This exotic green comet, named Lulin, will make its closest approach to Earth on 24th February (22.43 EST), when it will be a mere 60 million km (0.41 AU) away.

Astronomers have been avidly tracking its course over the last few months [spotting instructions, assorted links]. With a maximum brightness of between 4 and 5, it could be dimly visible to the naked eye, says Science News. NASA astronomer Stephen Edberg tells AP that Lulin will be paying a one-time visit only to our solar system: it rounded the Sun on 10 January, and is already on its way out, calculates Brian Marsden of the Smithsonian Astrophysical Observatory, at Space.com. And weirdly, Edberg adds, the comet is circling the Sun clockwise (all the planets, and most other objects in the solar system, circle anticlockwise). “It essentially is going backwards through the Solar System,” he says.

Lulin was spotted by 19-year-old Chinese meteorology student Quanzhi Ye at Sun Yat-sen University in 2007, from a photo taken a few nights earlier by Taiwanese astronomer Chi Sheng Lin at the Lulin Observatory. Ye is now tracking the comet on his blog.

Its green colour, Science@NASA explains, comes from the toxic cyanogen ((CN)₂) and diatomic carbon (C₂) gases that make up the comet’s Jupiter-sized atmosphere. Other notable green comets of recent years (whose colour originates from the same gases) include comet Swan (2006), comet Lovejoy (2007) – spotted from a photo taken by an off-the-shelf digital camera – and comet Machholz (2004).

Image: credit to Jack Newton, at the Arizona Sky Village