While comets are reasonably common in the solar system, it's pretty rare for one to be visible to the naked eye here on Earth. So, if you get the chance, try to spot comet Pan-STARRS in your western night sky this week. My southern-hemisphere readers may have already spotted it - it's been visible south of the equator for weeks - but tonight will be the first chance at a viewing for 90% of the world's population. To the unaided eye, it should look like a reasonably bright star, but even a pair of binoculars should reveal the dramatic tail characteristic of comets.
This body was recently discovered by the Pan-STARRS automated telescope in Hawaii. It's not uncommon for comets to be discovered only shortly before they pass by the Earth. Travelling from the outer reaches of the solar system, comets tend to be relatively dim until they approach the Sun. Heat from the Sun causes their famous tails, which make them appear larger and brighter as the approach the inner solar system. Because comets are basically big balls of ice, a close approach with the Sun can leave them significantly smaller as they journey back out towards the Kuiper Belt. These icy bodies have traditionally been thought of as a source for the Earth's water, but some recent simulations have cast some doubt on this idea.
There's more good news for avid sky watchers: Comet Pan-STARRS shouldn't even be the most visible comet of the year. Comet ISON, discovered last year, will make an appearance late this year. Although the brightness of comets is very difficult to predict, ISON could shine as brightly as the full moon, making it visible even during the day. Two bright comets in a single year is a fortuitous event, so don't pass up your chance to spot these beautiful objects!