As Comet 103P Hartley 2 cruises through the bright constellations of Cassiopeia andÂ 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.
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.
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!