Comet at 1000 km | Hacker News Reader

Comet at 1000 km

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48 pointsColinWright9 comments

I expected it to be smoother. I guess that makes sense, though, hurling through a vacuum isn't going to cause mechanical erosion.

Some are smoother. The population is pretty diverse but mostly since solar radiation causes outgassing over the surface, it's likely to get shaken up a bit especially as bits pop off.

Damn, that's not as pretty as they look in the sky. What gives?

Yeah, I wonder why the camera didn't catch the comet's tail.

There's a possibility all the volatile components have already been burned off by the solar wind (which is what causes the tail). That would make this either a very old, or very small comet. Or one that wasn't that icy to begin with.

Or the truth, which is that's it's far enough away from the sun that it's currently so cold that there are no volatiles being boiled off. Hence there is nothing for the solar wind to be blowing.

Technically the outgassing is considered a sublimation since it's not a liquid ;)

The outgassing is mainly a function of the magnitude of the solar radiation (which varies quite a bit) and the cross section of the comet, so it's doubly variable and may outgas really far from the Sun if the conditions are right.

The comet is currently not close enough to the sun to have the activity leading to a large, visible tail (this would be in conflict with the mission's aims). However, emissions have been detected since April 2014.

There's no set distance from the Sun for where a comet tail will develop, as it depends a lot on the size of the comet and the magnitude of the solar wind at the particular position.

This comet's aphelion is around Jupiter, and comets have been observed with tails (from telescopes, not naked eyes! :P) at that distance. This comet probably is too small though.

See my other comment for a better explanation of why the tail wouldn't be visible in this image.

Here's a cool image of Comet Humason that was taken while it was near Jupiter in the 60s, it's pretty epic, one of the most awesome comets I came across in my studies, this image is from an obscure journal that I can't find now, but I found the image on a forum :P

http://foro.tiempo.com/index.php?action=dlattach;topic=12940...

Partly the reason is contrast. The light reflecting off the surface is much brighter than the tail itself, if you wanted to catch the tail it would overexpose the nucleus.

Besides that, the comet tail is formed from the outgassing from the Sun-facing surface of the comet and then wrapped up in the magnetic field of the solar wind, so the bright stuff you see in the sky is usually something like 10 thousand (or more) kilometers away from the actual nucleus, the bright part you usually see from Earth around comet is the coma (nebulous gas) not the solid surface of the comet, with the tail extending out many hundreds of thousands of kilometers behind it, though technically the tail will extend as far as the solar wind carries it once it's entrained in it :)

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