Don’t worry. Although they don’t know for sure how much of a show this comet will put on, astronomers are certain of one thing: It’s not going to hit us.
Sunset watchers may get an extra treat beginning this weekend as a comet makes a pass near Earth. It should should be visible to the naked eye low in the horizon in the west for about an hour after sunset.

And unlike last month’s spectacular meteor in Russia, NASA promises this visitor will “definitely miss” us, said William Cooke, a meteor environment officer with NASA’s Marshall Space Flight Center in Huntsville, Ala.

The comet has been visible in the Southern Hemisphere for several weeks and is just becoming visible in the North, said Mark Hammergren of Chicago’s Adler Planetarium. “Some experienced amateur astronomers in Florida have already seen it.”

It should be fairly easily visible through the end of next week but will grow fainter in the days afterward as it heads away from the sun. Astronomers say using binoculars will give sky-watchers the best view.

The comet “will start to be visible just at sunset Friday night, and it’s going to get easier to see over the next few days,” said Amy Mainzer, a comet expert with the Jet Propulsion Lab in Pasadena, Calif.

The comet’s official name is C/2011 L4 but it’s been dubbed Pan-STARRS because it was discovered in June 2011 by astronomers using the Panoramic Survey Telescope & Rapid Response System, or Pan-STARRS telescope, in Hawaii.

Pan-STARRS will be visible very low in the horizon, “just a hand span above it,” said Hammergren. Because of this, it’s especially important to pick a vantage point with clear lines of sight where trees or buildings don’t interfere.

“It will look like faint jet contrails sticking up vertically in the sky; it won’t be very long,” he said.

So far Pan-STARRS has been something of a disappointment to comet watchers in the Southern Hemisphere, said Ben Burress, an astronomer at the Chabot Space and Science Center in Oakland.

But all that could change as the comet slingshots past the sun on Sunday, said Mainzer. “Right now the comet’s undergoing a very rapid temperature change as it gets close to the sun. So just because it is not doing much in the Southern Hemisphere doesn’t mean it won’t undergo a dramatic change as it gets closer to the sun.”

There’s just no way to know without looking. “That’s the trouble with comets: They’re finicky things. Sometimes they put on a great fireworks show, and sometimes they just fizzle,” she said.