See first of four comets tonight
BY DR. SCOTT M. LIEBERMAN
Special to the Tyler Morning Telegraph
Going back into ancient times, humans made note of the relative consistency of the night sky.
They noted the predictable movements of the sun and the moon and observed the planets in their slow movements among the stars.
While centuries were spent defining why these movements were what they were, they were, for the most part, consistent. When something unpredictable happened, there was often great concern.
Comets were just such an event. Often moving among the night or even the daytime sky with some rapidity over the course of days, their appearance often triggered concern.
They often were associated with calamities and the rise and fall of people in power. We know that this is myth, and that as long as they don't strike the Earth, the direct effects to the Earth and man are limited.
Of course, we now know a great deal more about comets, and 2013 is going to a very big year for them, starting this week, with the first of four notable visible comet passes.
The nature of comets is much better understood then at any time in history. We have been able to track them and predict their orbits for several hundred years now, and we have sent unmanned spacecraft to them and hit them with colliders to study their makeup.
There are about 4,000 known objects that are currently tracked. Comets are a combination of rock and ice, mostly left over from the formation of the solar system.
Short-period comets (orbits under 200 years) originate in the Kuiper belt, an area beyond the orbit of Neptune, but Long period comets (from 200 years to millions of years) originate in a region beyond the orbit of Pluto, in an area known as the Oort cloud.
They tend to stay there unless their orbit is perturbed by impact into another body, or a gravitational pull changes their path, which sends them into an elliptical orbit heading to the sun.
Some comets only make a one way trip, which can end with an impact into a planet as happened in 1994, when Comet Shoemaker - Levy 9 broke up and hit Jupiter, or fly a direct path into the sun.
Usually they travel into the inner solar system, swing around the sun and head back out to outer solar system.
Many will stay on that looping orbit and make many additional passes over the years, and are very predictable. For example, Halley's comet makes a pass every 75.5 years. As they approach the sun, the warmth from solar radiation causes the comets to warm and create plumes of water vapor and other gases to be ejected into space, forming a temporary atmosphere around the nucleus of the comet called a coma.
The solar wind carries the gas away from the coma, which then becomes the tail of the comet. Sometimes the comet can suddenly break up, or as was seen in 2007, a sudden release of gas can cause it to brighten greatly as was seen with Comet Holmes.
This is the week we will be able to see Comet Pan-STARRS. It is making what is believed to be its first orbital pass around the sun, and since it is considered a fresh comet, it might put on quite a show.
Astronomers discovered the comet in June 2011 with the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) telescope in Hawaii, for which the comet was named. It is believed that the comet will have an orbital period of more than 110,000 years. It is going to be visible in the western sky near the setting sun, for about an hour after sunset before it, too, sets.
Tonight, it will be very near the crescent moon, and on Wednesday it will be visible between the crescent moon and the setting sun. Binoculars will likely be necessary, but if conditions are right and the tail grows as expected, it might be visible to the naked eye.
It will likely be visible as it starts moving away from the sun for the next few weeks, however, but growing fainter over time.
There are three others to watch this year,
too. Comet Lemmon will pass around the sun on March 25 and likely be visible though May. Comet Encke, which visits every 3.3 years and will be making its 62nd known visit in October, will be rather faint. Comet ISON, named for the International Scientific Optical Network (ISON), is in a class of comets known as sun grazers and will come within 680,000 miles above the sun Nov. 28, which could cause it to be the most spectacular of the year, if it does not disintegrate into the sun.