Sat, 03 Sep 2011 23:37 CDT
As many readers already know, Comet Elenin has begun the irreversible process of breaking up. We spoke earlier about the probablility of such an outcome, but I considered it less than 50%. On the graph at left you can see a selection of ten comets that approach the Sun closer than 0.5 a.u. The red line shows the boundary, to the left of which, derived from J. Bortle's formula, is the safe zone, but to the right is the zone of disintegration. The yellow color shows Comet Elenin, with absolute magnitude obtained by visual observations, and the blue is from JPL-NASA data. As we see, Bortle's formula, all-in-all, doesn't work too badly. Although there is a bright exception - the green triangle belongs to the unique comet 96P/Machholtz, about which I will speak next time.
Now it is absolutely clear that the comet's drop in brightness, first noted by Michael Mattiazzo on Aug. 20th, was not coincidental - the decay process had already begun, and over the course of the next several days the comet changed greatly. Its pseudo-nucleus became diffuse and extended, and later vanished completely. On images from Sept. 1st in the comet's coma there was no condensation visible, and that meant the comet had already broken up into fairly small pieces, with a maximum size of not more than a hundred meters.
© L Elenin
Such a breakup of small comets passing near the Sun is not rare, and in that is nothing surprising. I note that this is a breakup, not an explosion. All the pieces continue to move on the comet's trajectory. The large fragments are likely to continue to disintegrate into smaller ones. It is possible that in October when the comet moves into the morning sky, we will no longer be able to see what once was Comet Elenin. It is possible that something will be visible to large earth-based telescopes. The breakup of a long-period comet fairly close to the Earth (on a Solar System scale) is a rather rare event. During such a breakup we can see the interior of the comet to better understand its construction and composition.
Overall, the most scientifically interesting thing is the breakup scenario, but unfortunately right now the comet is not visible to the largest telescopes or even the Hubble Space Telescope because of its close angular distance from the Sun (small elongation). On the other hand, amateur astronomers, awaiting this comet which might have been visible to the unaided eye, will now not see it, at least visually in their telescopes and binoculars.
We will wait for Sept. 23rd when the comet is due to appear in the field of view of the SOHO space coronagraph. Any result will tell us what we can expect at the beginning of October when the comet once again should appear in the pre-dawn sky. We will wait. The end of this story is near...