Bright comet headed our way?

A number of years have elapsed since Northern Hemisphere-sky watchers have been treated to a bright naked-eye comet. Many of us will remember comet Hale-Bopp that adorned our skies for several weeks during the spring of 1997. The comet was a dazzling sight, even from the light-polluted skies of large cities. It was probably the most widely observed comet of the 20th century. Discovered independently by Alan Hale and Thomas Bopp, the comet never really got close to Earth, but it was a monster. Astronomers estimated Hale-Bopp was about six times the size of comet Halley.

And comet Hyakutake (discovered by Yuji Hyakutake in Japan), making its appearance just a year earlier, was no slouch either. While not nearly as large as Hale-Bopp, this comet passed within about nine million miles of the earth in late March of 1996, the closest approach any comet had made in the previous 200 years. As a result, for about two days, Hyakutake's tail stretched some 50 to 60 degrees across the sky!

From my viewing location near the crest of the Coast Range just off Highway 26, it was one of the most spectacular celestial sights I have ever witnessed.

OK, fine, but that's old news. How long will we have to wait to see another bright comet? The answer is nobody knows for sure, but astronomers think they have a potential candidate headed our way.

Designated C/2011 L4 PANSTARRS, the comet was discovered on June 5, 2011 by a team of astronomers using the Pan-STARRS 1 telescope on Haleakala, Maui in Hawaii. PANSTARRS stands for Panoramic Survey Telescope & Rapid Response System and its purpose is to search for Earth-approaching comets and asteroids that could pose a danger to our planet.

When it was first discovered, C/2011 L4 PANSTARRS was located approximately half way between the orbits of Jupiter and Saturn. Calculations based on measurements made since that time indicate the comet's orbit is shaped like a parabola. This implies that the object likely came from the Oort cloud, a vast spherical halo of debris left over from the solar system's formation that may extend as far as a light-year from the sun. The material in this cloud is thought to be composed of ices, such as water, ammonia, and methane, plus varying amounts of dust. If this analysis holds up it will be the comet's first, and likely last, journey to the inner solar system.

The inbound comet is expected to come within about 100 million miles of the earth on March 5, 2013 - not a particularly close approach. On March 10 it will be nearest the sun, at 28 million miles; about the same distance the planet Mercury is from the sun. This is considered close enough for the sun's energy to boil off quite a bit of frozen gases and dust that produce a comet's tail.

So how bright is this comet going to be? To casual sky watchers, that is the most important question, and the hardest to answer. Astronomers currently estimate that the comet might become as bright as some of the most brilliant stars in the night sky, such as Vega and Arcturus.

But history has demonstrated that predicting how bright a comet will become is fraught with uncertainty.

Kohoutek is probably the most famous cometary dud in recent history. Before its closest approach to the sun in late December of 1973, Kohoutek was hyped by the media as the "comet of the century." There was even talk of being able to see it in broad daylight! While the comet did achieve naked-eye status by early January of 1974, it fell far short of nearly everyone's expectations. One reason for the dismal performance may have been the fact that instead of coming from the ice and dust-rich Oort cloud, as astronomers first believed, Kohoutek might have actually been a Kuiper belt object.

Scientists theorize the Kuiper belt is populated by mostly rocky bodies that are covered with relatively thin layers of ice and dust that orbit the sun beyond the planet Neptune.

Recent observations of C/2011 L4 PANSTARRS are encouraging. Photographs taken in June 2012 show a very compact coma, indicating active dust production even as far from the sun as 428 million miles.

Will this be the comet sky-watchers have been waiting for? Stay tuned.