I know that last answer doesnt really involve DL dynamics. I'm going by what I find that is based in the real world even at the expense of my pet ideas. So there is no real acceleration of the individual particles pre-x-ray in this case, it seems like. I cant see that there would be any greater attraction of the electrons across the coma from the nucleus. Thats not to say that a breakdown(across the coma) could not occur and produce jets or arcs. X-rays just seem to be a different mechanism. Then I would have to say we need to look the case of high energy particles comming in from the other side of a DL to completely understand DL dynamics.

__________________
"Only those who attempt the absurd will achieve the impossible." - M. C. Escher

"Freedom is popular." -Ron Paul

You get the dimensions from images of the comet taken by a spaceprobe, you get the mass from observing changes in the trajectory of the spaceprobe, you divide the (estimated) mass by the (estimated) volume, and out pops an estimate of the average density! B)

Now, for two gold stars, how many comets have been observed sufficiently well to permit an estimate of their average densities, to +/-30% (say; i.e. 0.5 OOM)?[/b][/quote]
The mass is calculated from trajectory changes by means of gravitation formulas only, I suppose?
So comets can have more mass if there is a compensating force, like an electric field maybe?

And now, Nereid, for the stars.

I found calculated densities for Halley, Shoemaker-Levy 9, Wild 2, Borrelly, Tempel 1 and Hale-Bopp.
From these six comets Shoemaker-Levy seems to have its density estimated based on the breakup by gravity. So I'm afraid this is not accurate enough.
My answer is 5.

<_<

gerards regards

__________________
Es gibt keine Tatsachen, es gibt nur Interpretationen. (There are no facts, only interpretations.) Friedrich Wilhelm Nietzsche

Seems our posts 'crossed in the mail':IOW, the devil is in the details.

So, just for fun, how about we work out what the charge difference between a spaceprobe and comet would need to be in order for the trajectory to mimic Newtonian gravity? IOW, let's assume they each have zero mass.

I'll start by sketching an approach:
1. What is the size of the gravitational force we are seeking to mimic?
2. Assume that distances are determined accurately, what can we say about the charges on the probe and comet, in order for the 'electric force' to be the same size as we found in 1?

Just for some fun - no guarantee of correct calculations even though I tried:

Comet 10^12 kg
Probe 10^3 kg
distance 500 km

Fgrav = 2,7 x 10^-6 N

Felectr = Fgrav

q = 8,5 x 10^-3 C where both have the same but opposite charge

To achieve the charge difference in this case 2 x 5,3 x 10^16 electrons are needed.

A 1 second current between the two bodies of 8,5 milliampere would be sufficient to create an attracting force equal to the gravitational force.

In this fun hour and a half (incuding discovering the equations, constants etc) I only used Newton's and Coulomb's equations. I have no idea how to account for double layers and their effects.

gerards regards

__________________
Es gibt keine Tatsachen, es gibt nur Interpretationen. (There are no facts, only interpretations.) Friedrich Wilhelm Nietzsche

Good stuff biknewb!

For completeness (not that I'm saying you got any of it 'wrong'!):
- what equations did you use?
- what values of the constants did you use?

Be sure to specify units (this can be a real minefield - as the engineers working on a certain Martian spaceprobe discovered to their considerable embarrassment).

BTW, the effect of DLs is to 'screen' charges ... in this case, the force between two 'naked charges' will be greater than that between those same two charges immersed in a plasma.

If you're interested, you might like to google on 'debye length'; if you do, you may find a simple formula that contains terms such as the (space) density of electrons (or other charge carriers), and distance (duh!)

Newton's Universal Law of Gravitation
F = G \frac{m_1 m_2}{r^2}

Gravity constant G= 6.67 10^-11 Nm^2 kg^-2

Coulomb's Law
F = k \frac{q_1 q_2}{r^2}

Coulomb force constant k= 9 10^9 Nm^2 C^-2

These laws look very similar, the constants are very different, but so are kg and C.


A little substitution shows that calculating the mass of a body depends also on its charge

m_1 = \frac{F r^2}{(m_2+q_1 q_2)k+G}

The calculated mass of planets and comets might be off if they do carry electrical charge.
Only spaceprobes have been measured directly.

If the EU ideas are correct, the laws of physics probably won't change but the values in the solar system will have to be recalculated.

gerards regards

__________________
Es gibt keine Tatsachen, es gibt nur Interpretationen. (There are no facts, only interpretations.) Friedrich Wilhelm Nietzsche

would you like to know what conspirationists think about this?

Well
that makes this excercise rather meaningless. If charges do not 'see' each other in a plasma, it is obvious that only gravity rules the motions of the heavenly bodies.

gerards regards

__________________
Es gibt keine Tatsachen, es gibt nur Interpretationen. (There are no facts, only interpretations.) Friedrich Wilhelm Nietzsche

Newton's Universal Law of Gravitation
F = G \frac{m_1 m_2}{r^2}

Gravity constant G= 6.67 10^-11 Nm^2 kg^-2

Coulomb's Law
F = k \frac{q_1 q_2}{r^2}

Coulomb force constant k= 9 10^9 Nm^2 C^-2

These laws look very similar, the constants are very different, but so are kg and C.


A little substitution shows that calculating the mass of a body depends also on its charge

m_1 = \frac{F r^2}{(m_2+q_1 q_2)k+G}

The calculated mass of planets and comets might be off if they do carry electrical charge.
Only spaceprobes have been measured directly.

If the EU ideas are correct, the laws of physics probably won't change but the values in the solar system will have to be recalculated.

gerards regards [/b][/quote]
Well done! {You might want to check your calcuations though; it seems your charge is 'off' by SQRT(10) }.No; but let me guess ... that the sooner we all adopt the same system of units (preferably SI), the better?But don't you want to check this out for yourself? I mean, the Debye length is pretty easy to calculate!

And here's another test - a charged object moving in a magnetic field will 'feel' a force that an object with no charge will not, right? And if the charged object is 'chunky' (e.g. a solid, with a decent average density), it won't matter what the tenuous ISM plasma does to screen the charge, right? So, another way to estimate the charge on bodies like comets and spaceprobes is to see how their trajectories deviate from that expected of a neutral body, when they go through a magnetic field, right?

Want to do an OOM calculation? an SL9 fragment colliding with Jupiter perhaps?

Something I found on the ESA Cluster web site. I theorized that there was a plamsa(current) flow from the heliosphere to a body in the solar system.
ex. comet ion tail. See my post about 3-tail comets.
"High-speed flows of plasma (BBF), propagating from the magnetotail to the Earth at velocities higher than 300 kms-1, are the carriers of decisive amounts of mass, energy and magnetic flux. A statistical study based on multipoint measurements by Cluster reveals, for the first time, their typical spatial scale. More than 1600 data points of Earthward flow events were used to deduce the size of the BBFs around 2–3 RE in the dawn-dusk direction and 1.5–2 RE in the north-south direction."
http://clusterlaunch.esa.int/science-e/www...fobjectid=37116

__________________
"Only those who attempt the absurd will achieve the impossible." - M. C. Escher

"Freedom is popular." -Ron Paul

I want to get back to the bright spots on comets Borrelly, Wild 2 and Tempel 1.
Here are the images:
Comet Borrelly:


In this image nothing specific is visible, only hints at brighter surface areas. Part of the caption reads:
The brighter areas were thought to be exposed icy surfaces at that time, but subsequent comet visits showed more detail and the picture changed.

Comet Wild 2


This image shows a stereo view of 2 bright spots, one of which appears to throw a shadow and is possibly "sunlight reflected of a short jet".
From Brownlee’s article in Science vol 304, p1768: Or , of course, they could be electric arcs.

The surface of comet Wild 2 is a hot surface near 300 K, the relief visible in the images shows that likely all of the surface has been active at one time.

From the article:
That’s because Brownlee et al. was looking for holes, crevasses and other vent-like openings, which means they don’t see what is right there in the images: the bright spots.
From another paper it was claimed that at least five jets were associated with bright spots. So, you have a heavily ablated object that is darker than soot, and you’re looking for the source of the dust jets. The one feature that shows correlation with the jets are the bright spots, wouldn’t it be logical to assume those "mysterious" patches are producing the jets?

Closer study shows them more likely to be "clouds" hovering above the surface, as also visible in the Deep Impact images:



Part of the caption reads:

Emphasis on "may", they may also be electric arcs and if some of them can’t be explained away as steep-sloped reflective surfaces, maybe none of them are.
They might as well be called "blind spots", because until now, they are just ignored. It happens whenever there is no explanation that fits into the current model; the observation is downplayed and swept under the carpet, set aside for future use.

You don’t hear from the people studying the data that these bright spots are important, they are just a curiosity, and they rather conclude that "as yet we have not been able to uniquely associate any specific type of landform as jet source".

Fortunately according to A’Hearn the nature of the bright patches will be revealed by the spectrometers. And without calculating a single thing, we could very well witness a breakthrough only by looking at pictures and "connect the dots". Let’s see what the spectral data tell us, arcs produce UV and maybe show emission lines from ions, at the very least they should be distinct from reflected sunlight (unless of course some material can be conjured that exhibits exactly the right "reflectiveness" under reasonable assumptions). Has anyone heard anything from the Deep Impact scientists after the story about what the spacecraft will be doing next?

Cheers.

Just wanted to add this quote to the previous post, it's from Sekanina et al. in Science vol 304:

There is a significant correlation between the bright spots and jets, apparently seven sources coincide ( my guess is that all jets coincide, but that some jet sources weren't imaged).
I'm a bit puzzled by this remark: "16 emanate from sources that are on slopes where the Sun's elevation is greater than predicted from the fitted triaxial ellipsoid". Does it mean they are in places that are not illuminated by the Sun?

Cheers.

Just the opposite VanderL, they eminate from areas the sun is hitting.

__________________
All civilizations become either spacefaring or extinct.~ Carl Sagan ~

Humanity must rise above the Earth, to the top of the atmosphere and beyond, for only then will we fully understand the world in which we live.~Socrates, 500 B.C. ~

Let every man judge according to his own standards, by what he has himself read, not by what others tell him. ~Albert Einstein~

Yep, thanks, you're right, it would have been a problem for both the "snowball" as well as the EU model if it wasn't. Another interesting comment from the Brownlee article is that all jets point radially away from the comet surface. In the "snowball' model this is not expected (as can be seen in many "artist's impressions" of comets ), on the other hand in the EU model they must, so if any cometary jets are found with different angles, it would be a problem for the EU model.

Cheers.

Whoa, there, cowboy! Are you seriously suggesting that we now star making scientific arguments based on artist's impressions?? Now I've seen it all ...

__________________
The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell

Yeah, that was an easy score, point taken.


Here is an article by Farnham et al. trying to model the jet from comet Borrelly and they don't see any of the expected rotational influence on the largest jet's direction , they conclude that the jet is aligned with the rotation axis. The expected curving of the jets was only observed at large distances from the comet's nucleus, they all start out radial. (Yeehaw)

Cheers.

Ok,

There is something to add to this thread, there is news from the Deep Impact team here , while the team expresses surprise about the results, I'm surprised even more.
I guess I'll have to wait a tad longer and read the articles in Science, but the summary they give doesn't bode very well.
Apparently they decided that comets are "dirty dustballs" with no more coherence than "a snowbank", despite all the evidence from this and other comets to the contrary;

• high relief

• impact structures

• the shape of the comet

• the fact that the earliest report was that something "solid" was hit

• cliffs and "spires" on comet Wild 2

They are doing a good job at making a comet disappear while retaining the water and organics that supposedly supplied early Earth with the ingredients for life.

Just a few days and hopefully some really interesting data are released.

Cheers.

I don't see the problem. Ever seen a snowball that had it's outer layer molten and frozen, over and over again? You can have a very fluffy interior beneath a solid crust, which can have clear and sharp features.

__________________
Knowledge is a curse, but ignorance is worse

I guess you haven't seen such a snowball either , all the features will become rounded and smooth eventually, and yes sure, you can have a fluffy interior with a solid crust, but we're talking comets here. Comets produce jets, highly collimated jets that eject dust and supposedly do so by converting heat into steam (ices, volatiles), but with the way a comet is described now, there's no way these jets can be collimated at all, it needs to have strength to build enough pressure.

And then there is the heat transfer, such a solid outer layer (I don't think the DI team said anything about a solid crust, but I might be wrong) would prevent any solar radiation to reach the ices. I also think that the only need to designate a comet "fluffy" is the amount of dust after the impact. Apparently this large amount was not anticipated, otherwise one of the goals of the mission, the crater size, would have been imaged. I'm curious to find out what the publications will bring.

Cheers.

How much pressure do you think is needed to send a narrow jet from the surface of a comet? The gravitational force at the surface of a 6 kilometer diameter comet with a density of 0.25 gm/cc is about 2/100,000 that of Earth's surface. Not much structure is required to maintain the little pressures required to launch dust and ions under those conditions.

__________________
Forming opinions as we speak

Highly collimated jets, probably leading to a rough surface with high relief in areas and having to break through a hardened outer shell of frozen ices.

Nope VanderL, I don't see a problem with the description either.

__________________
All civilizations become either spacefaring or extinct.~ Carl Sagan ~

Humanity must rise above the Earth, to the top of the atmosphere and beyond, for only then will we fully understand the world in which we live.~Socrates, 500 B.C. ~

Let every man judge according to his own standards, by what he has himself read, not by what others tell him. ~Albert Einstein~

Right.

"For example, comet Tempel 1 has a very fluffy structure that is weaker than a bank of powder snow. The fine dust of the comet is held together by gravity. However, that gravity is so weak, if you could stand on the bank and jump, you would launch yourself into space."
"The nucleus of Tempel 1 has distinct layers shown in topographic relief ranging from very smooth areas to areas with features that satisfy all the criteria for impact craters, including varying size,"
"Mission data indicate the nucleus of Tempel 1 is extremely porous."
http://www.nasa.gov/mission_pages/de...act-090605.htm


WHICH IS IT? Porous, fluffy or stiff enough to make surface features.

There should still be some rock left in the impact crater if it was fluffy....
Porous like pumice, never heard of porous dust. Yeah, ok you got me on that one.


First, assuming there is an ice pocket below the frozen crust of dirt? Or is the frozen crust ice and dirty snowball. Anyway its fluffy as a snowbank (underneath). That would be softer than sand. I would think that a jet of steam? or hot water would diffuse through the soft material, certainly before it, ahh, melted through the crust or would it melt the soft fluffy stuff? Do the experiment. Pour cold water on a fluffy snowbank(I snowboard, I know it well). Pour water on dirty road ice. Maybe its made of dirty road ice. If you know of a way to get water to not diffuse in a fluffy dirty snowbank, especially if its porous, please let me know.


Wait a minute, the water goes through frozen channels in the dust, or is it ice? Wait a minute, it has a porous surface that changes temperature quickly, so the jets should turn on and off on rotation. No????

I wonder if it comes with a remote control.....

Oh, I forgot to use comet physics.
I mean come on, you guys, just because somebody that is a "expert" tells you something that is clearly foul, your going to throw away logic?

__________________
"Only those who attempt the absurd will achieve the impossible." - M. C. Escher

"Freedom is popular." -Ron Paul

I suspect you did forget to use comet physics. Logic is, of course, vital. However, "common sense" and intuition can often lead you astray when used outside familiar subjects. I wasn't quite clear on just what you were suggesting, so if you have specific concerns or questions, could you clearly state them point by point?

A couple of general points -

(1) With extremely low gravity, little structural strength is required to retain surface features. This isn't on earth!

(2) The structure is not homogeneous. Some areas will be darker than others causing thermal variations and the material isn't just dust and ice. There are other materials, there will be gas pockets.

At the risk of repeating myself.
I mean come on, you guys, just because somebody that is a "expert" tells you something that is clearly foul, your going to throw away logic?

If you can explain to me how you keep the water(or steam) from diffusing in the porous, fluffy, icy, dirty, dusty, comet with other materials(rocks?), that would be great. AND I'm not asking for math, I'm just asking for a physical mechanism.

__________________
"Only those who attempt the absurd will achieve the impossible." - M. C. Escher

"Freedom is popular." -Ron Paul

The stuf being ejected in comet jets has a velocity of hundreds of m/s as gleaned from the Stardust mission, these jets are highly collimated, the source needs to be heated and still be small eneough and compact enough to produce the pressure needed for the acceleration. The surface material is now described as fluffy, with low coherence. How do you suppose this will work?

And what about the early reports that indicated a lack of water, the double flash, that a hard surface was hit and showed the emergence of new jets in addition to the existing jets? I'll wait for the publication, but I have the suspicion that the DI team has a lot more to tell us. For now it's just a "dusty no-ball".

Cheers.

Same here. I agree that there is more to tell, and I also agree that the mechanism for propelling the dust out of normal cometary jets (not the Deep Impact crater) is something that could use some more observing to get a good model.

__________________
Forming opinions as we speak

That's a misleading statement. There was less water than expected, not "a lack of water."

Inconclusive, but a curious event. I don't think we have enough data to ever determine what that was.

... I'm not sure what you're getting at here. Yes, the outer surface appears to be "hard," similar to how a packed snowball's outer surface is hard while the interals are still mostly loose powder.

Emergence of new jets? I'd expect that if the looser material were suddenly exposed to solar winds by the impactor. I didn't see much reason for surprise, there.

Oh, I'm sure they have tons of data to tell us. And I'm looking forward to the results.

__________________
"Reality is that which, when you stop believing in it, doesn't go away."
Philip K. Dick, Do Androids Dream of Electric Sheep?
"A lie can travel half way around the world while the truth is putting on its shoes."
Mark Twain

Avatar courtesy of Bunny.

No, there was a statement that the amount of water decreased upon impact (Sweden's Odin telescope), it certainly did not increase after impact:

Convenient, my thoughts are that the DI team does have the data, only no good explanation, so it'll have to wait until it can be explained. Where are the UV, X-ray and gamma ray data? Hopefully they are included in tomorrow's publications.


Here is the page with images from the different telescopes http://deepimpact.umd.edu/collab_pub/imagep.shtml click on the jet image of the NOT telescope of La Palma, Spain. I was hoping the jets would have been deemed important enough to comment on by the DI team.


Yep.

Cheers.

You're making assumptions about the structure and contents of the comet. I see no reason to think there couldn't be gas pockets. As far as that goes, packed snow on earth can do a very nice job of blocking the movement of gas. I would expect uneven heating to warm gas (such as methane) or cause gas pockets to form. Near the surface, even relatively low pressure gas could rupture a pocket and cause a plume.

But, I find it interesting that you think "experts" are speaking untruths. Do you think they are incompetent and you understand the subject better than they do, or do you think they are lying?

I think the whole structure, with a crust, possible gas pockets, harder elements in the centre, etcetera, can just as well explain the double flash. Imagine the projectile hitting a thin crust (flash!), breaking through (gas pocket or so), and hitting another, harder or thicker element (flash 2). This way, you get two flashes, and no clear impact crater (as it lays beneath the hole in the upper crust).

This is of course pure speculation, but I don't see any reason why a double flash is such a big problem. It's surprising, it's puzzling, but it is not unexplainable (and there are probably better explanations than mine).

Any news from the flash that supposedly happened microseconds before impact, as predicted by EU proponents?

__________________
Knowledge is a curse, but ignorance is worse

What Thornhill predicted was a pre-impact flash, what was actually observed was this (quoting Schultz):
The key is the moment of the first flash and the time of the delay. I can't check the timing, but earlier reports stated that there was an early flash in one frame of the flyby camera's movie, then one frame delay, followed by the second flash. Frames were 50 msec apart, so with a speed of 10.2 km/s, the probe travelled at least 510 meters (one frame) between the flashes. I'm not sure how to explain this without moving the first flash before the impact.

A double flash is not unheard of when bullets are fired on a solid surface, it's the timing and the spectral signature that is the key. I haven't heard any details yet, but the flash should have been strong in UV to fit EU predictions.

Btw, do you think "fluffy no-balls" can have impact craters?

Cheers.