Thursday, April 7, 2011

Will C/2010 X1 Elenin, the Comet of Doom, Evaporate?

Elenin as seen from the southern hemisphere at closest approach in October.

As I’ve written before, I am not sure why comet C/2010 X1 Elenin is attracting so much attention from the Doom and Gloom brigade, when C/2009 P1 was predicted to be about as bright (it’s been downgraded now). But ironically there has been a new spate of videos (and here) claiming Elenin is a Bad Thing ™ at the same time as there has been lots of discussion in the comet observers lists as to whether the comet will survive its passage of the Sun.

The various claims about Elenin include a) it will hit us, b) it will cause earthquakes c) we will pass through the tail and unspecified Bad Things ™ will happen.

The Earth as seen from Elenin at closest approach on 16 October (in this Celestia simulation)

Elenin will not hit us. The closest it will come is around 0.24 AU (where an AU is the distance from the Earth to the Sun). That’s only a little closer than the closest approach of Venus to Earth, and roughly 100 times the distance from the Earth to the Moon. That’s really, really far away (you think it’s a long way to the shop if you want a sausage roll, but that’s nothing compared to space*).

As the comet passes through the asteroid belt as it is doing now, and is exposed to the small gravitational tugs its orbit will change very slightly. This is important to us guys with big telescopes who need to point them really, really accurately, but even if the orbit changes by the distance of the Earth the to Moon (which would be a pretty big shift), then Elenin is still nowhere near the Earth. Nor is the debris in the comets orbit going to hit. We might get a decent meteor shower, but I wouldn’t hold my breath. As it is, the comet has passed it closest encounters, and its orbit hasn’t changed appreciably.

Eleinin will not cause earthquakes, as I’ve written before, even at closest approach Elenin has less than a billionth of the tidal force of the Moon. It will do nothing. Passing through Elenin’s tail will do nothing, its tail will be doing a very good approximation of a vacuum (and its not even clear its tail will reach Earth). We’ve been through cometary tails before of bigger and brighter comets (eg Halley), and nothing has happened.

How big is comet Elenin, then. Lots of people are claiming it is “big” for example “..when i look at the size of the orbit that this thing is huge, heavy…”. Well, orbital radius has nothing to do with the size of a comet, lots of long period comets are quite small. In the case of Elenin, it’s around 3-4 km in size, a bit smaller than Halley (6x 16 Km). We have a reasonable idea of its size from its brightness. Roughly speaking, at a given distance from the sun, the bigger an object is, the brighter its is. Of course, this also depends on how dark it is as well. Comets mess things up a bit as they are brighter than you expect from their size alone, as they are surrounded by a coma of gas and dust reflecting the light of the Sun. Now, comet Elenin is currently roughly magnitude 14, if it were the size of say, the asteroid Vesta, with a diameter of 530 Km, it would be at least magnitude 7 at its current distance. So Elenin has to be fairly small.

We’ve seen all this panic before. In 2006 Comet 73P/Schwassmann-Wachmann came within 0.08 AU of Earth, 3 times closer than Elenin. The comet disintegrated spectacularly, and Earth passed through its tail. There were cries of doom, destruction and catastrophic earthquakes …. and nothing happened. Just as no disaster struck in 1996, 1983 (two very close comets), 1961 and so on when other, more impressive comets came closer to Earth than comet Elenin will.

Now I mentioned that comet Elenin might evaporate. The cometary community has been keeping a close watch on Elenin. It’s not spectacular but still interesting and should be a nice binocular comet. If it survives perihelion. It is an intrinsically faint comet, and its light curve is developing in a manner similar to other comets that have vaporized on perihelion approach. After all, they are just dirty snowballs, or icy dirt balls, often barely hanging together. The gas pressure that develops as the comet comes close to the Sun can tear it apart into small icy chunks which rapidly evaporate.

Currently opinion on the comets list is that Elenin has a 50% chance of surviving. But as with all comet predictions we will need to keep a close eye on Elenin as enters the inner solar system. I hope it makes it; I’m looking forward to imaging the comet. Still, it would be interesting to see the conspiracy theorists reactions if Elenin evaporated when it got close to the Sun.

* Obligatory Hitch-Hikers guide to the Galaxy reference.


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