The comet heads to the southeast during October. On the night of October 8/9, it floats next to the famous Double Cluster in Perseus, a pair of adjacent bright star clusters that will form a beautiful backdrop for the comet. It then passes the bright star Capella in Auriga in mid-October. It comes closest to Earth October 20, about the time it will appear brightest in the sky. Unfortunately, a nearly Full Moon then brightens the sky, making the comet less conspicuous.
Comet 103P/Hartley should show two tails emanating from a roughly circular glow, known as the "coma," which masks the comet's nucleus. The nucleus is a giant ball of ice and dust that measures about a mile across. As sunlight hits the nucleus, the ice boils off and carries dust with it. This cloud of gas and dust forms the coma.
Sunlight ionizes the ejected gas molecules, causing then to glow with a bluish color. The solar wind carries this ionized gas away from the comet, creating a straight, bluish gas tail. The ejected dust gets pushed away from the Sun more gently, so it forms a curving tail. The dust particles simply reflect sunlight, so the dust tail has a white to pale-yellow color.
Although October marks the peak for Comet 103P/Hartley, the comet will continue to make news in November. NASA's EPOXI mission
will fly past the comet and return stunning images of its nucleus. EPOXI comes closest to the comet November 4, and NASA should release fresh images soon thereafter.