Year of the Comet: completely unique type of cosmic entity
First, and perhaps most importantly, comets are different from meteors- which are interplanetary rocks that very briefly hit earth's upper atmosphere at extreme speeds- nearly always vaporizing in less than a second. Comets, on the other hand, are much larger and don't hit the earth..usually. Some believe the last comet that struck mother earth created the Gulf of Mexico and caused extinction of the dinosaurs.
Comets are treats, much rarer than the nightly stars and planets. They are a completely unique type of cosmic entity, which makes viewing them both intriguing and unpredictable. A comet derives its name from the "coma", a small atmosphere around the central body which transitions to a tail. The term comes from a Greek word- translated "long-haired". Comets generally orbit the sun in extremely elliptical orbits, coming in 2 broad categories: short period comets that orbit the sun in less than 200 years and long period comets whose orbital period can range into the thousands and even millions of years.
Comets come from the Oort cloud, a spherical shroud around the outer reaches of our solar system about 1 light year from the sun. It is a region of planetesimals (small icy objects) and was theorized in 1950 by Jan Oort, a Danish astronomer.
Comets are composed of rock, dust, water and a variety of gases such as carbon dioxide and methane. Comets range in size from a football field wide to about 25 miles across. Their tail is a distinguishing characteristic, which also comes in 2 parts: the ion or gas tail and the dust tail. When a comet approaches the sun, heat and the solar wind begin to vaporize the body, which causes a small atmosphere to form, which is then "blown" away from the comet's nucleus by the solar wind. This gas tail always points AWAY from the sun, even when the comet is outbound- so that the tail leads the comet.
Throughout history, comets have gotten a bad rap. Their unexpected appearance in the night sky has generally been perceived as an omen of doom. Ancient people often believed that such a strange celestial object was a sign from the gods. Most of space has a clockwork regularity to it, but not comets. They were perceived as hairy stars with tails, not very consoling. Some felt they might be celestial monsters.
Since their spooky luminous tails stood out among the rest of the stars, it clearly meant something bad was coming. Everyone who looked up could see the comet. From early
Chinese records through the Middle Ages, comet sightings have been noticed by earth dwellers. The famous French Bayeux tapestry records the Norman conquest of England in 1066 AD, with what would later be called Halley's Comet in the sky- a comet heralding momentous change.
Common thought among philosophers from Aristotle in 350 BC through the Renaissance was that comets were events of the upper atmosphere, grouped together with the aurora borealis and even the Milky Way. But from 1609, when Johannes Kepler introduced his laws of planetary motion, comets were then understood as eccentric occupants of the solar system.
Sir Edmond Halley linked 3 prior viewings of a bright comet and then predicted its return in 1759. He was right, and the first Periodic comet was named in his honor- that is a comet whose orbit is calculable and can be anticipated. Other comets are initially discovered as dim wanderers among the planets. More of them are found every year through better technology and increasingly more powerful telescopes being built.
A new object was discovered only last September and will be gracing our morning skies with tantalizing potential later this year. Comet ISON (C/2012 S1) was discovered by 2 Russians, Nevski and Novichonok, serving as part of the Russian based International Scientific Optical Network (ISON). A dim image of the distant dark icy object was found moving slowly against the background of stars. Since then, its movements have been precisely calculated revealing not only a new comet but a "Sun Grazer" as well. Comet ISON will boldly dive toward the sun all year, until coming within about 1 million miles of the solar surface. But this may be the one and only time anybody will see the beautiful sky object.
Comet ISON is in a "hyperbolic orbit" and is expected to be flung far into space following its very close
encounter with the sun. As a result, some astronomers believe this comet will escape the sun's gravitational field forever.
And when the comet flies around the sun with a potentially bright tail to dazzle everyone, what might we expect? The sky measures 180 degrees from horizon to horizon. Comet ISON's tail could extend out nearly 60 degrees, about one third of the sky.
An amazing sight..if it happens. Some estimates have touted a brightness of magnitude -12 (the lower number the number, the brighter). That would beequivalent to a full moon, perhaps even bright enough to see during the day, like the Great Comet of 1680 or more recently Comet McNaught. The brightest comet in several generations was the 1965 appearance of Comet Ikeya-Seki, at magnitude -10. For comparison, the dimmest night object human eyes can perceive under ideally dark skies is magnitude +6.
There are two other possible fates for this year's star player "ISON". It could break up into a "String of Pearls", like Comet Shoemaker-Levy 9 did in 1994 when it crashed into the planet Jupiter. Or, it could just not perform well at all and fizzle. Long range comet predictions are like long range weather forecasts. They are fuzzy at first, but become clearer as time goes on.
If all goes well, here is what we may see in November and December. The comet will gradually grow brighter through mid-November rising in the eastern skies around 4 AM. As late November arrives, a long tail may develop, visible before sunrise. Following perihelion, Comet ISON's close brush with the sun, it should emerge in the morning sky with a rapidly lengthening bright tail, easier to view for the first week of December. By mid-month, the tail will begin to lose its bright sheen. By Christmas the comet moves into the northern skies, visible after 1 AM in the northeast. Finally by 2014, ISON will once again become a telescope object only, traversing an area of the sky near Polaris, the north star. This year's big show is still a big question mark. But the potential is there for perhaps the greatest comet event in many decades. Time will tell, but until then..keep watching the skies.