Identifing a track

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freyda rose
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Identifing a track

Post by freyda rose »

I have a simple question. Are tracks always straight?
fjgiie
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Re: Identifing a track

Post by fjgiie »

Hi freyda rose,

What makes you say that?
freyda rose
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Re: Identifing a track

Post by freyda rose »

The samples all looked straight, but in some movies a line looks almost exactly like the samples except for a small curve. So I want to know if it is possible that a track could have a curve it it.
fjgiie
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Re: Identifing a track

Post by fjgiie »

My hope was that you would say since the stardust was traveling so fast the tracks should be straight. I think most tracks are straight. I believe one track was not straight but that was caused by distorting the Aerogel during extraction. We have seen tracks split in the Aerogel with each part going in a slightly different direction but not a curved path.
freyda rose
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Re: Identifing a track

Post by freyda rose »

Thanks so much for reply about whether or not a track is likely to have a curve! That was very helpful information. Happy hunting.
jsmaje
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Re: Identifing a track

Post by jsmaje »

Fjgiie is right in my experience of all the tracks so far seen, but I have a naive physics question:
If the impacting particle were to be spinning at a significant rate for whatever reason, could that theoretically impart a curving trajectory?
gebils
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Re: Identifing a track

Post by gebils »

A bullet fired into gell or water always travels in a straight line and it is spinning.
jsmaje
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Re: Identifing a track

Post by jsmaje »

gebils wrote: A bullet fired into gell or water always travels in a straight line and it is spinning.
Gebils, are you sure? Having done a bit of googling, what about the Magnus_effect for example?

And from here I quote (my emphasis):
  • In 1976 Peter Bearman and colleagues from Imperial College, London, carried out a classic series of experiments on golf balls. They found that increasing the spin on a ball produced a higher lift coefficient and hence a bigger Magnus force. However, increasing the velocity at a given spin reduced the lift coefficient. What this means for a football is that a slow-moving ball with a lot of spin will have a larger sideways force than a fast-moving ball with the same spin. So as a ball slows down at the end of its trajectory, the curve becomes more pronounced.
Would that last sentence not be applicable to what happens when a tiny light dust particle hits aerogel? In contrast, I imagine that a relatively large, dense and heavy bullet will still be travelling enormously fast even after entering low-density gell or water, so any curving tendency would be small or at least undetectable as you say. What's more it will be spinning around the same axis as its direction of flight, rather than perpendicular or whatever.

Presumably the devil lies in the details - weight, densities, speed, Reynold's number, spin axes, degree of turbulence etc. And even if there is any theoretical curving tendency, it could be so small as to be unresolvable in the VM movies.

Meanwhile, there must surely be other effects on the trajectory, such as aerogel condensation to one side, and/or particle fragmentation, which look to me possible reasons for the terminal deflection in CM track 29, along with distortion of the aerogel during extraction, as evident in Elaine's image here from ISPE Update 6 (the dark curving slash).

Over to you Dan and the experts.
John
gebils
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Re: Identifing a track

Post by gebils »

What are the results in a vacuum at zero gravity?
jsmaje
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Re: Identifing a track

Post by jsmaje »

gebils wrote:What are the results in a vacuum at zero gravity?
Interesting question! I wonder if they've tested that issue on the International Space Station. I'd assume there'd be no curving tendency.

But after hitting the aerogel any impactor will of course no longer be in a vacuum environment. OK, there'd be no air (assuming prior complete out-gassing in space) but plenty of silica and other molecules to have chemico-physical interaction with. While initially in static solid form, aerogel rapidly becomes heated and fluidised, even gasified?, as the result of the impact. Hence those classic 'carrot tracks' and the blobs of condensed aerogel in the current CM tracks and enveloping the impactor. This must surely have some effect on a spinning particle's trajectory, though whether enough to notice within the few microns of penetration is another matter.

John
DanZ
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Re: Identifing a track

Post by DanZ »

jsmaje wrote:Meanwhile, there must surely be other effects on the trajectory, such as aerogel condensation to one side, and/or particle fragmentation, which look to me possible reasons for the terminal deflection in CM track 29, along with distortion of the aerogel during extraction, as evident in Elaine's image here from ISPE Update 6 (the dark curving slash).
I'm pretty sure the extremely curved and very dark line here is not a track. The curvature in this example is only very slight and the LABELED track extends back from the particle "almost" straight (but it does curve just a bit, which happened during extraction).

Otherwise, I'll see if there's any other data on curving tracks (or if I'm entirely wrong here!). But so far, everything we've encountered that I know of is basically straight.

Dan
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Re: Identifing a track

Post by DanZ »

Turns out the "dark line" you see here appears to be a region of denser aerogel -- it doesn't seem to have anything in it except silicon and oxygen. We don't, however, understand how it formed or whether it is somehow associated with the formation of the track.

The track itself is straight except for some barely perceptible bending due to the mounting procedure. And although the other stardust tracks we've found are, for the most part, straight; our experience with the cometary tracks and with light gas gun experiments is that they can curve substantially, so that the track near the terminal particle is a few degrees off of the initial trajectory. So, as Dr. Westphal has stated before, expect the unexpected!

Nevertheless, right now, straight seems to be the correct search image. What I'm not sure of though is why Track 29 (and others) have a split appearance where the track appears to come in along one line, “terminate” (or condense some aerogel), then continue on a bit off to the side. I will therefore continue to push for answers.

Dan
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