That’s just how the cookie crumbles, I guess. Things don’t look good for our once steadfast friend and/or ‘agent of doom’ (perspective your choice) Comet Elenin. It’s faded to 10th magnitude according to recent observations by Australian amateur astronomers including astrophotographer Michael Mattiazzo, and now appears as little more than a smudge low in the western sky as viewed from the southern hemisphere. While moonlight and low altitude make Comet Elenin appear dimmer than it would be if higher overhead, the photo says it all.
Comet Elenin’s nucleus, the small, solid body at the center of the fuzzy coma estimated at several kilometers across, has almost certainly broken into icy bits due to the powerful gravity and heat from the sun en route to perihelion. Small comets, which are a friable mix of ice and dust, just aren’t built to withstand the intense solar heating they experience when swinging near the sun after eons in the deep freeze of the outer solar system.
The breakup appears to have begun about August 19. That’s when the comet’s brightening trend stopped and slipped into reverse. At the same time, the core or nuclear region began to fade and elongate. Radio astronomers using the Greenbank Bank Telescope in West Virginia examined Comet Elenin in the “light” of radio waves on September 7 and reported no water, a good sign of the comet’s breakup.
My hunch is that disintegration would cause a rapid increase in the amount of water at first as fresh material was exposed to sunlight. This would then be followed by a quick decline as the fragments vaporized away. Since these were the first radio telescope observations, the peak of water production may have passed, leaving little left to detect. I’m basing this on similar behavior observed in Comet C/1999 S4 LINEAR in July 2000. You can read the fascinating story of S4 LINEAR HERE.
As Comet Elenin drops into the sunset sky in the coming days, our next chance to see it will be about the 24th, when its dregs will cross the field of view of the coronagraph in the orbiting Solar and Heliospheric Observatory (SOHO). Perhaps we’ll see a glowing dust trail. As for viewing the comet in the pre-dawn sky in October, it really looks like you’ll need a telescope … if anything’s left to see at all.
One thing you’ll have no problem seeing is the beautiful Harvest Moon tonight and tomorrow night. Your calendar may say it occurs on the 12th, but for North and South America, the moment of full moon is early tomorrow morning. That means it will be as full tonight (‘fuller’ actually) than tomorrow night.
Harvest Moon is the full moon closest to the fall equinox which this year falls on the 23rd. Because the moon is moving steadily northward along its monthly path at full phase in September and October, it rises at nearly the same time for several nights in a row. Check out the diagram below and you’ll see moonrises are only about 20 minutes apart. The opposite happens in spring, when the moon’s path at full takes it steeply to the south. That steep angle means the moon requires more time to ‘climb up’ to the horizon and rise above it. The white arrows in each view will help you picture the difference. Spring rising times are over an hour apart.
Before the advent of modern technology, farmers used the additional moonlight in the early evenings to harvest crops, hence the name. You can find out when the Harvest Moon is rising for your town by clicking over to the U.S. Naval Observatory’s Sunrise/Sunset/Moonrise/Moonset tables for the year.