Comet PanStarrs

A comet that was discovered while still in the deep freeze of the outer solar system is thawing out as it approaches the Sun. That may make it visible to the unaided eye next week, quite low in the west after sunset.

Comet Panstarrs was discovered two years ago by a robotic telescope that scans for objects that might come close to Earth. The comet will pass closest to Earth tomorrow, but at a healthy range of a hundred million miles. It’ll pass closest to the Sun on Sunday, at just 30 million miles.

As a comet approaches the Sun, the heat vaporizes some of the ice at its surface, spewing gas into space. That releases small solid particles as well. This material spreads out to form a giant cloud around the comet’s tiny nucleus. The Sun pushes some of this material away from the comet, creating a long, glowing tail.

Unfortunately, though, it’s impossible to predict how much material will spew into space, and how bright it will make the comet appear. As we recorded this episode, expert