Comet 103P/Hartley 2, a small periodic comet, was discovered in 1986 by Malcolm Hartley, an Australian astronomer. It orbits the sun about every 6.5 years, and on Oct. 20, the comet will make its closest approach to Earth since its discovery. In this case, "close" means 11 million miles, or 17.7 million kilometers. A moonless sky will make for promising viewing conditions in the northeastern skies, especially just before dawn.
Comet Hartley 2's nuclear diameter is estimated at 0.75-0.99 of one mile -- 1.2-1.6 kilometers -- and it's believed to have enough mass to make approximately 100 more apparitions, or appearances, near Earth. The 2010 appearance also marks one of the closest approaches of any comet in the last few centuries.
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.
The comet will pass within 0.12 AU of the Earth on October 20, 2010, only eight days before coming to perihelion (closest approach to the Sun) on October 28, 2010. During this passage, the comet may be visible to the naked eye at apparent magnitude 5, in the constellation Auriga, near its open clusters, if an observer knows where to look and is viewing from a dark sky location. Binoculars should make it an easy target.
Comet 103P Hartley from as seen from a four inch telescope, October 6th, 2010.
The comet passed near the position of NGC 281 in Cassiopeia on October 1. On the night of October 7 at New Moon, the comet is expected to pass near the Double Cluster in the sky and near the open cluster NGC 1528 by October 14, both in Perseus. In early November the comet will be visible around midnight without interference from the Moon.
Comet 103P passing within 0.12AU of Earth on October 20, 2010.