Typically during the course of a year about a dozen comets will come within the range of amateur telescopes. Most quietly come and go with little fanfare, but during the upcoming weeks one rather small comet will be making an unusually close approach to the Earth.
On Oct. 20, Comet Hartley 2 will pass just over 11 million miles (18 million km) from Earth. During October it should be easily visible in small telescopes, binoculars and ? from sites with dark enough skies ? even with the naked eye.
This video sky map shows how to spot the comet.
Comets are composed of rock, dust, water ice and frozen gases such as carbon monoxide, carbon dioxide, methane and ammonia. [Best Comet Appearances of All Time]
Because of their low mass, comet nuclei do not become spherical under their own gravity as larger bodies in space do, and thus have irregular shapes. They are often popularly described as "dirty snowballs," though recent observations have revealed dry dusty or rocky surfaces, suggesting that the ices are hidden beneath the crust.
In addition to its close pass of Earth, Comet Hartley 2 will be visited in early November by the Deep Impact spacecraft, which already had a previous encounter with Comet Tempel 1 in 2005.
Back on March 15, 1986, astronomer Malcolm Hartley discovered a new comet on photographic images taken at the U.K. Schmidt Telescope Unit at Siding Spring, Australia.
At the time Hartley discovered it, the newfound comet was an exceedingly faint object, with just a hint of a tail; it was about 25,000 times dimmer than the faintest stars that can be seen with the naked eye. After further images were obtained over the next several days, Hartley announced his discovery to the Central Bureau for Astronomical Telegrams in Cambridge, Mass.
This was Hartley?s second discovery of a comet and so was designated as Comet Hartley 2. Orbital calculations by Brian Marsden of the Smithsonian Astrophysical Observatory in Cambridge, Mass., indicated that Hartley 2 had already made its closest approach to the sun nine months earlier and that at that time it was too close to the sun's glare to have been detected.
Marsden also calculated that the comet made a close approach to Jupiter during 1982. The comet takes roughly 6 1/2 years to circle the sun and it has since been observed again in 1991, 1997 and 2004.
Rare close encounter
This fall, Comet Hartley 2 will again be passing through the inner solar system, reaching its closest point to the sun (called perihelion) on Oct. 28 at a distance of 98.4 million miles (158.4 million km).
And while en route to the sun, it will also make a very close approach to the Earth. In fact, at 3 p.m. ET on Oct. 20, the comet will be at its closest point to our planet at a distance of 11.2 million miles (18 million km).
It's quite unusual for any comet to approach this close to Earth. Such an event only happens on average perhaps three or four times a century.
If this comet were reasonably large, it would likely put on a very spectacular show around the time of its closest approach.
Halley's Comet passed a similar distance from Earth in the year 1066 and was described in ancient records as appearing "like a moon" as well as being depicted in the Bayeux Tapestry, a long embroidered piece of cloth that depicts the Norman conquest of England.
A small, faint comet
Unfortunately, Hartley 2 is a very small and intrinsically faint comet. Observations by the Spitzer Space Telescope in August 2008 showed that the comet's nucleus has a diameter of just 0.7 miles (1.1 km). Its nucleus is only about one tenth the size of Halley's Comet and perhaps only one thirtieth that of Comet Hale-Bopp.
Nonetheless, Hartley 2 will come close enough to us to become dimly visible to the unaided eye during early to mid October.
Astronomers use magnitude to define the brightness of sky objects; the lower the magnitude, the brighter the object. The brightest stars are zero or first magnitude, while the faintest stars are sixth magnitude.
Current expectation is that the comet will reach a peak magnitude of perhaps 4.4 at the time of its closest approach. However, around that time the comet will probably appear very large in overall apparent size, perhaps similar to the apparent size of the full moon.
As a result, much of the comet's brightness will be "spread out" over that area of the sky. So visually to the eye under a dark sky it will appear not as a sharp star-like image, but more like a dim, circular patch of light.
Binoculars or a small low-power telescope will provide a somewhat more pleasing view: a dim, circular, grayish-blue ball of light with a star-like condensation (the nucleus) at the center.
If the comet develops a tail of any kind, it likely will be of the gaseous variety ? very thin, faint and narrow, giving the comet the appearance of what comet expert John Bortle likens to "an apple on a stick."
Where to find it
As October begins, Comet Hartley 2 will be in the constellation of Cassiopeia, which at dusk will be positioned halfway up in the northeast sky; through Oct. 5 it will be passing below and to the right of the famous "W"-shaped formation composed of five bright stars.
The comet then moves into the constellation of Perseus on Oct. 6 and before dawn on the morning of Oct. 8, it will be situated only 0.7 degrees below and to the right of the famous Double Star Cluster. The cluster supposedly marks the sword handle of Perseus and is often touted as one of the most impressive star clusters in the entire sky.
On the morning of Oct. 10, the comet will appear to almost touch the 4th-magnitude star, Eta Persei. On Oct. 17, it will enter the boundaries of the constellation Auriga, and on the morning of Oct. 18, Hartley 2 will be about 1.2 degrees above and to the right of the star Epsilon Aurigae and 3 degrees below and to the right of the brilliant zero-magnitude star Capella.
Between Oct. 15 and 20, within about a half hour of 4:00 a.m. local time, the comet will be passing almost directly overhead.
Moon muscles In
But one problem that will become an increasing factor during the middle of October is the waxing moon that will light up the sky during the first part of the night and will seriously interfere with observations of the comet.
The moon will set later in the night leaving the sky dark during the predawn hours, but as it approaches its full phase on Oct. 22, the amount of time between moonset and the first light of dawn will get noticeably shorter.
For example: From mid-northern latitudes on Oct. 16, moonset is at 12:56 a.m. local time and dawn breaks at 5:37 a.m., which means the sky will be dark and moonless for 4 hours and 41 minutes.
Just four mornings later, however, the nearly full moon will set at 4:51 a.m. with morning twilight beginning at 5:41 a.m., leaving just a scant 50 minutes of dark sky for comet viewing. On Oct. 21, the moon will not set until 5:51 a.m. or 9 minutes after dawn.
Even after Hartley 2 makes its closest approach to Earth, it will become part of another close approach of a different kind. On Sept. 5, more than five years after its July 4, 2005, rendezvous with Comet Tempel 1, NASA's Deep Impact spacecraft will begin beaming down the first of more than 64,000 images it's expected to take of Comet Hartley 2.
The spacecraft will continue imaging Hartley 2 during and after its closest approach on Nov. 4, when it will pass to within 430 miles (692 km) of the comet's nucleus, providing an extended look at the comet.
The flyby of Comet Hartley 2 is the second leg of the Deep Impact spacecraft's two-part extended mission known as EPOXI. During the flyby a science team from the University of Maryland will study the comet using all three of the spacecraft's instruments ? two telescopes with digital color cameras and an infrared spectrometer.
- Gallery: The Great Comet McNaught: Part 1, Part 2
- The Best Comets of All Time
- Astrophotography Telescopes for Beginners
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.