The Comet Doth Please The Queen

Comet Hartley 2Â zips through Cassiopeia the "W" this coming week. With binoculars, look for a dim cloudy patch at the nightly positions shown. Illustration created with Stellarium
The comet on September 20. The green color, from fluorescing gas, and is only faintly visible in a larger telescope. Credit: Michael Jaeger

As Comet 103P Hartley 2 cruises through the bright constellations of Cassiopeia and  Perseus this month, it will be relatively easy to see in binoculars from the outer suburbs and countryside. At present, it looks like a dim misty patch of light among the rich star fields Cassiopeia the Queen. Yesterday night I estimated its size at 2/3 that of the full moon and brightness at magnitude 8. That’s two levels below the faintest star visible to the naked eye for most of us, but don’t think that means you can’t see it. Once my eyes were adapted the darkness, I spotted it in both 8×40 and 10×50 binoculars without difficulty.

Comet Hartley 2's orbit (in blue) takes it near the Earth in October. Closest approach will occur on the 20th. Credit: Caltech/JPL

It helps that Cassiopeia is high in the sky for northern hemisphere sky watchers, clear of horizon haze and much of the typical city light pollution. Hartley 2 should brighten further in the coming weeks as it makes its closest approach to our planet on October 20. On that day, only 11 million miles will separate us, or about 1/8 the distance of Earth to the sun. The hazy glow visible in binoculars is called the comet’s coma (KOH-ma). Those with keen eyes and dark skies will also be able to pick out the brighter nuclear region at the center of the coma. It’s unclear if much of a tail will develop, but if one does, I’ll report it here. Telescopic observers will see the nuclear region with ease as well as be able to tell that the coma is not perfectly circular.

The heart of all cometary activity is the nucleus itself, in this case, a small body made of ice and dust only about 1/2 mile across. When a comet’s orbit takes it into the inner solar system, heat and light from the sun vaporize some of the material, which then expands outward to form a temporary “atmosphere” or cloud around the nucleus. This is the coma we see in our binoculars. The nucleus itself is deeply shrouded within its dusty outflow and invisible except to the eyes of space probes able to get close enough for a good view. Thanks to NASA’s excellent planning, that’s exactly what will happen when the re-purposed Deep Impact probe, now called the EPOXI Mission, flies by the comet with its shutter clicking in early November.

Trouble finding the W of Cassiopeia? Face north and look to the left to locate the Big Dipper. A line drawn from the two end stars in the Dipper's Bowl extended upward will take you first to Polaris, the North Star, and then on up to Cassiopeia. The map shows the sky around 8:30-10 p.m.

With the moon out of the evening sky and Cassiopeia well-placed, now’s the time to hitch your wagon – and eyes – to a comet. I encourage you to share your Hartley observations, seen or unseen, on the blog using the comments link. Thanks!

5 Responses

  1. John Brophy

    I have started to take an interest in astronomy in the last couple of months and have found your column to be very interesting and helpful.

  2. Bob Cavanaugh

    Good article and pictures this week to show where the comet is. I looked for it last night and although it was very clear, I still could not see it from NC. Perhaps later this week if it brightens up to mag.5-6? Thanks Bob!

    1. astrobob

      Hi Bob. Reports are coming in from a few observers who’ve seen with the naked eye the past couple days. They must have some incredible skies and vision. I find that it’s a dim but obvious glow through 10×50 binoculars. The star fields in Cassiopeia are so rich, it’s a little tricky on nights to distinguish the comet from the stars. Keep at it though, and I bet you’ll see it soon. It is definitely brightening.

  3. Pingback : Dawn to dusk remix | Astro Bob

Comments are closed.