In June, 2012,une we earthlings had the chance to see a rare act by the planet Venus when it trekked across the face of the Sun for the last time in our lifetimes. Our figurative stars have aligned again this year to give us not one, but two celestial spectacles; comets speeding up towards our skies. The first one, Comet Pan-STARRS, is already visible from the southern hemisphere, but to us northerners the best time comes around mid-March. The second comet, Comet ISON, is arriving later in the year in November-December. However, ISON has the potential to more than make up for its ‘late’ arrival by putting up one of the greatest shows by becoming visible even in broad daylight like the full moon, according to some predictions.
Comets dwell predominantly in two different neighborhoods around the Sun. The comets that frequently visit our skies, such as Halley's Comet, come from the Kuiper Belt, which likes way out in the realm of Pluto, a range of thirty to fifty times as far away from the Sun as Earth is. Hundreds of thousands of comets in Kuiper Belt orbit the Sun just like the planets. The second region forms a large spherical halo around the Sun and the planets, lying more than a 1,000 times farther away from the Kuiper Belt. Known as the Oort Cloud, its comet population is estimated to be in the billions! At such great distances, from which the Sun is just another twinkle in the sky, the gravitational tug of passing stars and gas clouds can occasionally nudge a comet towards the inner solar system, throwing it into a new trajectory. The orbits of both Pan-STARRS and ISON show that they come from the Oort Cloud and that they are both on their very first visit to the inner part of the solar system. Their current orbits are hyperbolic which means that both are also likely on their last pass, unless some other body gives them another gravitational nudge in the right direction. Both comets are among the most distant visitors we can possibly receive.
A comet is nothing but a mixture of dust, water ice, and frozen volatiles such as carbon dioxide, methane, ethane, nitrogen, etc. But, scientists believe that comets colliding with earth in the early stages of the solar system deposited the water that eventually formed the oceans. In fact, it is thought that they may have even brought organic material to earth from which life itself evolved.
Far away from the Sun, comets fade away into a miniscule existence. Even as it approaches Earth, a comet’s nucleus, at most only a few miles in width, would be nearly impossible to detect if not for the effects of Sun's energy that transforms the volatiles and ice into gases and stirs up dust, forming a halo that glows in energetic solar radiation. This halo, known as the coma, can be a few hundred thousand miles in size, and may even approach the size of the Sun itself. As a comet gets closer to the inner solar system its most famous feature begins to emerge: the tail. Precisely speaking, there are two tails; one formed by dust particles scattered behind the comet by the pressure of solar radiation. The other, formed by gas ionized by a stream of charged particles coming from the sun (solar wind), points directly away from the Sun. These tails, which grow to millions of miles in length, and comas that enlarge with the comet’s approach, allow small, inconspicuous bodies to make astronomical impressions.
Comet brightness is notoriously hard to predict. For first time comets from Oort Cloud such as Pan-STARRS it is even harder. The date to watch would be March 10 when the comet will get the most intense heat from the Sun as it gets inside Mercury's orbital distance. The solar heat it will receive during its approach may just be what the comet needs to kick its brightness up a notch or two. After March 10th it will appear low in the western sky after sunset. The best dates would be March 12th, 13th and 14th when Pan-STARRS will emerge west in the sunset sky along with the waxing crescent moon. A view of the western horizon will be needed as the comet will stay low in the twilight sky. Naked eye visibility is likely to be limited to the comet's coma. The comet is predicted to appear about as bright as the stars in the Big Dipper. Binoculars or small telescopes may show the comet's tail.
The biggest blast seems to be coming from Comet ISON whose orbit takes it as close as 680,000 miles to the solar surface making it a “sungrazer”. And ISON's orbit has similarities with the orbit of the Great Comet of 1680 leading scientists to think that both comets may have come from the same parent body. Also known as Newton’s Comet, the 1680 comet brightened rapidly grew a tail nearly forty times the size of the moon in the sky. When it got closest to the Sun the comet astonished spectators all over the world with its brilliance, and a fiery tail 70 degrees long, or about 140 times the width of the moon, visible after twilight. If ISON is anything like its ancestor it will be immortalized in the cometary hall of fame!
For now, ISON is just a chunk of dirty ice a few miles long lying near the orbit of Jupiter. But get ready! It is best to consult a magazine such as Astronomy Magazine, and Sky & Telescope for the latest on what ISON and Pan-STARRS are up to. And of course there are your local astronomy clubs are also an excellent source of info. Some of them will likely hold observing sessions or expeditions. United Astronomy Clubs of New Jersey operating at Jenny Jump State Forest hosts a website with links to a number of local astronomy associations. Morris Museum Astronomical Society, associated with Morris Museum in Morristown and New Jersey Astronomical Association located at Voorhees State Park are other active organizations.