By Irene Klotz
CAPE CANAVERAL, Florida (Reuters) - The first of two comets heading toward the sun this year made its closest approach to Earth on Tuesday and will be visible in the Northern Hemisphere beginning on Thursday.
Skywatchers in the Southern Hemisphere have been able to see Comet Pan-STARRS for weeks at twilight, even without binoculars or a telescope. The comet came about 100 million miles (161 million km) from Earth on Tuesday.
"As Comet Pan-STARRS was setting on the southwestern horizon, its nucleus was visible to the naked eye," photographer Michael White from Manawatu, New Zealand, wrote to accompany a stunning image of the comet posted on the SpaceWeather.com website.
The comet, officially known as Comet C/2011 L4, was discovered in June 2011 by the Panoramic Survey Telescope and Rapid Response System, or Pan-STARRS, in Hawaii.
Comet Pan-STARRS is believed to be a first-time visitor to Earth after being gravitationally bumped out from the Oort Cloud, a repository of small icy bodies located beyond Pluto in the solar system's back yard.
Comets, which are comprised of minerals, rocks and ice, are believed to be remnants from the formation of the solar system some 4.5 billion years ago.
As a comet approaches the sun, some of its ice vaporizes, creating an envelope of gas and dust, called a coma, around its body. The heating also generates two tails, each of which can be more than 1 million miles (1.6 million km) long.
One tail is comprised of dust and the other is made of molecules ionized by sunlight.
Comet Pan-STARRS currently is inside the orbit of Mercury and brightening as it heads toward the sun.
"Observers in the Southern Hemisphere say the comet can be seen with the naked eye even through city lights. Currently, it is about as bright as the stars of the Big Dipper. The comet could become even brighter when it moves into Northern Hemisphere skies in the second week of March," SpaceWeather.com reports.
Northern Hemisphere sky-watchers will get their chance to see the comet beginning on Thursday, though the best views may come later in the month.
"To see it, you will need an unobstructed, cloudless view of the western horizon. It is best to pick a dark spot, away from street lights," University of Hawaii's Institute for Astronomy wrote in a press release.
The comet should be visible in the direction of the setting sun just after the sun slips below the horizon. Twilight and moonlight may make viewing the comet difficult. The best opportunity to see it may be on March 13 when the comet appears just beneath a thin crescent moon, astronomers said.
By the end of the month, Comet Pan-STARRS will appear in the eastern skies just before sunrise, but it will be farther from the sun and Earth and fainter.
Comet Pan-STARRS may just be the warm-up act for another celestial visitor due to arrive in November. If it is not destroyed by the sun, Comet ISON has the potential to be as bright as a full moon, possibly even visible in daylight.
Comet ISON, which was discovered last year by two amateur astronomers in Russia, is expected to pass as close as 684,000 miles (1.1 million km) from the surface of the sun - about four times closer than Comet Pan-STARRS will pass during its closest approach to the sun on Sunday.
(Editing by Cynthia Osterman)