Comet to be visible in western sky after sunset

ROY GEPHART, STARGAZINGMarch 10, 2013 

Recently, my 2-year-old granddaughter started mimicking my reading to her, and excitedly kept pointing out and saying her two favorite letters -- X and O. It's a joy to watch children discover their world. Each day holds new adventures waiting to happen.

Comets are a lot like young children. Both do what they want, when they want.

This week, Comet PanSTARRS climbs above the western horizon to either showcase itself or remain faint and hidden inside the afterglow of dusk.

Predictions of comet brightness ebb and flow like morning tides.

I recommend using your computer and searching for "Updates on Comet PanSTARRS" to tap into the latest news and download charts showing the comet framed against the western sky. One of the best websites is maintained by Sky and Telescope magazine.

If the comet brightens and skies remain clear, the comet could be visible to the unaided eye any evening this week starting 30 to 40 minutes after sunset (7 p.m.).

I look forward to this Tuesday, when the thin crescent moon enters the evening sky about one hand width (held at arm's length) above the point of sunset. With the moon serving as a benchmark, glance about one finger width left of the crescent moon. That's where the fuzzy image of Comet PanSTARRS will shine.

On Wednesday evening, the comet is sandwiched halfway between the now higher crescent moon and the horizon.

These two evenings likely will offer the best chance for most casual observers to spot the comet before the moon brightens the sky. If the comet brightens, it will happen this week.

Astronomers from the University of Hawaii discovered Comet PanSTARRS in June 2011. Its name is an acronym for Panoramic Survey Telescope and Rapid Response System -- a project that uses telescopes to photograph the sky to discover and characterize comets and asteroids that might threaten Earth.

Such warning systems are a great idea, but I've always wondered what we would do if Earth were threatened by a large chunk of space debris. After all, no nation has the launch and interception capability to divert or destroy a multi-mile-wide object hurdling at a speed of miles per second.

Want to enjoy a Hollywood movie about Earth getting severely wacked by a comet or asteroid? Try "Deep Impact" or "Armageddon." They are fun to watch but forgo critiquing the science.

Anyway, Earth rests safe from Comet PanSTARRS. On March 5, the comet missed our planet by a whopping 102 million miles.

Today, the comet reached its closet point to the sun -- a scorching 28 million miles above the solar surface. This could severely alter the comet's persona. We'll witness the results this week.

When astronomers first traced the comet's path, they discovered it followed a hyperbolic or open-ended orbit. This meant it originated far away -- likely in the hypothesized Oort Cloud. This is a spherical region about one light year from the sun where a vast reservoir of icy objects marks the cosmic rim of our solar system.

Several million years ago, Comet PanSTARRS began its journey toward us after being jostled by a passing star or interstellar cloud of gas. This is the comet's first tour through the solar system.

The gravitational pull from the sun and planets now is redrafting the comet's orbit into an elongated, closed ellipse. This means the comet will not remain forever tossed outward into the cosmic abyss, but will slingshot back about 100,000 years in the future.

As I write this, we know that observers in the Southern Hemisphere are spotting Comet PanSTARRS with their unaided eyes. The comet is not bright but is visible. This is good news for us in the Northern Hemisphere.

To improve your chance of spotting Comet PanSTARRS, I recommend finding an observing site with an unobstructed, flat view of the western horizon where the sun sets. A clear sky also is essential. Use a pair of binoculars to patiently scan the area of interest. The comet should pop into view before noticing it with the unaided eye.

Comet PanSTARRS will resemble a small fuzzy ball with a tail of dust and gas extending upward into the sky.

Depending on where you observe, the comet will set 60 to 90 minutes after sunset.

Comet hunting is an adventure. You're braving the elements and playing the odds to glimpse an ancient object perched inside the glow of twilight.

* Roy Gephart is a retired environmental scientist and avid amateur astronomy.

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