For backyard stargazers, now is the best time to see green Comet 103P/Hartley 2 as it approaches Earth for an 11-million-mile close encounter on Oct. 20th. Set your alarm for the dark hours before dawn, go outside, and look straight up. You will find Hartley 2 not far from the bright star Capella: sky map. Although the comet is barely visible to the unaided eye, it is easy to find in binoculars and looks great through a backyard telescope.

Rolando Ligustri has been observing the comet nightly using a robotic 4-inch telescope in New Mexico.

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© Rolando Ligustri
Many readers have asked, why is the comet green? Answer: Hartley 2'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.

NASA scientists say 103P/Hartley 2 is one of the most active comets they've seen, with copious outgassing from jets in the nucleus. Amateur astronomers are encouraged to monitor the action and submit their images here.

More images: from Nick Howes of Cherhill, Wiltshire, UK; from Abe Schwartz of San Juan, Puerto Rico; from Dr Paolo Candy of Ci.A.O. Cimini Astronomical Observatory - Italy; from John Chumack of Dayton, Ohio; from Keith Johnson of Durham, England,UK; from Joao Porto of Ponta Delgada, Azores islands, Portugal