WASHINGTON – This week, the western sky will have a visitor from the depths of the outer solar system: the Comet PanSTARRS.
March 12 through 18 is the prime time to view the comet, since it will start to fade after this week. Officially known as Comet C/2011 L4 in order to distinguish it from the other comets discovered by the automated sky survey observatory Pan- STARRS in Hawaii, the comet will finally be visible to observers in the Northern Hemisphere.
Comets are named after their discoverers, so Pan-STARRS gets the credit for finding this icy interloper from the outer solar system in June 2011.
Comets are best described as dirty snowballs. They are frozen leftovers from the formation of our solar system 4.6 billion years ago. They are thought to have a rocky or rubble pile center known as the nucleus a couple of kilometers in diameter that is covered with frozen water and gases.
Over a trillion comets are thought to form the Oort Cloud, a vast spherical reservoir of comets very distant from the sun. A passing molecular cloud, a collision with another cometary body or another gravitational encounter can cause a cometary body to begin its long fall into the inner solar system. Comets such as these and PanSTARRS pass through the solar system only once and are known as long period comets.
Another reservoir of comets lies near Pluto and contains what are known as short period comets. These comets can become regular visitors to the inner solar system, with “Halley’s Comet” being the most famous of our time.
When a comet passes through the inner solar system, the surface of the nucleus is heated by the sun, which causes the water, gases and dust to form an atmosphere around the comet called a coma. The coma can grow to be hundreds or thousands of kilometers in size. Solar wind and pressure from sunlight act upon the coma and can cause a tail to form that flows away from the comet. These tails can be grand in their size and scope, sweeping across whole sections of the sky in spectacular fashion.
Comet Hyakutake in 1996 and Comet Hale-Bopp in 1997 had decidedly different and distinct tails. Comet PanSTARRS has a reported tail, but it is rather small in the sky due to the large distance between us and the comet.
Comet PanSTARRS has been visible to Australian observers for a few weeks now and has been spotted fairly easily in their western sky at twilight, sporting a short but visible tail. The comet is over 105 million miles from Earth, which is farther than the distance to the sun, and is pretty far when it comes to encounters with comets that can be seen.
To see the comet this week, you need to have a clear western horizon that is free of trees, bright lights and buildings. You will likely need binoculars as well to find the comet, which may be visible to your unaided eye once you have found it. The thin crescent moon will be a guidepost to see the comet on March 12 and 13 as you can see in this diagram.
Start looking 30 to 45 minutes after sunset. Slowly scan the sky with binoculars or your eyes. Comet PanSTARRS will look like a fuzzy star with a short tail. If you have a camera and tripod, try taking some pictures of the comet. It truly will be a once-in-a-lifetime shot. Read more about Comet PanSTARRS here.
Your experience in hunting down Comet PanSTARRS may prove useful in the fall, as you might have the chance to witnesses what may be a historic flyby of Comet ISON. Some are speculating that this will be THE comet of the 21st century. It could be spectacular as a sight to behold, or a dud to be forgotten.