Charts are now available for the entire time when Comet Hartley 2 is expected to be 10th magnitude or brighter. Click on one of the links below for:
Update, October 27: For more than a week the comet has been washed out in moonlight, but dark-sky viewing will return before the first light of dawn at your site around the morning of November 1st — in time for the EPOXI spacecraft encounter on the morning of November 4th (see section below).
Meanwhile, two unusual fireballs have raised the possibility that the comet's debris stream is wide enough to intersect Earth. This is, after all, one of the closest Earth approaches by a comet in recent centuries. If there's going to be a "Hartley-id" meteor shower, its peak would most likely come on the evenings of November 2nd and 3rd with a radiant in Cygnus. See the NASA story out this morning: Scientists Watch for a "Hartley-id" Meteor Shower.
Incidentally, this idea isn't new. S&T's longtime meteor writer Joe Rao discussed it on the meteorobs list 13 years ago today.
Update, October 18: With the comet now about at its closest to Earth, its dim coma has grown huge, as seen above — if you have a dark sky! Through light pollution only the inner portion and the tiny, faint central condensation can be seen, leading to wildly divergent visual magnitude estimates.
With the Moon now waxing gibbous and the comet in Auriga, you're got only one or two more mornings (until November) to catch the comet in deep darkness before dawn's first light. Use our online almanac to find your local times of moonset and the start of astronomical dawn.
Update, October 14:Associate editor Tony Flanders viewed Comet Hartley 2 shortly after moonset from his favorite inner-suburban site in Arlington, Massachusetts. It was visible though challenging in 12×60 binoculars, and fairly attractive at 40× through his 7-inch Dob. Only the bright inner halo, about 4' across, was visible. From a suburban location, Hartley 2 is comparable to a typical 8th-magnitude galaxy. Pinpointing the precise location is the key to spotting it.
For the next week, the comet scoots southeast at a breakneck 3° per night, making it ever lower in the evening sky. Moreover, the Moon doesn't set until midnight or later. So most people will find it easiest to observe the comet in the pre-dawn hours.
Update, October 11: "Comet Hartley is now visible with the unaided eye," writes Alan Whitman this morning. Observing from a dark site south of Penticton, British Columbia, he calls the comet "slightly easier than M33," the Triangulum Galaxy. For most people that still means invisible.
S&T's Alan MacRobert called the comet "pretty easy" last night in 8×50 binoculars under moderately dark skies in southern New Hampshire. In 20×80s, he says, "the comet is a big, round, almost uniform glow of dim gray-green with only the barest hint of any central condensation."
Update, October 8: With its small central condensation but a big, diffuse coma, Comet Hartley 2 appears quite different through different amounts of light pollution. Accordingly, observers are reporting it as anywhere from magnitude 5.3 to 7.2 overall.
Ernesto Guido and Giovanni Sostero have created a wide-field animation (7MB) of their images of the comet moving near the Double Cluster last night.
Update, October 6: On Thursday and Friday night, October 7th and 8th, Comet Hartley 2 sails just south of the magnificent Double Cluster in Perseus — a wonderful photo-op.
The comet's halo is quite diffuse, with relatively low surface brightness, so you will probably need fairly dark skies to spot the it through binoculars or a telescope. Click on the finder chart at right for a larger black-on-white version suitable for use in the field.
Update, October 4: Many people viewed or attempted to view Comet Hartley 2 over the weekend. Associate editor Tony Flanders spotted it under a fairly dark sky through 10×50 binoculars. It was a little hard to locate in the super-dense star field of Cassiopeia, but obvious enough once found — a small, medium-bright fuzzblob.
Through his 12.5-inch Dob at 151×, the comet was quite attractive, sporting an elliptical halo perhaps 8? by 6? in size, with a bright, minuscule pseudo-nucleus.
The comet's halo is fairly diffuse, and most people who attempted to spot it from suburban skies either failed or had considerable difficulty.
Original article: An icy visitor is positioning itself for good evening viewing. Periodic Comet 103P/Hartley 2 will be high in the evening sky when at its best during October 2010, glowing at perhaps 6th or 5th magnitude. It should be dimly visible to the unaided eye from very dark, moonless sites, and visible in binoculars and telescopes from less ideal locations throughout 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.
October 1st finds the comet passing 1.5° south of Alpha (?) Cassiopeiae, high in the northeast during moonless evenings. Perhaps 7th magnitude by then, it should remain at least that bright for the next nine weeks. But with the comet just 0.18 a.u. from Earth and closing, much of its light will appear spread out rather than concentrated into a tiny knot. So even if you can sight a 6th-magnitude star with the unaided eye, Hartley 2 will be tougher.
On the evenings of October 7th and 8th in the Americas, the comet is within about 1° of the Double Cluster in Perseus; the two clusters are magnitudes 4.3 and 4.4. This will make for a wonderful wide-field sight and a great astrophoto opportunity — particularly since it's new Moon!
Moonlight becomes a significant factor again starting around October 15th or 16th.
On October 20th the comet is closest to Earth, at a distance of 0.121 a.u. On that date the fuzzy visitor is passing 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 October 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. In November it's a midnight-or-later object heading south across Gemini, Monoceros, and into Puppis. It ends November at perhaps 7th or 8th magnitude, east of Sirius.
By the end of December it fades to 10th magnitude, still east of Sirius.
Spacecraft Flyby November 4th
Early on the morning of November 4th, NASA's EPOXI spacecraft will fly within about 450 miles (700 km) of the comet's little nucleus. This is a continuation of the spacecraft's Deep Impact mission, in which it sent a projectile slamming into the nucleus of Comet Tempel 1 during a flyby in July 2005. The probe will image the nucleus's surface at resolutions as fine as 7 meters per pixel. And as with the Tempel 1 encounter, you can watch on NASA TV.
Deep Impact's new name “EPOXI” is an inelegant contraction of “Extrasolar Planet Observation and Characterization” and “Deep Impact Extended Investigation (DIXI).”
Evidence from Hubble Space Telescope observations on September 25th confirms that Hartley 2’s nucleus is small, probably 0.9 mile (1.5 km) wide. Hubble also confirmed that Hartley 2's nucleus seems to be spewing gas and dust from 100% of its surface, not just isolated vents. Because Hartley 2 is smaller but more active than Tempel 1, scientists are keen to explore the similarities and differences between these two short-period comets.
TV Schedule for the Flyby
From the EPOXI encounter schedule:
"Closest approach of comet Hartley 2 is expected to occur at about 6:50 am PDT [9:50 am EDT Nov. 4] at a distance of 700 km. NASA TV will be covering the EPOXI Flight and Science Teams at JPL from 6:30 am - 8:10 am PDT [9:30 am - 11:10 am EDT].
"The first data downlink after encounter is scheduled to start at E+30 minutes, once the spacecraft's high-gain antenna is pointed to the Earth. The data will continue to download from the spacecraft through November 6th. A post-encounter live press briefing is scheduled for 1:00 pm PDT [4:00 pm EDT] at JPL and will include members from the Flight and Science Teams as well as a NASA HQ representative.
"Prior to the press conference, 5 raw images from closest approach will be released on the EPOXI website, epoxi.umd.edu, as soon as they are available from the spacecraft. Processed versions of these images will be presented during the press conference. The press conference will be aired live on NASA TV." You can watch on the web.
Backtracking the Comet's History
How could a short-period comet that may become visible to the unaided eye go undiscovered until just 24 years ago?
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 well-determined 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 discovery.
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.
Greg Bryant, editor of Australian edition of Sky & Telescope, fondly recalls his first sighting of Comet Hartley 2 in 1991. S&T editors Tony Flanders and Alan MacRobert contributed to this report.