Talk:Extinct comet

Lead and subsequent sections

A recent edit of this article removed 'duplicate' material from the body of the article, and transferred the key citations from that material to appear against relevant sentences in the lead.

Here are two quotes from WP:LEAD

(Lead section)

...The lead should be able to stand alone as a concise overview of the article. It should establish context, explain why the subject is interesting or notable, and summarize the most important points—including any notable controversies that may exist...

Citations

The lead must conform to verifiability and other policies. The verifiability policy advises that material that is challenged or likely to be challenged, and quotations, should be cited. Because the lead will usually repeat information also in the body, editors should balance the desire to avoid redundant citations in the lead with the desire to aid readers in locating sources for challengeable material. Leads are usually written at a greater level of generality than the body, and information in the lead section of non-controversial subjects is less likely to be challenged and less likely to require a source; there is not, however, an exception to citation requirements specific to leads. The necessity for citations in a lead should be determined on a case-by-case basis by editorial consensus. Complex, current, or controversial subjects may require many citations; others, few or none. (emphasis added)

I am reverting the edits because the material was 'duplicated' (though not, I believe, identical) to reflect thte fact that material should not appear only in the lead and not in the body of the article, and leaving the refs in the body because i didn't think it was particularly controversial content that should be immeidately cited. The thing about where the refs should be is a judgement call and I'm happy to see them moved if other editors think differently, but I'm pretty sure keeping the content as it was is consistent with WP policy. Cheers.hamiltonstone (talk) 04:44, 20 March 2009 (UTC)

Category:Comets vs. Category:Lists of comets

Category:Lists of comets is itself a category within Category:Comets — Robert Greer (talk) 14:13, 20 March 2009 (UTC)

Speculative content?

As the article reads now, it claims that comets become asteroids upon depleting their volatile constituents. This is speculative claims and might even be categorized as Original Research. The article needs proper sources and references on these claims and even then it will be categorized as speculative, as there is no scientific consensus about this issue. There is not enough data on comets to back up these claims and our understanding of comets is to pre-mature to draw any conclusions about this process. Indeed several known facts contradicts the claim. Some are already discussed in the article (thank you), but at least three facts should be mentioned as well:

• Comets has a mean density that is very far from that of asteroids. This holds true also for the dormant and extinct comets. The uncertainty of mean density calculations/estimates of comets should however be included in this issue.
• The chemical constituents of comets and asteroids are very different, even when the volatile compounds of comets are excluded. It is impossible to create an asteroid out of a comet, as the materials are simply not there.
• The organic compounds that covers the surfaces of comets are not present in asteroids.

Perhaps a lot of asteroids has been wrongly categorized as asteroids and are in fact extinct (or dormant) comet nuclei. I don't disagree, but does it even makes sense to give estimates on this? we need to document on what grounds these estimates has been made and to address the uncertainty of these "grounds". I see none of that in the article.

RhinoMind (talk) 03:56, 16 November 2014 (UTC)

An object is a minor planet if it has not been seen outgassing. An object is a comet if it has been seen with a coma. Though comets generally have a much lower density than asteroids, comets can have higher densities and asteroids can be rubble piles. Unless an object has a know satellite with a secure orbit determination, you will not know the objects precise mass or density. If an object has a highly eccentric orbit that crosses many planetary orbits, it is currently impossible to tell exactly where the object originally accreted or what the original orbit was 4 billion years ago. The dividing line between comets and asteroids has become much blurrier over the last 20 years. -- Kheider (talk) 05:37, 16 November 2014 (UTC)

Hi and thanks for adressing a few of the concerns I list above. I think the "blurrier lines" should be explained a lot better than it is right now in the article. And with proper sources and referencces to more elaborate and detailed discussions of course. I have the impression, that you know where to find such sources and refs? As the article-text is now, it specifically needs to mention, that the category "asteroid" is blurred. As it reads now, it is very confusing and could even be interpreted that some comets consists of densely packed metals and rocks like the meteorites. This is clearly wrong and this misunderstanding should be avoided by providing a better text. I might write it up myself one of these days, but is grateful to have a discussion like this thread as well. About the mean densities: I don't see how it could be so hard to determine really. If you know an objects trajectory around the Sun, including its velocity, even a highschool student should be able to calculate the mass of said object. To get to the mean density, you simply needs to know about the objects dimensions. Nevertheless I have read, that many estimates on comets mean densities come with an exceptionally large uncertianty, but without a ref to a proper discussion I really canot understand how it can be that difficult. without an explanation and refs, it doesnt make any sense. -- RhinoMind (talk) 21:17, 16 November 2014 (UTC)

You can not securely determine the mass of a comet unless you can measure how it perturbs a similar or smaller sized object. The Sun and planets are so massive compared to the comet that the comet is effectively a massless point source. There are only a handful of comets that we accurately know the diameter, mass, and density of, not to mention that most comets are oblong. I also see no reason that an outer main-belt asteroid with a density of ~3 g/cm³ could not be caught outgassing near surface volatiles that were accreted before the protosun matured. Semantics say such an object would be dually classified as a comet. -- Kheider (talk) 21:59, 16 November 2014 (UTC)

Hi. Do you have some refs and sources about these blurred lines between asteroids and comets? The article needs them really bad. RhinoMind (talk) 23:57, 25 November 2014 (UTC)

• Is That an Asteroid or a Comet? It’s Getting Harder to Tell (12.08.14) -- Kheider (talk) 01:44, 9 December 2014 (UTC)

All good. But Wired? We/you can do better than that. I guess some proper refs will be widely available in a month or two. There is already officially talk about it on ESAs own pages. Haven't looked to see if we can use this as a source in itself. RhinoMind (talk) 19:38, 13 December 2014 (UTC)