Talk:Comet

Hill sphere dimensions - very outdated

The article gives a size of 230,000 AU for the Hill Sphere of the Sun, but the reference it cites for that is a a 1964 Paper by Chebotarev. That's very old, and as it depends crucially on the estimated mass of the galaxy - an estimate that's changed greatly with advances in astronomy and astrophysics in the nearly half century since that paper was written. That paper is reproduced in English here - the relevant section is section 6 "solar gravitational sphere", pp621-622 of the original journal. There Chebotarev says "For the mass of the galaxy we assume the value M=1.3 × 10¹¹ solar masses." And indeed if one runs that through the formula given for a simple (non-eccentric) system given at Hill sphere, one gets a value for r of 225837 AU (with the value for the semimajor axis Chebotarev cites); that's consistent with the 230,000 value he gives. --165.155.196.123 (talk) 20:12, 7 November 2011 (UTC)

That paper is 46 years old, and it looks like the current estimates of the galaxy's mass are quite different. Looking first at Wikipedia's numbers - Milky Way says its mass is roughly 1.4×10⁴² kg, and the Sun article says its mass is 1.9891×10³⁰. Going by those figures, the mass of the galaxy is 7.038×10¹¹ solar masses, making the galaxy 6 times more massive than Chebotarev's numbers. Running that through the equation (with a slightly refined value for a, again from Sun) gives us a Hill radius of 130,261 AU (down from Chebotarev's 230,000).

But there's more. If you ask Wolfram Alpha for "mass of the galaxy" it says 6×10⁴² kg which it says is "based on 2009 velocity data from Mark Reid using the Very Long Baseline Array (VLBA)" (I guess that's the finding discussed here). That gives us a mass ratio of 30.2×10¹¹, making the Hill radius about 80,000 AU - that's about a quarter of Chebotarev's number.

Moreover, ask Wolfram Alpha "distance from the sun to the centre of the galaxy" and it says 2.349×10²⁰ m (it cites a bunch of astronomical sources). That's quite a bit lower than the values given by Wikipedia or Chebotarev; plugging that into the equation and we get an estimated Hill radius of 75,350, a third of Chebotarev's number.

I'm not proposing that we use the numbers from my calculation, but it does look like Chebotarev's calculation is based on a very wrong estimate of galactic mass. We need a modern reference with a calculation based on modern observations. -- Finlay McWalter ☻ Talk 18:49, 16 January 2011 (UTC)

I used Gillessen2009 for the distance to our galactic center as 8.33 parsecs (27.2 light-years) Template error: {{Convert}} no longer supports abbr=none. Use abbr=off instead.. I have done some Wikipedia work using barycentric coordinates for the apoapsis distance of bound comets listed at non-periodic comet. I notice that JPL Horizons shows Comet West has a barycentric apoapsis distance (AU) of "AD= 6.977038306313576E+04 (epoch 2031-Jan-01)" (~69770AU). That is very close to your suggested number. Unbound comets seem to travel out to 10^91 AU. I certainly agree that we need a more recent reference for the solar hill sphere. Besides, I would really like to know how close Comet West is to the edge. -- Kheider (talk) 02:54, 2 February 2011 (UTC)

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Spacecraft Imagery

Some spacecraft photos of some nucleuses with dust or particles jetting out would be a nice addition.--Tablizer (talk) 05:37, 16 February 2011 (UTC)

Contradiction tag

The following has been cutNpasted from the contradiction tag in the article for better exposure here: "reason = Contradicts following statement (and statement made in the intro) that a few comets are permanently (mathematically) thrown out of the Solar System after a single (observed) pass. Also contradicts literal meaning of "single apparition". The explanation seems muddled about extremely long period vs single-apparition comets. (kheider notes: Readers may be confused by difference of Parabolic is e=1 and Hyperbolic is e>1. I may remove this whole sentence and comment as redundant.)" (I am not the author of the tag.) --S. Rich (talk) 02:02, 23 February 2011 (UTC)

Yes, that section needs a clean-up. The problem is that there is a very small difference in overall velocity of a comet like Comet McNaught that should return in ~92,600 years (semi-major axis=~2050AU) and a comet like C/1980 E1 that should get lost beyond the Oort Cloud (>80,000AU or so). The Osculating perihelion eccentricity is never a good indication whether a comet leaves the solar system since often the osculating eccentricity is larger than 1 in the inner solar system, but the future (or proper) eccentricity is smaller than one when it gets beyond the influence of the planets. I have been slowly trying to build-up List of non-periodic comets and Barycentric coordinates (astronomy) so that there might be some good sub-articles and references to pull from. But we still need more references. -- Kheider (talk) 03:04, 23 February 2011 (UTC)

List of Visible Comets

I think that a "List of Visible Comets" (i.e. of above Mag 6), in order of date of greatest brightness, would be of use. If it would be too long, make it a "List of Easily Visible Comets", with a cut-off of Mag 3 or 4. 94.30.84.71 (talk) 13:00, 3 May 2011 (UTC)

I think this page use to have such a thing. But unless it is updated ever month, it would quickly become dated and incorrect. Such information can be located at: http://www.aerith.net/comet/weekly/current.html -- Kheider (talk) 15:41, 3 May 2011 (UTC)

Distance from the sun

There is no average distance from the sun listed. If an expert could do this it would improve the article--174.34.41.239 (talk) 13:55, 14 June 2011 (UTC)J28

There is no average distance to the Sun because comets like 2P/Encke take 3.3 years to orbit the Sun, while comets from the Oort Cloud can take upwards of 30 million years to orbit the Sun. -- Kheider (talk) 16:11, 14 June 2011 (UTC)

Recent comets documented at Flickr

Upon a cursory search at Flickr to see what comets have recently been easily visible, I came up with:

• McNaught 2006 mag -4
• Comet Hale–Bopp 1997 mag 0
• Comet Hyakutake 1996 mag 0
• Comet Holmes 2007 mag 2.5
• Comet Lulin 2009 mag 5

I was also reminded of how confusing the difference between a comet and a meteor can be, because pictures of them are so similar. Lots of pictures of meteors came up under a search for comets, so it's apparent that folks really don't know the difference. I've never seen a comet, and I would've flown to New Zealand to see McNaught, if I had known about it. Heyzeuss (talk) 13:59, 8 August 2011 (UTC)

You might be interesting in Brightest comets seen since 1935 (International Comet Quarterly). -- Kheider (talk) 17:54, 9 August 2011 (UTC)

Page protection?

Based on all the recent vandalism, I think it is time to protect this page. -- Kheider (talk) 18:51, 15 November 2011 (UTC)