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  #1  
Old 12-30-2010, 11:12 PM
Stink Fish Pot Stink Fish Pot is offline
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Halley's Comet - when will it finally melt away?

A couple of questions regarding Halley's Comet:

1) As it nears the Sun every 76 years or so, tons of ice, dust, etc. fly off the comet to produce the comet's tail. How much material does the comet lose on a single pass of the sun, and

2) is there any estimate on how much material is left and/or how many more trips around the sun the comet has before it's disintegrated completely?

3) Is there any way to guess how big Halley's comet was at the beginning of its life? I guess there is no way to know this for sure, (since HC has been seen, some think, as far back as 467 BC). However, in the 2500 years since, the comet must have reduced size significantly. Or perhaps not.

If the comet is estimated to be the size of our moon (which I'm pretty sure is a generous estimate), given the amount of material that blows off the comet during each pass, we should be able to figure out how many passes the comet has before it is gone.

4) Will the comet actually be gone, or will it continue to orbit as it does now, only not showing itself off with a tail? Is the comet a complete ball of ice, or is there a large rocky core?

Thanks in advance.
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  #2  
Old 12-30-2010, 11:39 PM
enalzi enalzi is online now
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From Wikipedia

Quote:
In 1989, Boris Chirikov and Vitaly Vecheslavov performed an analysis of 46 apparitions of Halley's Comet taken from historical records and computer simulations. These studies showed that its dynamics were chaotic and unpredictable on long timescales.[40] Halley's projected lifetime could be as long as 10 million years. More recent work suggests that Halley will evaporate, or split in two, within the next few tens of thousands of years, or will be ejected from the Solar System within a few hundred thousand years.[35] Observations by D.W. Hughes suggest that Halley's nucleus has been reduced in mass by 80–90% over the last 2000–3000 revolutions.[15]
Also, it's nowhere near the size of the moon, (and never was). The comet it's 15 km across on it's long side. The Moon is about 3500 km across.
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Old 12-30-2010, 11:46 PM
dracoi dracoi is offline
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I think the amount of material coming off the comet is a lot less than you think. 18 mL of water turned into gas will fill up 22.4 liters of space at 1 atmosphere of pressure. Many of the nebulae that look so spectacular are still so diffuse that we'd consider them a vacuum here on Earth. It won't take very much frozen material to produce the halo and tail we see.

While waiting for better answers, I can answer the size and composition questions using Wikipedia: http://en.wikipedia.org/wiki/Halley's_Comet
In short: More dust than ice, with the ice mostly water and carbon monoxide. It's density suggests a loosely packed rubble pile rather than a solid object. 11 km is the diameter - nowhere close to the moon, but plenty big enough that even a few thousand tons of gas wouldn't affect its total size much.

Since it is primarily dust, you wouldn't expect Halley's comet to ever go away entirely except by breaking up or colliding with something. But once the ice goes away, it'll be virtually invisible, like most asteroids.

Last edited by dracoi; 12-30-2010 at 11:46 PM..
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Old 12-30-2010, 11:47 PM
Stink Fish Pot Stink Fish Pot is offline
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Quote:
Originally Posted by enalzi View Post
From Wikipedia



Also, it's nowhere near the size of the moon, (and never was). The comet it's 15 km across on it's long side. The Moon is about 3500 km across.

How does anyone know the size of what halley's comment was at its origin? Granted, I figured the moon was way too large, but is there some sort of an upper limit on how big a comet can be?

That comment was also fascinating in that it said that between 85-90% of the comet has been removed over the last 2000-3000 revolutions. If that's true, how in the world does the comet have a lifespan of 10 million years?
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  #5  
Old 12-31-2010, 04:51 AM
Khadaji Khadaji is offline
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I can't remember the exact date, but it will be a Tuesday. In the morning.
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  #6  
Old 12-31-2010, 07:30 AM
Alessan Alessan is online now
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Have all of the comets out in the Oort cloud been there since the creation of the solar system? Or are new comets being created, and if so, how?
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Old 12-31-2010, 10:47 AM
Stink Fish Pot Stink Fish Pot is offline
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Quote:
Originally Posted by dracoi View Post
I think the amount of material coming off the comet is a lot less than you think. 18 mL of water turned into gas will fill up 22.4 liters of space at 1 atmosphere of pressure. Many of the nebulae that look so spectacular are still so diffuse that we'd consider them a vacuum here on Earth. It won't take very much frozen material to produce the halo and tail we see.

<snip>

Since it is primarily dust, you wouldn't expect Halley's comet to ever go away entirely except by breaking up or colliding with something. But once the ice goes away, it'll be virtually invisible, like most asteroids.
Thanks for the info, dracoi. Your last paragraph implies that at the core of a comet is a rocky center (basically, an asteroid). Is this something we know to be true or something we are just guessing? I always figured this was the case also, but if it is just a layered, giant snowball with a core of more compact ice/dust, then theoretically, it could just disappear one day while it's ejecting matter?

Also, I can see what you are saying regarding matter removed from Halley's comet each time it nears the sun, but the point is, eventually, that stuff would have to reach a mass of zero (i.e nothing left). Which again raises the question of how large HC could have been when it was "born", how many passes of the sun will destroy it, and how scientists on earth could possibly predict how long it has left. If they can do that, they should be able to work backwards to get to some sort of approximate originating size. Or at least how big it might have been at approx. 500 bc. Is that correct?
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Old 12-31-2010, 10:53 AM
Johnny L.A. Johnny L.A. is offline
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There goes that rotten Halley's Comet. It makes me sick, I want to vomit!
-- B. Kliban
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Old 12-31-2010, 11:31 AM
Alka Seltzer Alka Seltzer is offline
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Quote:
Originally Posted by Stink Fish Pot View Post
Also, I can see what you are saying regarding matter removed from Halley's comet each time it nears the sun, but the point is, eventually, that stuff would have to reach a mass of zero (i.e nothing left).
During each pass it loses much more of the volatile material than dust or rock, so it won't reach zero mass due to normal cometary processes.

Quote:
Originally Posted by Stink Fish Pot View Post
Which again raises the question of how large HC could have been when it was "born", how many passes of the sun will destroy it, and how scientists on earth could possibly predict how long it has left. If they can do that, they should be able to work backwards to get to some sort of approximate originating size. Or at least how big it might have been at approx. 500 bc. Is that correct?
It's not possible to work out it's original mass, as we don't know when it was perturbed into it's current orbit. Halley's comet spent most of it's life in the outer solar system, far from the sun. It would be possible to make a very rough estimate of it's mass in 500 BC.
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Old 12-31-2010, 12:11 PM
Markxxx Markxxx is offline
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Poor Halley's comet

Of course considering the poor showing he/she (what gender is a comet?) put in, in the 80s I guess he/she is already making plans to check out.

I also hear in a few million years the big dipper will be gone too. How will we find the North Star? And I also heard the North Star of today is not the same North Star of previous times.

Oh what a mess this is!!!!

Please won't someone think of the astronomers!!!!
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Old 12-31-2010, 12:54 PM
Stink Fish Pot Stink Fish Pot is offline
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Quote:
Originally Posted by Alka Seltzer View Post
During each pass it loses much more of the volatile material than dust or rock, so it won't reach zero mass due to normal cometary processes.
I'm not sure I understand this. Can you add to this?

Quote:
Originally Posted by Alka Seltzer View Post
It's not possible to work out it's original mass, as we don't know when it was perturbed into it's current orbit. Halley's comet spent most of it's life in the outer solar system, far from the sun. It would be possible to make a very rough estimate of it's mass in 500 BC.
I agree with this. I've come across a few items that suggest that it might be possible to back into the length of time the comet has been in its current orbit, implying that a comet has a known, maximum size.

The wiki article states:

In 1989, Boris Chirikov and Vitaly Vecheslavov performed an analysis of 46 apparitions of Halley's Comet taken from historical records and computer simulations. These studies showed that its dynamics were chaotic and unpredictable on long timescales.[40] Halley's projected lifetime could be as long as 10 million years. More recent work suggests that Halley will evaporate, or split in two, within the next few tens of thousands of years, or will be ejected from the Solar System within a few hundred thousand years.[35] Observations by D.W. Hughes suggest that Halley's nucleus has been reduced in mass by 8090% over the last 20003000 revolutions.[15]

How these guys could work out Halley's lifetime, or how much Halley's nucleus has been reduced is beyond my understanding of what seems to be known. These could just be astronomical WAG's, but they may have some actual science associated with their computations.
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  #12  
Old 12-31-2010, 01:08 PM
Der Trihs Der Trihs is offline
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Quote:
Originally Posted by Stink Fish Pot View Post
I'm not sure I understand this. Can you add to this?
The volatile elements are the ones that are being boiled away by the Sun; dust and rock survive sunlight just fine, so little of them leaves the comet.
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  #13  
Old 12-31-2010, 02:37 PM
dtilque dtilque is offline
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Quote:
Originally Posted by Alessan View Post
Have all of the comets out in the Oort cloud been there since the creation of the solar system?
More or less. Current thought is they were ejected from the inner parts of the system very early on. However, the Sun's gravity is quite weak out there and it's quite possible that there may have been some comet swapping with other stars that have passed nearby over the eons. It's also possible that some comets come directly from interstellar space. Such comets would have distinct orbital characteristics (hyperbola instead of a parabola) and none have been found yet.

But note that periodic comets such as Halleys do not come from the Oort Cloud. Instead they come from the Kuiper Belt.

Last edited by dtilque; 12-31-2010 at 02:39 PM..
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