An icy visitor is positioning itself for easier viewing in the coming weeks. Periodic Comet 103P/Hartley 2 won't have the pizzazz of Comet Hale-Bopp or the unexpected spectacle of Comet Holmes. But it will be high in the evening sky when at its best, glowing at perhaps 5th magnitude. It should be dimly visible to the unaided eye from very dark locations, and visible in binoculars and telescopes from almost anywhere in the Northern Hemisphere. (Most of you in the Southern Hemisphere will be able to observe it from mid-October onward.)
Hartley 2's brightness, and its unusually fast slide across the constellations, both result from how closely it will approach Earth: by just 0.12 astronomical unit (11 million miles; 18 million km) on October 20th. This will be its closest approach since its 1986 discovery and one of the closest approaches of any comet in the last few centuries.
The comet has reached 9th magnitude and is brightening by 0.1 magnitude per day. So right now, before the Moon washes the sky with light, is an especially good time to look for this faint visitor.
Comet Hartley 2 spends September gaining speed in Lacerta and Andromeda southwest of Cassiopeia. In October it races high across the northern sky from Cassiopeia to Capella. Click here for a large version suitable for field use. S&T diagram
The waxing Moon will brighten the evening sky from about September 15th to 26th. On the 22nd Hartley 2 should be 7th or 8th magnitude and within a few degrees of Lambda (&lambda) Andromedae.
October 1st finds the comet passing 1.5° south of 2.2-magnitude Alpha (&alpha) Cassiopeiae, high in the northeast during moonless evenings. Perhaps 6th magnitude by then, it should remain at least this bright for the next nine weeks. But it's important to note that, with the comet now just 0.18 a.u. from Earth and closing, its light is no longer concentrated into a small dot but instead is more spread out. So even if you can sight a 6th-magnitude star with the unaided eye, Hartley 2 will be tougher. It's closest to Earth on October 20th at a distance of just 0.121 a.u.
On the night of October 7th in the Americas, when the comet should be 5th or 6th magnitude, it creeps less than 1° south of the Double Cluster in Perseus, magnitudes 4.3 and 4.4. This will make for a wonderful wide-field sight and a great astrophoto opportunity — particularly since it's again new Moon!
From here on Hartley 2 turns southeast, passing near the head of Perseus. On October 20th the fuzzy visitor passes just south of brilliant Capella. By the end of October the comet should still be around 5th magnitude — but now in Gemini. So it doesn't gain a high altitude until later in the night. Perihelion, 1.06 a.u. from the Sun, comes on the 28th — but that morning the nearly last-quarter Moon is just a few degrees away.
Moonless viewing times return around November 1st. But now, with the comet moving away from both the Sun and Earth, it fades by about a magnitude every two weeks. Besides — by then our attention should surely be turning to the exploits of (and pictures from) NASA's EPOXI spacecraft, which swoops by it on November 4th at a distance of just 600 miles (1,000 km).
Observer Rolando Ligustri remotely photographed 103P/Comet Hartley 2 on September 6th (Universal Time) using a 10-inch f/3.4 Takahashi Epsilon 250 astrographic telescope.
Rolando Ligustri
How could a short-period comet, visible to the unaided eye, go undiscovered until just 24 years ago? Read on.
Malcolm Hartley first spotted it on March 16, 1986, at magnitude 17 or 18 during a sky survey by the 1.2-meter U.K. Schmidt telescope at Siding Spring, Australia. A series of position measurements soon revealed it to be a short-period comet orbiting the Sun about every 6 years. It was the second short-period comet discovered solely by Hartley, hence the "2" in its name. The appellation 103P indicates that it was the 103rd comet with a known orbital period.
A backtrack of Hartley 2's path revealed that it fell into its current orbit only recently. Three close encounters with Jupiter (0.33 a.u. in 1982, 0.09 a.u. in 1971, and 0.23 a.u. in 1947) had shifted the comet's track closer to the Sun. Prior to those encounters, Hartley 2 never came closer than 2 a.u. from the Sun, leaving it beyond visual detection.
Hartley 2's next return came in 1991, when it brightened that September to 8th magnitude. It did so again at its following return in December 1997. The 2004 apparition was a poor one, with the comet far from Earth.
Now it's arriving front and center for its best showing yet. So enjoy it while you can!
Greg Bryant, editor of Australian edition of Sky & Telescope, fondly recalls his first sighting of Comet Hartley 2 in 1991.
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