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The comet Pan-STARRS, left, is visible next to a crescent moon in the western sky after sunset over Westminster, Md. Tuesday(AP Photo/The Carroll County Times, Dylan Slagle)

Stargazing: Brightness of comets hard to predict

March 16, 2013 at 02:44 PM

The State Journal-Register

If the weather – or some other reason – kept you from seeing the comet Pan-STARRS earlier this week, you still have some time.

It should be visible in the west for a couple of more weeks.

However, as time goes on, the comet will be racing away from Earth and becoming dimmer even as it appears higher in the night sky.

Wednesday night, stargazers gathered on the sledding hill at Centennial Park, hoping for a glimpse of the comet.

It was hard to see without magnification or help locating the faint object.

Fortunately, those with telescopes were willing to share the view with others.

Pan-STARRS, named for the telescope that discovered it two years ago, has been visible in the southern hemisphere for some time, and just this week became visible near the horizon at twilight.

Another comet, ISON, is expected in November, and that one is predicted to be very bright.

John Martin, associate professor of astronomy and physics at the University of Illinois Springfield, said how bright comets will appear is difficult to say ahead of time.

“We can predict where they will be in the sky, but how bright they are? That is more of a dark art than a science,” he said.

Newer comets, those more recently dislodged from the Oort Cloud, a ring of debris at the fringes of the solar system, have made fewer orbits and have more debris to shed.

“If (ISON) hasn’t made hundreds of thousands of trips, that leaves us hopeful it could be a brighter comet,” Martin said.

Comets essentially are “dirty snowballs,” he said a “couple of 10s of kilometers across, about the size of Springfield.”

As the comet gets closer to the sun, the material heats up and starts to “ablate into space.”

That forms a cloud much larger than the rock at the center, known as the coma.

Charged particles streaming off the sun sculpt that coma into the tail we associate with comets.

Because the “solar wind” is coming from the sun, the tail always is directed away from the sun.

Some comets display a second “ion” tail. Ions have electrons knocked off them and therefore have an electric charge that is influenced by the sun’s magnetic field.

That means the ion tail forms separately from the dust tail and goes in a slightly different direction.

Martin said sometimes tails disconnect from comets and fly off. The comet then forms a new tail.

Even though comets may travel to the edge of the solar system the sun’s gravity always brings them back.

“It’s about escape velocity,” Martin said. “If you take a ball and you throw it up in the air, it keeps coming back,” he said.

Even if you throw it harder, each time it still comes back down.

“To escape it has to be thrown hard enough that gravity can’t slow it down enough to bring it back.”

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