There’s sure been a lot of excited talk about the disintegration and disappearance of Comet Elenin this week, but we must be careful not to jump to conclusions based on only a small amount of evidence. I don’t think there’s any doubt that the small, several kilometer-wide comet body itself has broken into pieces, which are now vaporizing into a cloud of dust and gas. That began around the 19th of August when Comet Elenin started to fade and its coma became much more diffuse. The breakup was caused by heat and gravitational stresses from the sun in advance of its closest approach to the sun on September 10th. This behavior is not unusual and has been observed in other comets in the past. It demonstrates to us again that comets are not dense, rock-hard bodies like many of the asteroids. Being composed of ices and dust, they’re subject to break up and dissolution from the sun.
As a single, solid body, the comet has not survived, however the fragments have obviously continued to spew dust and gas, enough to still see a what’s left of the comet in the image taken on the 14th, four days past perihelion. Its brightness at that time was about magnitude 10.5. Mattiazzo also has a photo he took with a telephoto lens on that date which doesn’t show the comet at all. This isn’t surprising since a 300mm telephoto doesn’t have the light gathering power of a telescope.
His observing conditions also have to be taken under consideration. In both photos, Comet Elenin was only 7 degrees above the horizon, which is very, very low. At that altitude, light from stars and other celestial objects is absorbed by the thickness of our atmosphere like a sponge soaking up water, making them appear much fainter than when higher up. Since his photo shows a 10.1 magnitude galaxy, all we can say for certain is that the comet is fainter than the galaxy or roughly below about magnitude 10.5.
So is the comet there or gone? I’m sure that the dust cloud is still there but faint and difficult to photograph because of low altitude and evening twilight. A larger telescope and better conditions would undoubtedly show it. Indeed the comet may be 11th magnitude and just below the limit for Mattiazzo’s scope. Because it will get even closer to the sun from our perspective in the coming days, it may well be impossible to photograph again until it swings into the dark morning sky in mid-October.
As I’ve mentioned before, the comet may appear in the Solar and Heliospheric’s LASCO C3 coronagraph photos beginning a week from today on the 23rd, but I doubt it. The remnant dust cloud will likely be too dim and diffuse to show.
I want to make clear that whatever is left of Comet Elenin when it passes nearest the sun on September 26 will have no effects on Earth. This date shouldn’t be confused with perihelion, which is the time of closest solar approach. It will only appear close because Elenin will lie approximately between the Earth and sun. There will be no three days or even three seconds of darkness like some of you have been reading online. Not only will the comet completely miss the sun by 2 full degrees (equal to four full moon diameters), but the dusty cloud is so exceedingly rarefied it will be undetectable even in a telescope. Rest easy.
I’m eager to see what October will bring. It certainly looks like a large telescope and dark skies will be needed to make out what’s left of the comet. Pity. But there’s a lesson here. No matter what machinations and purposes humans may want to attribute to this little comet, it’s a piece of the natural world and will do what it does. If we turn down the noise, we can better see a comet or any celestial object for what it is rather than what we want it to be. Nature is the best teacher, but we need to sit still and pay attention if we’re to fathom her creatures.