# Comet Tail ? **URL:** \ **Category:** Factual Questions\ **Created:** [October 27, 2000, 3:34am UTC](https://boards.straightdope.com/t/comet-tail/38546 "2000-10-27T03:34:32Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![DeutschFox](https://avatars.discourse-cdn.com/v4/letter/d/49beb7/32.png) [@DeutschFox](https://boards.straightdope.com/u/DeutschFox)\ **Post date:** [October 27, 2000, 3:34am UTC](https://boards.straightdope.com/t/comet-tail/38546/1 "2000-10-27T03:34:32Z")
Why does a comet tail point away from the sun (when near it), regardless of its own motion? ---
**Author:** ![Lance\_Turbo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lance_turbo/32/6156_2.png) [@Lance\_Turbo](https://boards.straightdope.com/u/Lance_Turbo)\ **Post date:** [October 27, 2000, 3:37am UTC](https://boards.straightdope.com/t/comet-tail/38546/2 "2000-10-27T03:37:27Z")
I’ve always heard it described as solar wind. The solar wind originates at the sun and pushes stuff off the comet. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [October 27, 2000, 3:43am UTC](https://boards.straightdope.com/t/comet-tail/38546/3 "2000-10-27T03:43:48Z")
Wow, third question in a row from you that I can answer… (though correctly or not, I don’t know) Comet tails are made up of gas and dust particles that evaporate off the nucleus. The motion of the comet (nucleus) doesn’t cause it to go in any particular direction - it will just stay beside it. When an astornaut takes a step outside the shuttle, will he be left behind? No, he just flies right along, because there is nothing to slow him down. However, gas and dust particles are very very tiny, and even the smallest force will scatter it in a particular direction. That force comes from the sun. Actually I can’t remember if it’s photon pressure of solar wind (electrons) - I think it’s both. Anyway, that’s why the tails always point away from the sun. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [October 27, 2000, 4:13am UTC](https://boards.straightdope.com/t/comet-tail/38546/4 "2000-10-27T04:13:42Z")
I think it is both. In fact, I seem to remember that there are actually two tails, one caused by each source. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 27, 2000, 5:04am UTC](https://boards.straightdope.com/t/comet-tail/38546/5 "2000-10-27T05:04:00Z")
The two tails are the gas tail and the dust tail. The gas tail is much lighter and more easily affected by the solar wind, and so always points straight away. The dust tail is heavier, and thus tends to curve back a bit due to tidal effects. ---
**Author:** ![DeutschFox](https://avatars.discourse-cdn.com/v4/letter/d/49beb7/32.png) [@DeutschFox](https://boards.straightdope.com/u/DeutschFox)\ **Post date:** [October 27, 2000, 5:18am UTC](https://boards.straightdope.com/t/comet-tail/38546/6 "2000-10-27T05:18:14Z")
“There are only two things that are infinite: The Universe, and human stupidity-- and I’m not sure about the Universe.” –A. Einstein Why would it have a dust trail while traveling in a vacuum? What tidal effects? The Earth’s? ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 27, 2000, 5:47am UTC](https://boards.straightdope.com/t/comet-tail/38546/7 "2000-10-27T05:47:37Z")
> [@](#): > > \*Originally posted by DeutschFox \* > Why would it have a dust trail while traveling in a vacuum? What tidal effects? The Earth’s? When a particle escapes from a comet, it enters into its own orbit around the sun. A gas molecule is so light that the orbital effects are swamped by that of the solar wind and light pressure. So they move directly away from the sun. Dust particles are much heavier and don’t get pushed around so easily as gas. They still get pushed back, but gravity also has some effect on how they move. This puts them in a slightly different orbit around the sun than the comet, so they don’t move directly away from the sun like the gas does. This separates the two tails. It’s this moving in a separate orbit that Chronos meant when he refered to tidal effects. Tides are basically caused by the waters of the oceans moving in a slightly different orbit around the moon than the earth. Of course on earth, the gravity of the earth keeps the ocean from floating away, but there’s not enough gravity on a comet to keep the dust down. ---
**Author:** ![DeutschFox](https://avatars.discourse-cdn.com/v4/letter/d/49beb7/32.png) [@DeutschFox](https://boards.straightdope.com/u/DeutschFox)\ **Post date:** [October 27, 2000, 7:12am UTC](https://boards.straightdope.com/t/comet-tail/38546/8 "2000-10-27T07:12:43Z")
Dtilque and Chronos … thanks … great explanations! I was wondering about the momentum effects of the sun’s photons on the gaseous molecules released by the comet as it approached the sun and as “frozen” warmed up due to radiative heat transfer from the sun. It would seem that any dust released is secondary, due to the vaporization of potentially gaseous molecules first, which bind the non-gaseous molecules. Thanks again. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 27, 2000, 7:56am UTC](https://boards.straightdope.com/t/comet-tail/38546/9 "2000-10-27T07:56:00Z")
> [@](#): > > It would seem that any dust released is secondary, due to the vaporization of > potentially gaseous molecules first, which bind the non-gaseous molecules. Pretty much. The simplest way to describe a comet is as a “dirty snowball”, largely water ice with some gunk (technical term) mixed in. Evaporate the ice, the gunk gets released. ---
**Author:** ![pldennison](https://avatars.discourse-cdn.com/v4/letter/p/9fc29f/32.png) [@pldennison](https://boards.straightdope.com/u/pldennison)\ **Post date:** [October 27, 2000, 3:23pm UTC](https://boards.straightdope.com/t/comet-tail/38546/10 "2000-10-27T15:23:50Z")
[hijack]Anyone interested in cometary science might be interested in reading about the [Deep Impact](http://deepimpact.jpl.nasa.gov/)mission, which plans to launch a copper impactor at the core of Comet Tempel 1 on July 4, 20054. The project is designed to study cometary composition and crater-formation theories. NASA and the University of Maryland are recruiting the help of proficient amateur astronomers with CCD imaging capabilities to provide preliminary information.[/hijack] ---
**Author:** ![right\_said\_fred](https://avatars.discourse-cdn.com/v4/letter/r/9d8465/32.png) [@right\_said\_fred](https://boards.straightdope.com/u/right_said_fred)\ **Post date:** [October 27, 2000, 4:12pm UTC](https://boards.straightdope.com/t/comet-tail/38546/11 "2000-10-27T16:12:25Z")
Anyone in the GAO know exactly how NASA expects to expense an 18,000 year mission to a dirty snowball? That’s got to be an interesting amortization schedule. Just teasing, **pldennison**! ---
**Author:** ![mrblue92](https://avatars.discourse-cdn.com/v4/letter/m/5daacb/32.png) [@mrblue92](https://boards.straightdope.com/u/mrblue92)\ **Post date:** [October 27, 2000, 4:12pm UTC](https://boards.straightdope.com/t/comet-tail/38546/12 "2000-10-27T16:12:56Z")
> [@](#): > > July 4, 20054 Well, at least there’s no hurry. ---
**Author:** ![Munch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/munch/32/5281_2.png) [@Munch](https://boards.straightdope.com/u/Munch)\ **Post date:** [October 27, 2000, 4:26pm UTC](https://boards.straightdope.com/t/comet-tail/38546/13 "2000-10-27T16:26:15Z")
> [@](#): > > \*Originally posted by pldennison \* > July 4, 20054. Yeah, but at what _time_? ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [October 27, 2000, 4:36pm UTC](https://boards.straightdope.com/t/comet-tail/38546/14 "2000-10-27T16:36:22Z")
I was once told that the dust or molecules coming off the comet are ionized by the sunlight which knocked them off, and that the two tails are caused by the magnetic field deflecting the positive and negative particles differently. Was this a load of hooey? ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [October 28, 2000, 1:10am UTC](https://boards.straightdope.com/t/comet-tail/38546/15 "2000-10-28T01:10:37Z")
> [@](#): > > \*Originally posted by Chronos \* > \*\*The two tails are the gas tail and the dust tail. The gas tail is much lighter and more easily affected by the solar wind, and so always points straight away. The dust tail is heavier, and thus tends to curve back a bit due to tidal effects. \*\* Thanks for the clarification.