Comets are small solar-system objects. They are often referred to as dirty snowballs because they are believed to mainly consist of ice and dust. But they also contain things like methane, ammonia, carbon dioxide etc.
|Illustration of a comet's tails. Image credit: NASA|
When comets come close to the sun, the radiation from the sun causes some of the comet material to be released, i.e. the ice is turned into gas and any dust within it is freed. This forms a coma of material around the comet's core; it's a little bit like an atmosphere. Due to the solar wind this material is pushed away from the comet and leaves a visible tail behind. In fact comets can have more than one tail. There is a gas tail that points in the opposite direction of the sun which consists mostly of gas atoms that are ionized by the suns radiation and a tail of dust grains that leans a little bit more towards the comets trajectory (see the illustration). The core of a comet is thought to be a few tens of miles/kilometers in size or smaller, the coma on the other hand can reach a million miles or more; that's about the size of the sun! Some comets can also have very long tails with some of the longest reported tails being as long as about 1 Astronomical Unit; that's the distance between the Earth and the sun!
The tail and coma are what makes comets easily distinguishable from asteroids. However, every time a comet passes by our Sun it loses some of its material until eventually all the ice has gone and the only remainder might be a piece of rock.
It is believed that most comets originate from the formation of our solar system. They are left-overs that didn't make it into a planet or moon. They mostly live in the Kuiper Belt and what is called the Oort Cloud. The Oort Cloud is a described as a sort of spherical area far out around our solar system that harbors a vast number of icy objects. Occasionally some of these collide or encounter other massive objects (such as the gas planets) that disturb their regular orbit. And sometimes the new path the comet adopts will lead through the inner solar system. When these objects come close to the Sun we observe them as comets. Some comets come by on a regular basis, such as the famous Halley's comet. They found a stable new orbit that can take the comet anything from a few years to more than 100,000 years to complete once. Comets with periods shorter than about 200 years are called short-period comets and are believed to come from the Kuiper Belt, those with longer periods on the other hand are called long-period comets coming most likely from the Oort Cloud. Rarer are those comets that only pass by once and are kicked out of the solar system forever. These comets are called hyperbolic comets, named after the shape of their trajectory.
|Path of Comet PanSTARRS, Image credit: NASA|
In the past we have had the pleasure to see many great comets in the night sky, the greatest ones even with the naked eye. This year, 2013, promises to be another great year for bright comets. There will be 2 very bright comets, one moderately bright one (Comet C/2012 F6 Lemmon) and one regular visitor (Comet 2P/Encke). Let me tell you here about the 2 brightest ones. The first one predicted to be relatively bright is comet PanSTARRS (official designation C/2011 L4 PanSTARRS) this month. It's got its name from the PanSTARRS survey which discovered this comet. Scientists predict that the brightness of this comet is going to be around as bright as the stars in the big dipper but brightness predictions are difficult. While being visible from the Southern Hemisphere already, comet PanSTARRS will be visible from the Northern Hemisphere starting March 7th (this Thursday), just above the horizon after sunset.
|Comet McNaught in 2007, Image credit: ESA/NASA|
Another great show will be put on at the end of November by comet C/2012 S1 ISON, which some say is expected to be brighter than the full moon!! This is indeed a rare occasion. Two Russian amateur astronomers discovered Comet ISON while observing for the International Scientific Optical Network (ISON) which gave the comet its name. Calculations of the comet's orbit revealed that it will pass very close to the sun (less than 1 million miles distance). Comets that come this close to the sun are called sungrazers. In the case of surviving this close pass to the sun, the view of this comet should be spectacular, possibly similar to that of Comet C/2006 P1 McNaught in 2007 (see picture). I will sure keep an eye out this year and try to spot one of these passers-by!