Did you know...

ManDay

Did you know...

...that a comet's tail doesn't indicate the direction in which the comet is flying?

I didn't, but having come across "Deep Impact" on TV yesterday made me wonder how a comet could have a tail formed from its coma when there is no friction to extend the dust to such a tail. And assuming that I understood Wikipedia (where I looked it up) correctly, the tail is caused solely by "solar forces", so to say, and extends away from the sun, which is not necessarily the direction in which the comet is heading.

So, did you know?

(feel free to post other things of that kind)

pallidin

Yeah, it can give a false sense of comet direction to those who don't know how this works.

turbo

You didn't specify which tail. The dust tail does indicate the direction of the comet's trajectory, while the ion tail points away from the Sun. These tails can appear to be opposite in some situations.

ManDay

Then coukd you explain how that dust tail extends without forces such as friction?

turbo

The dust tail is made of material that is ablated from the comet due to radiation from the Sun. As the material leaves the surface of the comet, it can have a different speed and direction than the core of the comet. This is why the dust tail looks flared-out. Material farther behind the comet has had more time to disperse from the travel-path of the core.

The gas tail (ion tail) is made of gases ionized (set aglow) by the Sun's radiation, and it is driven by the Solar wind in the direction directly away from the Sun. The ion tail is often quite well-defined and pointed.

protonchain

If you're asking us to post "did you know" questions, then I have one.

Did you know that summer in the northern hemisphere (aka the time when it gets the hottest) is when we're furthest away from the sun? And that winter (aka when it gets the coldest) is when we're closest to the sun?

Reason being is the tilt.

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LURCH

I remeber this from the last time Halley's Comet came by. TV shows and stories kept showing the comet racing by with its tail streaking the sky behind it. Drove me nuts!

turbo

Ah, yes, the old "comet streaking across the sky" syndrome. Some comets (like IRAS) can move across the sky "relatively" rapidly if they are close to Earth. Most, though, require long photographic exposures to show any motion relative to the background stars and can be seen in about the same place in the sky night after night.

Depending on our viewing angle and the path of the comet relative to us and the sun, the dust tail can often appear curved, while the ion tail will be pointed straight away from the Sun with no discernible curvature. Some old depictions of comets show them as scimitars in the sky.

Edit: I should note that my description of the ion tail is general, and that the tails can be kinked or even disconnect from the coma in some circumstances, but generally they point directly away from the Sun, while the dust tail can be dramatically curved. Google on Comet McNaught for images of one of the most dramatic dust-tails in years.