Comets poised to grace Sisters' skies

Many years from now, when folks reminisce about significant events that have taken place in their lives, they may very well remember 2013 as "the year of the comet."

Two potentially bright comets are currently headed toward the sun. The first one is due to arrive by mid-March, the other, perhaps much brighter, bursts on the scene in November.

What are comets, and where do they come from?

Our solar system came into existence some 4.6 billion years ago from the collapse of a giant molecular cloud. The sun, planets, moons, asteroids, and comets formed from this cloud.

Surrounding our solar system is a vast cloud of icy objects, called the Oort cloud, which extends outward one light-year from the sun. Another disk of icy bodies, called the Kuiper Belt, extends from the planet Neptune out billions of miles from the sun. The dwarf planet Pluto is a member of the Kuiper Belt.

The Oort cloud contains trillions of comets and other icy bodies. Objects in the outer portion of the outer Oort cloud are only loosely bound to the solar system, and thus are easily affected by the gravitational pull both of passing stars and of the Milky Way galaxy itself. These forces occasionally dislodge comets from their orbits within the cloud and send them toward the inner solar system. Most long-period comets, with orbital periods of 200 to millions of years, come from the Oort cloud.

Short-period comets usually come from the Kuiper Belt and have orbital periods of about 200 years. Halley's Comet is one of these, with an orbital period of 75-76 years. Its orbit extends just past the orbit of Neptune.

Comets are dirty snowballs. They are composed of loose collections of ice, dust, and small rocky particles. The ice is mostly water; but you also find frozen carbon monoxide, methane, and ammonia. Comets also contain a variety of organic compounds.

Most of the water on Earth came from comets hitting the earth early in its history. Some of the building blocks of life on Earth also came from comets.

Comets range in size from several hundred feet to approximately 24 miles across. The nucleus of Halley's Comet is nine miles long and four miles wide.

In the outer solar system, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size.

As a comet approaches the inner solar system, radiant energy from the sun causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the sun's radiation pressure and solar wind cause an enormous tail to form, which always points away from the sun. The coma may be larger than the sun, and ion tails have been observed to extend 93 million miles or

more.

Comet C/2011 L4 PANSTARRS was discovered in June 2011 using the PANSTARRS telescope located at the summit of Haleakala on Maui, Hawaii. PANSTARRS, which stands for Panoramic Survey Telescope And Rapid Response System, consists of four 70-inch telescopes equipped with the world's largest digital cameras, capable of recording almost 1.4 billion pixels per image.

This comet, which is a long-period comet with an orbit of approximately 110,000 years, will pass closest to Earth on March 5 at distance of 93 million miles, and be nearest the sun on March 10.

Original estimates predicted the comet would brighten to roughly magnitude 0. This is as bright as some of the most brilliant stars in the sky, making the comet easily visible to the naked eye. However, in January 2013 there was a noticeable slowdown in the comet's brightening, suggesting that the comet may be a magnitude dimmer than originally thought. Even so it will still be very

bright.

The comet's rapid northward motion, owing to its orbital inclination of 84 degrees to the plane of the solar system, will gradually carry it away from the sun and into the western evening sky. If there is a significant tail, it would probably be seen standing almost straight up and perhaps tilted slightly to the right, relative to the horizon.

On the evening of March 12, just two days after its closest approach to the sun, the comet will be positioned less than five degrees from a slender crescent moon, making for a very picturesque scene.

While on successive evenings in March comet PANSTARRS will grow fainter as it recedes from the sun, it will also set later, allowing for greater viewing opportunities.

But comet PANSTARRS is just a warm-up.

Discovered on September 21, 2012 by two Russian astronomers, comet C/2012 S1 ISON has the potential to become the "comet of the century" later in the year. ISON stands for the International Scientific Optical Network observatory located near Kislovodsk, Russia.

The orbital path of comet ISON suggests that it is an Oort cloud object making its first journey to the inner solar system. If the comet survives a very close approach to the sun (to within just 680,000 miles on November 28), it will leave the solar system, perhaps never to

return.

If this comet performs as expected, it might become as bright as the full moon, according to some astronomers. This would truly be a spectacular sight! But that event is down the road apiece. Stay tuned for further information regarding progress of this comet.

The Sisters Astronomy Club (SAC) will set up telescopes and invite anyone to come by to view these comets. Visit SAC's web page at http://sistersastronomyclub.org.