication Carl Sagan's book "Comet" and the 1986 appearance of Halley's comet. Sadly, Halley's was a bust, and the chief interest centered around the pronunciation of Edmund's name. After decades of calling him Hey-le, he suddenly became Hal-le. Comets are notoriously unpredictable, and Halley's has not been the only dud. Kohoutek fizzled in 1973, and there was PANSTARRS in 2013. I drove 30 miles east of Nashville hoping to see it. PANSTARRS was low in the west after sunset and managed to elude me. There is always hype around comets. We read that they may be as bright as Venus or a full moon. Never the case! Comets have been called "dirty snowballs." As I write, astronomers are anticipating Comet ISON in November, 2013. I will make an effort to see it! I have seen three.

It was the unpredictability that inspired fear in ancient peoples. The sun, moon, planets and stars had regular patterns, but comets appeared seemingly for no reason. They were considered bad omens.

Comets are stuff left over when the solar system formed. There may be a trillion of them in the Oort Cloud in the outer reaches of the solar system. The lighter stuff was blown out when the sun turned on. This spherical region is named after Dutch astronomer Jan Oort. There are about 4000 known comets.

Comets circle the sun like the planets. Their orbits are wildly elliptical. As comets approach the sun, frozen methane and ethane gases vaporize to form tails millions of miles long.

Comets have two parts, heads about six miles in diameter and fan-shaped tails. There are two kinds of tails, dust tails that reflect sunlight and blue gas tails that shine when excited by the solar wind. Ices vaporize as comets come near the sun.

Comets approach the sun head first with their diffuse tails trailing. When leaving the sun, it is the other way around. Tails take the lead. Either way, tails point away from the sun, pushed back by the solar wind.

Comets cause meteor showers. As comets orbit the sun, particles get strung out along their orbits in streams. When our planet crosses these streams, meteoroids no larger than apple seeds enter our atmosphere and flare up because of friction.


1 The Great Comet of 1744
This one had six tails.

2 Halley's Comet 1910
Halley's Comet was recorded in 467 BCE (before the common era). Edmund Halley's research led him to predict that the same comet was returning every 76 years. When it appeared in 1758, it made him famous. Halley's is a short-period comet. Long-period comets take thousands of years to return. Halley's was spectacular in 1910, and the earth passed through its tail.

3 Comet Ikeya-Seki 1965
I saw this comet, Halloween morning. Ikeya-Seki was a "sun-grazer," coming close to the sun. It broke into two pieces after emerging from behind the sun. Its tail was 90 million miles long, one of the longest ever.

4 Comet Bennett 1970
Passing near the Cygnus Star Cloud, this comet sported a 10 degree long tail. I was in the army and out of astronomy. Discovered by John Bennett.

5 Comet West 1976
The head of Comet West was as bright as Jupiter, and its tail was 20 degrees long. I was at Peabody College in Nashville and unaware of it. Discovered by Richard West.

6 Shoemaker-Levy 1994
The Hubble Space Telescope imaged this comet as it crashed into Jupiter. It was discovered while orbiting Jupiter, captured by the gas giant's gravity. 21 fragments hit Jupiter. Gene Shoemaker died in a car accident in Australia in 1997.

7 Hyakutake 1996
I saw Hyakutake. Its head reached zero magnitude and was seen by many people around the world. Its blue gas tail was 12 million miles long. Dust tails are brighter than gas trails.

8 Hale-Bopp 1997
Discovered by Alan Hale and Thomas Bopp, Hale-Bopp became the darling of comet enthusiasts. It was brighter than zero magnitude for two months and visible in amateur telescopes for two years. Hale-Bopp showed two distinct tails. Its dust tail and blue gas tail were widely separated. I saw it from my parents' front yard on Aiken Road. Hale-Bopp was bright because of its size. Its head was 25 miles in diameter. Comets are brighter when closest to the sun because more ice is vaporized.

9 Comet ISON 2013
ISON is coming in November. .