C/2010 X1 (Elenin)

A comet is usually a very large object, from a few hundred yards to several miles in diameter. Comets orbit the sun like the earth does. Our orbit is almost a circle, but a comet's orbit is very elliptical. If you traced a football on a sheet of paper, that shape would be elliptical. A comet's orbit will bring it in close to the sun where we might be able to see it like Hale-Bopp did and then it will swing way out to the farthest part of our solar system. Comets return time and time again. Sometimes it takes a very long time, thousands of years and sometimes it takes only a few years. Comet Halley will return to near earth each 76 years. When we see a comet in the sky, it will appear not to move very much while we watch even though it is moving at several thousand miles per hour. That is because it is far away and is usually moving towards us or away from us. A comet is like a dirty snowball. It is made of water ice, carbon dioxide ice, dust, rocks and other molecules. As it approaches the sun, the sunlight warms the ices and it sublimates, (that is, a solid turns to a gas without turning to a liquid). This gas points away from the sun and we see it as the comet's tail. The comet and its tail will be visible for several nights, sometimes even weeks. Its position will change a little each night.
http://www.madsci.org/posts/archives/2000-01/947298651.As.r.html

A celestial object that orbits the Sun along an elongated path. A comet that is not near the Sun consists only of a nucleusa solid core of frozen water, frozen gases, and dust. When a comet comes close to the Sun, its nucleus heats up and releases a gaseous coma that surrounds the nucleus. A comet forms a tail when solar heat or wind forces dust or gas off its coma, with the tail always streaming away from the Sun. Short-period comets have orbital periods of less than 200 years and come from the region known as the Kuiper belt. Long-period comets have periods greater than 200 years and come from the Oort cloud.
http://www.thefreedictionary.com/comet

Comet C/2010 X1 (Elenin) is a long-period comet discovered by Russian astronomer Leonid Elenin on December 10, 2010 at the International Scientific Optical Network's robotic observatory near Mayhill, New Mexico, U.S.A. At the time of discovery Elenin had an apparent magnitude of 19.5,[2] making it about 150,000 times fainter than the naked eye magnitude of 6.5.[5] The discoverer, Leonid Elenin, estimates that the comet nucleus is 3–4 km in diameter.[6] 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.[7]

C/2010 X1 will come to perihelion (closest approach to the Sun) on 10 September 2011 at a distance of 0.4824 AU.[3] On 16 October 2011, the comet will pass within about 0.233 AU (34,900,000 km; 21,700,000 mi) of the Earth[3] at a relative velocity of 86,000 km/hr.[3] The Minor Planet Center ephemeris shows this relatively bright comet will reach about 6th magnitude near mid-October 2011,[8] but until the activity level of the coma is better known it is still uncertain just how bright this comet will become.[9] Elenin will make its closest apparent pass in the night sky to Comet 45P/Honda-Mrkos-Pajdušáková on the morning of October 8, before moving apparently close to and in front of Mars on October 15. The comet came to opposition at 178° from the Sun on March 14 and will come to opposition again on November 22 at 175° from the Sun. The minimum angle between the Sun and comet will occur September 26 (1.9°), and between July 28 and October 10 the comet will be less than 45 degrees from the Sun.

Given the orbital eccentricity of this object, different epochs can generate quite different heliocentric unperturbed two-body best-fit solutions to the aphelion distance (maximum distance) of this object. Near perihelion using an August 2011 epoch, Kazuo Kinoshita shows C/2010 X1 to have a heliocentric orbital period of 600,000 years,[10] but being on a highly eccentric orbit, the comet will be frequently perturbed by the planets as it leaves the inner solar system.[11] For objects at such high eccentricity, the Sun's barycentric coordinates are more stable than heliocentric coordinates.[12] The orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the solar system. Using JPL Horizons with an observed orbital arc of 147 days, the barycentric orbital elements for epoch 2020-Jan-01 generate a semi-major axis of 515 AU and a period of approximately 11,700 years.[4]

Before entering the planetary region (epoch 1950), Elenin had a calculated barycentric orbital period of ~3.5 million years with an apoapsis (aphelion) distance of about 46,400 AU (0.73 light-years).[4] Elenin was probably in the outer Oort cloud with a loosely bound chaotic orbit that was easily perturbed by passing stars.
http://en.wikipedia.org/wiki/C/2010_X1

C/2010 X1 (Elenin), Orbit Diagram:
http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=elenin;orb=1;cov=1;log=0;cad=1#cad

Where to look:

http://www.sott.net/articles/show/221672-Comet-Elenin-is-Coming-