Yet another respectably-bright comet is nearing Earth and will put on a good show for binocular observers. Comet Hartley 2 will be visible in Cassiopeia and Perseus over the next few weeks, and may grow bright enough to see with the unaided eye as it comes within 11 million miles of Earth, one of the closest approaches of any comet in the past century.
Comet Hartley 2 is a periodic comet, returning to its closest approach with the sun every six years. It was first detected by Malcolm Hartley in March of 1986 at the U.K. Schmidt Telescope in Siding Spring, Australia. This is Hartley’s second comet discovery, hence the name “Hartley 2”
But it’s strange for a such a bright comet to go undetected until 1986. Why didn’t astronomers find it earlier? A little number crunching revealed the comet took up its current orbit only recently, after three gravitational interactions with Jupiter in 1947, 1971, and 1982. Before that, the comet was in a longer-period orbit which never brought it close enough to the sun to make it visible. It was quickly picked up by Hartley after it settled into its new orbit.
Because of its unusually close approach this year, Comet Hartley 2 will put on its best showing since its discovery. Over the next week, the comet speeds through the constellation Andromeda, then into Cassiopeia through the first week of October when the Moon is waning and casts less glow.
On October 1, the comet is just 1.5 degrees south of the star Shedir (alpha Cassiopei). A week later, the comet is in Perseus where it passes just below the magnificent Double Cluster on October 7-8.
Here’s a map to help you find the comet’s location.
The location of Comet Hartley 2 through late October (from Sky and Telescope)
In dark sky, you might spot the comet with your unaided eye in early October, when it reaches 6th magnitude. But because the comet is close to Earth and apparently large, its light is diffuse and harder to see. Binoculars will give you a better view, as will a telescope at low magnification. The comet may grow as bright at 5th magnitude by mid-October. The image at the top of the page was taken on September 20 by Michael Jäger in Austria.
I am sorry to tell our southern-hemisphere readers than Comet Hartley 2 will be visible only in northern hemisphere through mid-October, when it’s at its brightest. Though if you’re north of 30 S latitude, you might glimpse the comet in Perseus on the northern horizon (see map above).
But if you can’t see it for yourself, don’t worry. The comet will be met by the NASA Deep Impact spacecraft on November 4. The craft, now on an extended mission called EPOXI, will take images and other measurements of the comet’s 1-km-wide nucleus. Check out NASA’s website as the day approaches. You may get to see live mission updates as they come in.