Many of you (myself included) have likely missed one of the celestial shows of the decade. Comet McNaught, shown above in a picture taken by Andrew Drawneek in New Zealand and published on Sky Tonight's webpage, has become the brightest comet in decades. Due to some bad geometry it was hard to see in the northern hemisphere, and now it has moved so that it can only be seen well south of the equator. I tried to see the comet a few times, but the weather was typically bad.
Why did this comet get so bright so fast? Comet brightnesses are hard to predict. Some comets put out a lot of dust, others put out more gas (like water vapor and ammonia). Dust readily reflects sunlight, so dusty comets are brighter. Distance also plays a role in how bright a comet appears. Comet McNaught came about 76 million miles of the Earth, while Hale-Bopp (the bright comet in 1997) never came closer than 122 million miles. In 1996, Comet Hayakutake became very bright as it passed within 10 million miles of the Earth; had it been at Hale-Bopp's distance, it would have been invisible to the unaided eye.
Comet McNaught has one unique property for a comet -- its orbit. Most comet orbits are very elongated ellipses (ovals) that take the comet tens of billions of miles away from the sun, and then bring it back to within a few tens of millions of miles. But Comet McNaught's orbit seems to be a hyperbola, meaning that it may completely escape from our solar system, never to return. Jupiter is probably responsible for this travesty -- its gravity can give a comet a little extra energy and kick it out of the solar system completely.
For a real treat, check out the videos of the comet on this SOHO Spacecraft website. The SOHO spacecraft constantly watches the sun. In the movie, you see the comet come in so bright that it nearly blinds the camera. The comet passes the planet Mercury (the bright star in the lower left) and slips out the other side of the camera. The disk that you see in the middle blocks the sun's light; the little white circle shows you the actual size of the sun. The blue color is not real -- this is really a black and white image.