A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei are themselves loose collections of ice, dust, and small rocky particles, ranging from a few hundred meters to tens of kilometers across. Comet Elenin is coming to the inner-solar system this fall of 2011. Comet Elenin (also known by its astronomical name C/2010 X1), was first detected on Dec. 10, 2010 by Leonid Elenin, an observer in Lyubertsy, Russia, who made the discovery using the ISON-NM observatory near Mayhill, New Mexico. At the time of the discovery, the comet was about 647 million kilometers (401 million miles) from Earth. Over the past four-and-a-half months, the comet has — as comets do — closed the distance to Earth's vicinity as it makes its way closer to perihelion (its closest point to the sun). As of May 4, Elenin's distance is about 170 million miles. It is scheduled to come as close as 22 million miles.
Comets and other extraterrestrial visitors fairly often come relatively close to the Earth. For example on Oct. 20, 2010, Comet Hartley 2 passed just over 11 million miles from Earth.
At the time of discovery Elenin had an apparent magnitude of 19.5, making it about 150,000 times fainter than the naked eye magnitude of 6.5. The discoverer, Leonid Elenin, estimates that the comet nucleus is 3—4 km in diameter. As of April 2011, the comet is around magnitude 15 (roughly the brightness of Pluto), and the coma (expanding tenuous dust atmosphere) of the comet is estimated to be about 80,000 km in diameter.
"We're talking about how a comet looks as it safely flies past us," said Yeomans. "Some cometary visitors arriving from beyond the planetary region — like Hale-Bopp in 1997 -- have really lit up the night sky where you can see them easily with the naked eye as they safely transit the inner-solar system. But Elenin is trending toward the other end of the spectrum. You'll probably need a good pair of binoculars, clear skies, and a dark, secluded location to see it even on its brightest night."
Comet Elenin should be at its brightest shortly before the time of its closest approach to Earth on Oct. 16, 2011. At its closest point, it will be 22 million miles from the Earth.
"Comet Elenin will not only be far away, it is also on the small side for comets," said Yeomans. "And comets are not the most densely-packed objects out there. They usually have the density of something akin to loosely packed icy dirt.
"So you've got a modest-sized icy dirtball that is getting no closer than 35 million kilometers," said Yeomans. "It will have an immeasurably miniscule influence on our planet. By comparison, my subcompact automobile exerts a greater influence on the ocean's tides than comet Elenin ever will."
"This comet may not put on a great show. Just as certainly, it will not cause any disruptions here on Earth. But there is a cause to marvel," said Yeomans. "This intrepid little traveler will offer astronomers a chance to study a relatively young comet that came here from well beyond our solar system's planetary region. After a short while, it will be headed back out again, and we will not see or hear from Elenin for thousands of years."
For further information: http://www.jpl.nasa.gov/news/news.cfm?release=2011-135&rn=news.xml&rst=2989