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Cignus
2007-Nov-01, 06:16 PM
OK, aside from the impressive sight of the very sudden amazing brightening of Comet Holmes, I was observing it last night and it occured to me that its what, 165 million miles away? In the eyepiece its angular size is way, way WAY bigger than the planet Mars. Now I'm thinking that, as solar system objects go, the core of this comet is not all that large, a few tens of kilometers maybe (I have not seen any write-ups about that). So it occurs to me to see if anyone has calculated the explosive power it required to blow enough dust and gas out to a diameter of what has to be 6 or 8 times the diameter of Mars, enough dust and gas to glow brighter than mag 3! It is mind boggling to me that a cold hunk of dust and ice moving away from the sun, that was mag 17 and did not really do much of anything during its travel close to the sun, and now as it exits halfway to Jupiter it explodes with enough force to through out the volume of dust needed to create the size and brightness. That's not just a couple pounds of evaporating ice! We're talking some serious blast power are we not? Anyone besides me wondering where that power came from?

Tucson_Tim
2007-Nov-01, 06:19 PM
It is now larger than the planet Jupiter.

http://www.spaceweather.com/



The comet's physical diameter is thus seven times wider than the planet Jupiter--and it is still expanding.

antoniseb
2007-Nov-01, 06:24 PM
Anyone besides me wondering where that power came from?
As I mentioned in another thread on this topic, I have assumed that this comet is a several kilometer wide dust ball sweeping through the asteroid belt. The chances of it hitting a 50 meter wide asteroid are small, but not that small. A fifty meter wide hunk of rock at a relative speed of five or ten kilometers a second would certainly be enough to eject a cloud that size.

astromark
2007-Nov-01, 06:37 PM
A perfect example of the most likely is the fact. What else could have done this...? If it were to just open and explode it would have done that when it was under more tension, ie nearer the sun. Only a collision with some other object could release such massive force so quickly... Suggesting it has had a collision is perfectly in line with the facts we can now see.

Swift
2007-Nov-01, 06:40 PM
So it occurs to me to see if anyone has calculated the explosive power it required to blow enough dust and gas out to a diameter of what has to be 6 or 8 times the diameter of Mars, enough dust and gas to glow brighter than mag 3! It is mind boggling to me that a cold hunk of dust and ice moving away from the sun, that was mag 17 and did not really do much of anything during its travel close to the sun, and now as it exits halfway to Jupiter it explodes with enough force to through out the volume of dust needed to create the size and brightness. That's not just a couple pounds of evaporating ice! We're talking some serious blast power are we not? Anyone besides me wondering where that power came from?
I haven't done the calculation. But maybe it isn't all that much force. As you point out, the nucleus of the comet is rather small - its not like there is a big gravitational field the dust has to overcome. What's the escape velocity from a 10 km diameter sphere of ice?

Once it escapes from the surface, there is no resistance (its a vacuum), so the particles would just keep going and the cloud would keep expanding. I suspect someone could calculate a velocity for the how the size of the cloud changes over time.

antoniseb
2007-Nov-01, 06:56 PM
I suspect someone could calculate a velocity for the how the size of the cloud changes over time.
The velocity isn't the issue so much as how much dust did it take to make the comet a million times more reflective of sunlight. The velocity could easily be calculated, but a small explosion or a large one would probably each impart the same velocity.

Tucson_Tim
2007-Nov-01, 07:01 PM
It's been just over a week and last night it was noticeably "fuzzier" to the naked eye, in contrast to its star-like appearance when it first exploded (outbursted?).

Also, it looks like it will make a nice equilateral triangle with Alpha and Delta Persei within another week.

Hornblower
2007-Nov-01, 07:48 PM
A perfect example of the most likely is the fact. What else could have done this...? If it were to just open and explode it would have done that when it was under more tension, ie nearer the sun. Only a collision with some other object could release such massive force so quickly... Suggesting it has had a collision is perfectly in line with the facts we can now see.

I don't think a collision is the only possible explanation.

Suppose some millions of tons of water vapor and other volatiles are bottled up under the crust, which is being progressively weakened by evaporation of surface ice. That evaporation will continue well past perihelion, and the breaking point could come at any time.

A good analogy would be a high-pressure boiler that is being weakened by creeping rust and metal fatigue. That sucker could blow up at any time, not necessarily when the pressure was at its highest.

Romanus
2007-Nov-01, 09:50 PM
^
That's my currently-favored explanation. Assuming its discovery was tied to a similar outburst over a century ago, it just seems a little too convenient for two significant impacts to have that kind of dramatic effect in that period of time.

In short, I'm thinking 17/P is a much more dramatic analogue of 29/P (Schwassman-Wachmann 2), which undergoes regular outbursts as well. Of course, it doesn't help that we don't know the cause of those, either...

Jens
2007-Nov-02, 02:06 AM
As I mentioned in another thread on this topic, I have assumed that this comet is a several kilometer wide dust ball sweeping through the asteroid belt. The chances of it hitting a 50 meter wide asteroid are small, but not that small. A fifty meter wide hunk of rock at a relative speed of five or ten kilometers a second would certainly be enough to eject a cloud that size.

What about the fact that the same comet also exploded in 1892 or whenever it was? Wouldn't it seem a little bit too coincidental that the same comet would happen to hit two asteroids in the space of one century? I'm not all that well informed, though. Are collisions between comets and asteroids frequent or not?

Tucson_Tim
2007-Nov-02, 03:27 AM
Are collisions between comets and asteroids frequent or not?

For long period comets, I'd say no. For main-belt comets, maybe. For a Jupter-Family comet like 17P/Holmes, not sure.

Wicked Lad
2007-Nov-02, 03:33 AM
So I brought the exploding comet to my 13-year-old stepdaughter's attention by e-mail, and she wanted to know why it's exploding. I did a quick Web search and wrote back:

It appears no one is sure why comet 17P/Holmes is brightening like this. I found a Web site that says it "could be due to a sudden exposure of fresh cometary ice or even the breakup of the comet nucleus. (http://antwrp.gsfc.nasa.gov/apod/ap071026.html) " I think the "exposure of fresh cometary ice" refers to the fact that most comets are covered with dust. If an area of dust got removed somehow, maybe the solar wind would blast away at the ice, and the ice would reflect a lot of sunlight.
Tonight she replies:

That could be it.... But would the ice really be that reflective? And is this comet very big? Why would the nucleus break up? I'm wondering how scientists know this....
I'm so proud of that kid. All excellent questions, and such economy of expression. And the only question I think I can answer is about the (approximate) size of the comet.

I have to take the blame for that last bit about "how scientists know this." I made her tatoo "How do you know?" on her forehead backward so she sees it every time she looks in the mirror. Okay, not really. But I've told her over and over she needs to ask that question. She needs to ask it a lot.

Anyway, I've been reading what I can find about what would explain the "outburst." I think I have to deploy that other sentence indispensible in scientific inquiry: "We just don't know."

What about her other question? Is ice really that reflective? If not, what's likely to cause the glow? And BTW, is it safe to assume it would be water ice?

Devino
2007-Nov-02, 12:47 PM
My questions are; what causes comets to emit/eject stuff out that creates it's head and tail? I would assume it's the sun's solar wind reacting with the comet body causing it to eject small pieces out and away from the sun but what is a solar wind, I mean besides ionized gas? Is it something natural that all stars do or is it a reaction to an outside force? Does this process strip away material from a comet decreasing it's size over time or do the particles just relocate never to venture far from the comet? Can a large planets magnetic field cause a similar reaction to a comet that comes too close traveling at a high velocity and perhaps in a retrograde motion?

I am having trouble with the idea of comet Holmes illuminating in such a uniform (spherical) manner due to an impact. I suppose it is hard to tell from here and it does seem plausible but could it be a reaction similar to our Suns solar wind that is the cause? I know this last idea just adds more questions then it could answer but thats what makes it all fun for me. :)

Another odd thing is that comet Holmes appears to have a small tail but not in a direction away from our Sun's solar wind. I don't know how accurate this info is but if there is a tail it looks like it may be pointing in our direction.

Michael Noonan
2007-Nov-02, 12:56 PM
Would an analysis of its spectra indicate whether it was an impact or a charge induced event?

One of the pictures showed fairly clearly what appeared to be jets.

Tucson_Tim
2007-Nov-02, 02:08 PM
My questions are; what causes comets to emit/eject stuff out that creates it's head and tail?

A good article on wiki:

http://en.wikipedia.org/wiki/Comet

Romanus
2007-Nov-02, 02:36 PM
<<What about her other question? Is ice really that reflective? If not, what's likely to cause the glow? And BTW, is it safe to assume it would be water ice?>>

My own limited understanding of the matter is that what we're seeing from Holmes is dust, not vaporized ice. The bright yellow color is characteristic of dust envelopes (gas envelopes tend to be fainter but more colorful, with blue or green hues depending on the compounds involved). In fact, the yellow color is from reflected sunlight, though cometary dust itself is very dark. However, if you put a *lot* of something poorly reflective together, you can end up with something very bright.

In any event, if water ice is involved, it's probably a secondary component only.

John Mendenhall
2007-Nov-02, 09:22 PM
No time to discuss this right now, it's dark, I have to go look at the beautiful comet.

I live in an urban area, the seeing is terrible, I'm getting great results with 7x50 binoculars. Go check it out!

Wicked Lad
2007-Nov-03, 12:43 AM
Thank you, Romanus. John, do you really have good viewing there? We're all clouded over 50 miles northeast of you.

JohnD
2007-Nov-03, 11:59 AM
We're seeing this as a globular cloud, because the tail of the receding comet happens to point away from the Earth. But cocmets can have two tails, one dusty and straight while the other is charged (?) and curved.
A curved tail would be visible, I presume as a pear-shaped extension of the cloud but it isn't there. What does this tell us about Holmes? Or rather, what is known about comets that have, or don't have, curved tails?

John

Hornblower
2007-Nov-03, 12:48 PM
We're seeing this as a globular cloud, because the tail of the receding comet happens to point away from the Earth. But cocmets can have two tails, one dusty and straight while the other is charged (?) and curved.
A curved tail would be visible, I presume as a pear-shaped extension of the cloud but it isn't there. What does this tell us about Holmes? Or rather, what is known about comets that have, or don't have, curved tails?

John

I think you have them backward. An ion tail, swept away at high speed by the solar wind, is nearly straight and points almost directly away from the Sun. The particles of a dust tail move much more slowly from solar wind and light pressure, and generally lag behind the ion tail in a curved pattern such as that of McNaught.

At Holmes' distance from the Sun, the light and wind pressure are relatively weak, and we are seeing a colossal amount of dust that was expelled in a roughly spherical pattern and is slowly being pushed back. An ion tail would be faint and pointing almost directly away from us. The dust would make it hard to see from this angle.

paul schroeder
2007-Nov-09, 10:27 PM
What's the chance that the source of the light is not from an explosion, but from the accumulation of photons? The concept would be Dopplewr in nature and similar to sound when the source is moving toward the observer. The rays of photons are emitted forward with a shorter wave lenght by C - speed of Holmes comet. The higher intensity would occur when the comet is aimed most directly toward the observer. Additionally, those rays sent out nearly forward would arrive in a group as their forward motion would exceed their sideways motion. They would thus produce the halo seen for Holmes.

antoniseb
2007-Nov-09, 11:11 PM
What's the chance that the source of the light is not from an explosion...
The light is not from an explosion. It is sunlight reflecting off of dust. The thing that is curious is the question of why so much dust departed from the nucleus in a brief time.

Hornblower
2007-Nov-10, 03:28 AM
What's the chance that the source of the light is not from an explosion, but from the accumulation of photons? The concept would be Dopplewr in nature and similar to sound when the source is moving toward the observer. The rays of photons are emitted forward with a shorter wave lenght by C - speed of Holmes comet. The higher intensity would occur when the comet is aimed most directly toward the observer. Additionally, those rays sent out nearly forward would arrive in a group as their forward motion would exceed their sideways motion. They would thus produce the halo seen for Holmes.

The comet is not moving toward us, but rather across our line of sight and slightly away. Even if it was moving directly toward us it would be too slow to have any visually significant Doppler effect.

galacsi
2007-Nov-10, 07:12 AM
The light is not from an explosion. It is sunlight reflecting off of dust. The thing that is curious is the question of why so much dust departed from the nucleus in a brief time.

Yes and in such an orderly manner , keeping spherical all the time. No jets or whirls. Even if we can see a short tail now.

Look like ,all of a sudden , a spherical layer of dust has been release to expand in space.

Very strange indeed !