"Spot and follow the year's brightest comet with Astronomy.com
," by Richard Talcott, Senior Editor
Lulin's green color comes from the gases that make up its Jupiter-sized atmosphere. Jets spewing from the comet's nucleus contain cyanogen (CN — a poisonous gas found in many comets) and diatomic carbon (C2). Both substances glow green when illuminated by sunlight in the near-vacuum of space.
In 1910, many people panicked when astronomers revealed Earth would pass through the cyanogen-rich tail of Comet Halley. False alarm. The wispy tail of the comet couldn't penetrate Earth's dense atmosphere; even if it had penetrated, there wasn't enough cyanogen to cause real trouble. Comet Lulin will cause even less trouble than Halley did. At closest approach in late February, Lulin will stop 38 million miles (61 million kilometers) short of Earth, utterly harmless.