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Astronomy: Astronomical objects FA‑class

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	Comet is a featured article; it (or a previous version of it) has been identified as one of the best articles produced by the Misplaced Pages community. Even so, if you can update or improve it, please do so.
	This article appeared on Misplaced Pages's Main Page as Today's featured article on May 18, 2004.
Article milestones Date Process Result January 19, 2004 Refreshing brilliant prose Kept

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Miscellany

There's another comet article at Comets, which needs to be intregrated with this one. --Zundark, 2001 Oct 14

Drat, should have checked before I wrote all that stuff. :) I'll get to work on it. -BD

This page could really use a ordinary picture of a comet, preferably one showing two tales. And maybe a picture of the Jupiter impact. Should be something we could use at NASA. I just can't ever find what I want there. Rmhermen 17:04, Mar 5, 2004 (UTC)

"Some modern Freemasons claim that Stonehenge and similar ancient observatories were used to evaluate whether comets would hit the earth."

Why "modern Freemasons" nobody else can believe it ? This need some explanatio/sources IMO. Ericd 12:10, 18 May 2004 (UTC)

Hmm. Gonna get meself in more @hit here.

Not everyone was surprised by the discovery of X-rays. One astronomer named Jim McCanney actually predicted them. He did so as early as 1981 in a scientific paper first published in the journal Kronos. McCanney even urged NASA officials to look for X-rays when the agency was preparing a fly-by of Comet Giacobini-Zinner in 1985. At the time, NASA's ISEE-3 satellite had already completed its original mission, and was being reprogrammed for comet study. The spacecraft had X-ray equipment on board, and McCanney urged NASA to use it. Instead, NASA shut down the equipment to conserve power. NASA's experts concluded that there was no point in leaving the X-ray detector on, since there couldn't possibly be X-rays coming from a cube of ice.

Noting that McCanney is clearly what Wikipedians in good standing (which I am not) class as a cuckoo, I have two simple questions: was such an article published; did NASA get requests to check for X-rays? Kwantus 05:39, 2005 Feb 1 (UTC)

Should there be something about near-earth comets that may pose a collision danger? According to the Near-Earth object article, at least 49 near-earth comets have been identified... (2005, Jul 6)

Request for references

Hi, I am working to encourage implementation of the goals of the Misplaced Pages:Verifiability policy. Part of that is to make sure articles cite their sources. This is particularly important for featured articles, since they are a prominent part of Misplaced Pages. The Fact and Reference Check Project has more information. Thank you, and please leave me a message when you have added a few references to the article. - Taxman 17:36, Apr 22, 2005 (UTC)

Proposed Edition

The text that follows is my proposed edition to this article. If anybody has a problem with it, identify it before I make the changes to the article.

A comet is a small astronomical object similar to an asteroid, hypothesized to be composed largely of ice. Unlike asteroids, comets typically move in highly elliptical orbits, the aphelia of which may be many times more distant than Pluto's orbit. Often described as "dirty snowballs", or, after the "Deep Impact" mission, "snowy dirtballs", comets are commonly believed to be composed largely of frozen carbon dioxide, methane and water with dust and various mineral aggregates mixed in.

As late as 2002, no conclusive evidence of water had been discovered on any comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. NASA remains confident the water is hidden just beneath the "crust". The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice."

This surprise was echoed in 2004, when comet Wild 2 was visited by NASA's Stardust spacecraft. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile. In addition, no ices were identified on Wild 2.

Comets are believed to originate in a hypothetical and as yet unobserved cloud (the Oort cloud) at large distances from the sun consisting of presumed debris left over from the condensation of a hypothetical solar nebula; the outer edges of such nebulae are assumed to be cool enough that water would exist in a solid (rather than gaseous) state. Asteroids are believed to originate via a different process, but close inspection of comets has revealed no water ices or volatiles, and show that comets appear to be very much like asteroids.

Hmm, well, what was wrong with my edits? I think I presented both POVs correctly. If you wish, you may add a reference to the section in the introduction but having so much disputed information in the intro seems to push it off balance. Also, the scientific community at large still believes comets are made mostly of ice and at the beginning it say "are believe to originate" etc. which is good enough. I think my edition is more balanced, however, you of course are entitled to disagree. User:Sasquatch

I checked the page again and saw that the information I put in was kept mostly intact, only moved. I still think the thrust of it, that data suggests comets have very little, if any, ices or volatiles, should go in the opening paragraphs. The opening paragraph still gives the reader the impression that comets are made of these ices and volatiles, but observations show ices and volatiles are present in miniscule amounts if they're present at all. I'm going to move some stuff over here from my talk page to support this. Plautus satire 14:33, 29 July 2005 (UTC)

This proposed version puts way too much emphasis on the issue of water detection, especially considering that the general consensus in the scientific community is pretty clear. Deep Impact was only a month ago, that's not enough time to claim that comets are now commonly called "snowy dirtballs" rather than "dirty snowballs" (a phrase that goes back decades). In general, there are too many weasel-words being used here. I think Sasquatch's version cleared up all these problems nicely. Bryan 06:27, 28 July 2005 (UTC)

I'm going to move some stuff over from my talk page that demonstrates again and again astronomers changing their views about specific comets being made primarily of ices or volatiles. Plautus satire 14:33, 29 July 2005 (UTC)

In the picture (which is now on the Article page) of Comet Borrelly, what do the colors represent? I assume temperature, but the picture needs a scale. Mcswell 18:51, 25 February 2007 (UTC)

Creationism section

Although contrary to my previous position on this issue, having come across this at least three times in my WikiTravels (recently on Talk:Oort cloud), it is clear a significant number of creationists believe the fact comets are around (and according to them have no natural source) confirms young earth creationism. As such I think a "Creationism" section outlining the argument, linking its primary source Walt Brown, and a short rebuttal is appropriate. - RoyBoy 18:37, 30 November 2005 (UTC)

As discussed in that talk page, this is not really needed in the Oort cloud article and in this one. It fits just fine in the article about the author, Walt Brown, but is not "mainstream" enough IMO to have its own space here. Awolf002 20:31, 30 November 2005 (UTC)

Alrighty, you won't get an argument from me. It does get tiresome to address these issues and have nothing to point to, in order to say, its there and has been addressed. I guess I should indeed do something about that in Walt Brown. - RoyBoy 00:26, 1 December 2005 (UTC)

Done. - RoyBoy 01:44, 1 December 2005 (UTC)

Pulp fiction

"Deeply implausible"? It was written in 1877! And I suppose a meteor shower rendering people blind is plausible? I'm deleting. Trekphiler 04:37, 23 December 2005 (UTC)

Improvement drive

Asteroid deflection strategies has been nominated on WP:IDRIVE. Support it with your vote if you want it to be improved.--Fenice 22:45, 29 December 2005 (UTC)

Etymology

I am including information about the origin of the word "comet" (Gr. "kometes", Aristotle.) CurtLindsay 2005.02.03 22:08 PST.

Comet symbol

Do we really need to have it in the opening sentence? it hardly seems important enough, many users will see it just as a question mark, and I'm not even sure it's very commonly used anyway - I've never seen it myself before, in the literature or elsewhere. I'm sure it could be included lower down though. Worldtraveller 12:59, 12 February 2006 (UTC)

I'd agree with that. There seems to be a tendency to use the lead as a bin for anything miscellaneous, which is roughly the opposite of what it should be. It's also very misleading, since I would read it as saying that the symbol in question is a question mark- we need some cunning way of saying otherwise. Mark 13:10, 12 February 2006 (UTC)

Yes, put it into the last section of the intro paragraph. I would have done that already, but I just see a question mark, so I can not tell if I mess things up moving the text. Awolf002 13:12, 12 February 2006 (UTC)

I suggest keep, the symbol is widely known, and for users read it as a question mark, it's their own problem, they should configure their computer properly first. — Yaohua2000 19:34, 12 February 2006 (UTC)

No-one's suggested deleting it. Mark 20:17, 12 February 2006 (UTC)

I suspect we could interpret Worldtraveller's deletion of the character as a suggestion that it should be deleted. :) Bryan 21:03, 12 February 2006 (UTC)

To help editors, at least, we could replace the character with its HTML unicode entity: ☄ (☄). Bryan 19:14, 12 February 2006 (UTC)

THE COMA IS LIGHT!!!

The statement saying that the comas would be made from dust and gases is more than funny. Think about how fast would such a comet consume if it would liberate dust and gases behind it. No, people! Comets don't leave dust and gases behind it. THE COMETS PRODUCE LIGHT, due to one of the natures laws veritable for all the quickening bodies: ANY BODY PRODUCES ELECTROMAGNETIC FIELD IN OPPOSITE TO ITS ACCELERATION. The properties of this field depends on the properties of that body and its movements. I can not find, the frequency of the light produced by the comets, yet, but I am convinced that it is an universally phenomenon. This theory says that even all the planets (mostly Mercury) has an opposite tail to the Sun, but this one is probably made from microwaves because the acceleration of the planets is smaller than comets. Even the radioactivity of the atomics kernels is due to this universally phenomenon! abel 15:46, 21 April 2006 (UTC)

This is the wrong place to propose new (and unorthodox) theories (dubbed original research in WP). Please, find a venue on a different web site for this type of discussion. Awolf002 22:53, 21 April 2006 (UTC)

Thank you for response. But NOT EXIST other place to propose new theories! And this text is JUST DISCUSSION for THIS place. abel 06:02, 24 April 2006 (UTC)

Not quite. This place/text is meant for discussing the article and possible improvements. So, when proposing such an improvement one should follow accepted policies, and your suggestion runs into WP:NOR. Therefore, I think you want to find a different website altogether to discuss your ideas. Awolf002 01:00, 25 April 2006 (UTC)

This is the most obsurd thing I have ever heard except for the folks that thought there was a space ship behind Hale-Bopp. — Preceding unsigned comment added by ] (] • ])

Most sources say that the coma's brightness comes from the reflection off the coma's dust and gas. The coma is not completely behind the comet, as a huge portion in in front of its acceleration. Also, if the coma was microwave energy, it would not be visible. The comet does, however, emit ultraviolet light, hydrogen, and X-rays, and this is mostly caused be the solar wind's effect on the comet. Also, the tail of the comet is dust and gas and the reflection of it, and so is the coma. It's not impossible that the coma emmits a bit of light, since there is some effects on the comet that cause it to emit energy, especially of high frequency. Actual light emitted directly from the coma, however, has not yet been observed. AstroHurricane001 00:38, 21 November 2006 (UTC)

Misplaced Pages:Version 0.5 Nominations

This article was selected for inclusion in Version 0.5 due to its quality and its importance; however, is it possible for the reference numbering to be fixed? Titoxd 06:27, 31 May 2006 (UTC)

Confirmation of water

Deep Impact confirmed that Tempel 1 at least has large amounts of water ice below the surface, though not much on it, since it would obviously be vaporised by the sun. I'm rewriting some things to reflect this.--Planetary 23:58, 18 August 2006 (UTC)

what is the next big one?

I realize it's not 100% possible to predict, but when will be the next great or significant comet to look out for? I don't want to miss it. Or I wonder how long I have to wait. (I live on Earth.)--Sonjaaa 02:15, 19 August 2006 (UTC)

Halley will be back in 2061, other then that, I don't know.--Planetary 02:45, 19 August 2006 (UTC)

That's actually a good way of putting it. The short-period comets, those with orbital periods less than 200 years or so, we can more or less predict their positions, but of them, only Halley consistently puts on a good show. Most of the really bright comets are long-period comets, that have never appeared before within human history, and we have no idea when one is coming until someone notices it. Shimmin 02:53, 19 August 2006 (UTC)

So if a really bright comet suddenly came, we would only get notice a month or so notice before it's visible? Or how long before it comes is it usually predictable?--Sonjaaa 04:25, 21 August 2006 (UTC)

It depends on how early the comet is detected and the comet's orbit. A number of automated telescopes, that are used for NEO searches, sometimes discover long period comets when they are very faint. Comet C/2002 T7 LINEAR was discovered when it was 17th magnitude and astronomers had almost two years before it reached perihelion. ~ User:LawfulGoodThief 06:03, 26 August 2006 (UTC)

I think the current visible comets section needs to be frequently updated. It is now November, not October. AstroHurricane001 00:41, 21 November 2006 (UTC)

"Dave's Virtual Sungrazing Observatory"

The newly added text with the 'advertising' for this web site is superflouos and it should go. This is not an article about how to find comets. Adding a link this is inappropriate. Awolf002 16:32, 3 December 2006 (UTC)

Comet NEAT

Hi. Is there an article on Comet C/2002 V1? Yes, it's notable, and I am able to provide reliable sources. Please respond so I could decide whether or not I create the article. Thanks. AstroHurricane001 23:23, 30 December 2006 (UTC)

Ok, I guess I'll just create one Here. I know where to find references. AstroHurricane001 23:10, 1 January 2007 (UTC)

Great comets

How bright does a comet have to be before it is classified as a great comet? Is mag. 0 to -7 bright enough? Thanks. AstroHurricane001 21:10, 12 January 2007 (UTC)

"Wide" Page

Is it just me or is the page very "wide?" Mike6271 03:24, 22 February 2007 (UTC)

Looks fine to me. Might just be your personal computer settings. Serendipodous 04:45, 21 March 2007 (UTC)

No he's right the page was too wide because some mentally challenged person decided to put a truckload of random letters together and create a wide page, slowing our computers down in the process. However, I have taken the initiative to delete that, and if you (mentally challenged person) are reading this, I suggest you stop and read the text above the edit screen, because it states there that we have your IP address in storage and can track you down. User: yctaabpjic 11:25, 4 June 2007 (PST)

Confused by this sentence

"One theory holds that as a comet approaches the inner solar system, solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven."

What other theories are there? How is this theory still in debate, even after we've sent probes into the comas of several comets? Serendipodous 04:44, 21 March 2007 (UTC)

Do comets shrink?

Comets trails are made by getting too close to the sun and bits burning off, right? Are they constently shrinking? Are they replenished somehow or will they eventually shrink down to nothing?--Viridis 06:07, 9 April 2007 (UTC)

Well, comets don't "burn" if you mean the chemical process. Instead their tails are formed by water vapor, dust etc. Comets can't replenish their material, so every time they come into the inner Solar System they indeed lose some material. After thousands of orbits a comet has lost all its volatile material and it becomes a "dead comet". Many near-Earth asteroids may have been originally comets.--JyriL 12:32, 9 April 2007 (UTC)

Motion & velocity

I have some questions regarding the above, it is not exactly clear from the article. How exactly do comets start to move, gain speed while approaching the sun, and loose it again when moving away from the it?

For example the outer cloud is only weakly bound to the Sun, but still rotating slowly, therefore it has some initial speed. So I presume a future comet has to be influenced by another object, making it move outwards of it's natural rotation in the Oort-cloud. Or is it caught by the gravity well of the Sun, and then pulled towards it?

Secondly, if moving towards the sun or jupiter, how does the comet's speed increase? I'm quite puzzled that mademade rockets are unable to push it beyond 17.5 km/s (Voyager 1), while comets can gain speeds up to 50 or 60 km/s, even without active propulsion. A comet doesn't have have some kind of propulsion, so the increase in speed can only be from gavitational pull, or am I mistaken?

thirdly, at perihelion the speed of a comet is about 500,000 km/h, but at aphelion it has slowed to only a few thousand km per hour. How it this possible? I can't think of what should slow it down in a near vacuum, except for some roaming dust. Evenmore, as the comet is moving away from the sun, shouldn't it actually gain speed, as the gravitational pull of the sun becomes weaker the further it travels? Or is it actually this pull that slows it down ;) I'm confused since the Voyagers and Pioneers have reached escape velocity, but comets (who appear to have a greater speed) do not? Comets appear to obtain great speeds at ease, even if their nucleus has a diameter of 50 km. So I'm wondering why or rockets can't. Thanks to the one(s) that can give some enlightment! --Patrick1982 10:20, 20 August 2007 (UTC)

Wow there is a lot in there to answer. The overall answer though is its all the sun's gravity doing the work.

It's peturbations by other object that set a comet on a path to the inner solar system. It could be orbiting the sun for billions of years and then finally pass quite close to another object sending one, the soon to be comet, on its way towards the sun. Now, remember what newton said? An object won't change speed unless a force is acting upon it. The object was already moving at a slow rate and now begins to fall towards the sun... and fall... and fall... and fall possibly for hundreds if not thousands of years. All the time it is falling it is accelerating as gravity is acting upon it, weakly at first but growing in strength all the time. It reaches its max velocity as it passes its closest point to the sun then the forces go into reverse - now gravity will no longer speed the comet but slow it as it moves away. Two things can happen now - either the comet can fly off into space and never be seen again like Comet McNaught or it can continue to orbit the sun as many comets do, like Halley's Comet. Now - let's compare to Voyager etc. While a comet reaches it max velocity close to the sun where it will also face the full braking effect of the sun's massive gravity as it moves away voyager reached it maximum speed way out by Jupiter, where the sun has a lesser hold. The Sun's gravity still slows voyager down every day - just a tiny bit, as it does to a comet all the time it moves away. Now escape velocity is an interesting thing... and its not a constant. It depends on where you are and the path you take. If we assume the sun isnt moving (which it is) a path directly away from the sun would require a higher escape velocity than a path at a tangent. And of course the further away from the sun you start the lower speed you need to escape. So 17km/s out past Jupiter may well be an escape velocity, but 50 or 60 km/s near the sun might not be.

Now gravity and rockets work to accelerate things quite differently. A rocket has a fixed ammount of energy - it can accelerate a mass to a given speed, or twice that mass to half that speed etc. Gravity doesnt care how heavy an object is it will accelerate it at a rate that depends only on how far away it is. Voyager has been travelling for 30 years or so now... and in all that time its rocket engines only fired for a couple of minutes. For the whole of the rest of the time it has been accelerated (and decelerated) by gravity. It has gained more momentum from gravity than any rocket we could build could have given it. How did it gain speed from gravity rather than just being slowed down? Well that was due to clever use of the alignment of the planets and getting a "gravity assist" from each one to the next.

It was realised a long time ago by Kepler that in eliptical orbits a line joining a planet and the sun sweeps out equal areas during equal intervals of time as the planet travels along its orbit. This means that for planets and comets the closer you get to the sun the faster you move and vice versa. For comets with highly eliptical orbits this effect is very pronounced.

Last but not least don't underestimate how powerful the sun's gravity is. Imagine how many rockets it would take to fly Jupiter, a planet so big it has storms that could swallow this planet whole, around in a big circle. The sun's gravity is powerful enough to do exactly that even though they are half a billion miles apart. --LiamE 13:36, 20 August 2007 (UTC)

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