2 or more apparent tracks on same movie?

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Nikita
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Post by Nikita »

JStevens,

In reply to your email, no, there is no way to hit more than one. You just pick the best canidate and go with that. But if there are more than one, then you can post in on the forum and it will be noticed faster by the team and it can be reviewed.

As stated earlier in the thread, it is highly unlikely that there would be two tracks on one slide, much less have 4 or 6. Whatever is there, odds are it isn't IDP.

Good luck!

bmendez
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Post by bmendez »

Hi All,

Based on what we have seen so far, there is no reason to expect the need to identify more than one feature in a single focus movie. The only examples we've seen of clustered strikes are high angle tracks that are likely results of ricochets off of the aft solar panels.

Current thinking is that the particle flux may have been lower than originally predicted because the interplanetary magnetic field carried by the solar wind was stronger during the collection period than originally expected. So we may have collected fewer than the estimated 45 interstellar particles. Just how many we do expect is being worked out by a graduate student here.

Based on statistics using the original estimates, however, we should expect at this point to only have identified 0.7 interstellar particles. So, since we have so far found 0 we are well within expectations.

Thanks,
-Bryan
"I am made from the dust of the stars, and the oceans flow in my veins"
- RUSH

DTF
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Post by DTF »

Bryan,

Considering what you have now stated, can we go back to when there were more participants contributing and more reasonable expectations of success for them, more precisely stated using Phase 1 rather than Phase 2 for scoring/rating purposes? I for one, and I'm pretty sure there are many others, do not think that the Phase 2 criteria was particularly helpful, although the greater resolutions were. The program lost a lot of participants during phase 2, didn't it? I know in my case that was not because I didn't want to participate, but because I was totally lost on what I should be looking for based on the new and somewhat confusing examples of "probable" tracks on testing/scoring movies. I doubt the Phase 2 testing/scoring movies were helpful in finding anything of value, but were actually negative influences to participation in finding something of value.

the moon
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Post by the moon »

Phase 2 wasn't all that bad. It was supposed to be a different way of examining the same movies again while we wait for the rest of the tiles to be scanned. The examples they gave us to look for did not likely resemble real tracks, but they were the most "probable" examples they could give us because, like I've suspected since the end of phase 1, there just aren't any interstellar particles in the tiles scanned so far. Bryan's new estimate would mean there's 2 particles in the whole collector. So chances are they haven't been scanned yet.

I think a real particle will be fairly obvious though. Hopefully when phase 3 starts we'll all laugh about how we tortured ourselves in phase 2 debating if these little specks are tracks or inclusions. That is if there will be a phase 3. I fear the tiles may be too contaminated after all these years and the tracks will lost among all the dust and cracks. But hey prove me wrong!

Star Gezzer
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Tracks

Post by Star Gezzer »

I have looked at over 1,000,000 movies and I do not believe I have seen anything resembling a particle track other than the hat tracks. Anything traveling at those velocities no matter how small is going to do some damage. Using the hat tracks as an example. Some of them traveled quite far horizontally to the surface of the aero gell after hitting the frame or whatever. Which would lead me to believe that with a direct perpendicular hit there would be obvious damage. Probably a slight eruption on the surface and a track going the full range of focus. Which may or may not be visable depending on what the material was which is why we are here. Come on phase 3

jsmaje
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Post by jsmaje »

the moon wrote:Bryan's new estimate would mean there's 2 particles in the whole collector.
Oh dear - just 2 predicted interstellar particles in the entire collector? Seems my 'foreboding' could sadly be justified. I've worried before (though for different reasons) whether the collector was ever in fact likely to have found what was initially predicted.

Over a magnitude less likelihood in spotting a track greatly changes what volunteers have been led to expect in exchange for their time and effort, and many might well now be discouraged from continuing (though I'm sure the inveterate top quartet of Kevin A Courtney, drush, zioriga & Star Gezzer can be relied on to keep the boat afloat).

Personally, I'd like more explanation, evidence and an estimate of reliability about this latest analysis of potential particle impacts. Those of us with a scientific background can take the gory details!

Thanks, John

the moon
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Post by the moon »

Yeah more explanation is always good, I understand this forum doesn't have a big audience though so don't stress out on it.

It does seem strange though that they would be revising their estimate now. Was there some new data about interplanetary wind fields, or what ever, that recently came to light?

Also maybe I misinterepted the 0.7 estimate. I assumed he got the number by using the prelimenary estimates from the new thinking that the particle flux was lower, and multiplying by the % of the collector scanned so far. That's why I said there'd be 2 in the whole thing. But now I think he based the number on the 45 estimate and was refering to how many tracks have been extracted so far, not how many movies have been seen.

jsmaje
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Post by jsmaje »

I'm as confused as the moon.
Bryan has stated: "Based on statistics using the original estimates, however, we should expect at this point to only have identified 0.7 interstellar particles".

Use of such terms as "statistics", "original estimates", "however" and "at this point" all in the same sentence always makes me feel queasy.
So:

(1) Just how many interstellar particle impacts are now estimated to have occurred during the spacecraft's IS collector deployment? Fewer than the initial figure of 45 it seems, but only 2? (i.e. 0.7 x 3, given that only a third of the collector has so far been examined)?

(2) Or is it that the statistics (assuming random impact distribution) are that 0.7 particles per third of the collector is the simply the minimum to be expected within 3 standard deviations or so?

(3) And/or (as the moon now wonders) is it that too few extractions (if not movie examinations) have yet been made to be sure of the final yield.

(4) Finally (as the moon also thinks strange), why has it taken this long to take account of the prevailing solar wind and magnetic field? Naively, I'd have thought they should have been of some relevance from the start. Were they not measured by the Stardust spacecraft itself, or are they retrospective measurements from other sources and therefore yet more derived estimates?

John

jsmaje
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Post by jsmaje »

Still no answers to the above questions of over a month ago (despite PMing Bryan twice) ...

ZackG
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Flux estimates

Post by ZackG »

I don't really know the full answer to your question, but I'll put a word in to Ryan and see if maybe I can't get him to come and pitch in his two cents. He is the postdoc who is working/did work on the flux calculations. (I think they're done now ... for the moment, with an intention to revisit them as we proceed).

Essentially the original prediction was made based on measurements from Ulysses and a couple other spacecraft. It looked good, it made sense. Then Stardust came back and ... where were the particles. Knowing that most likely some assumption must be awry, Ryan started digging through the models and "reassuming." By modeling the environment of the system, he made new predictions as to how many particles made it in to the collector during the specific times the collector was collecting. This was no longer a generic "x particles / m^2*year exist at 1.8 AU." It actually takes into account flux changes over the years and determines the counts for the specific months when the fluxes matter. The model still has assumptions, and unfortunately, we can't get around that because we're a bit off into the forest, where the path is well lit experimental ground. :-)

So like I say, maybe I can invite him to pipe up.

Zack
Zack Gainsforth
Space Sciences Laboratory
UC Berkeley

ogliore
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Interstellar Dust Simulation

Post by ogliore »

Hi John and everybody -
I am the aforementioned Ryan. I understand your confusion, let me try to explain the situation.

The original estimate of the ~45 particles in the collector was done by Landgraf in a Planetary and Space Science paper in 1999, which was of course prior to the Stardust launch. As Zack mentioned, this was based on interstellar dust observed by the Ulysses spacecraft and confirmed by Galileo. Their work is very convincing - it really can't be doubted that there is a stream of interstellar dust particles present in the solar system. The flux of interstellar dust particles greater than 0.6 micrometers in width that they calculated Ulysses saw was 0.00015 per square meter per second. So if you multiply 0.00015 by the size of the Stardust interstellar collector (0.1 square meters) multiplied by the collection time (~180 days = ~1.5 x 10^7 seconds) you get ~200 particles.

So now we enter the magnetohydrodynamic reality of the situation: interplanetary space isn't a quiescent peaceful place for small charged particles (ionized by solar UV) to hang out. The charged interstellar dust particles experience a Lorentz force due to the solar wind magnetic field. Since the charge on a dust particle is proportional to the surface area of the dust grain and the mass of the particle is proportional to its volume, the larger particles have a smaller charge to mass ratio and are less affected by this phenomenon. The smallest interstellar particles are effectively evacuated from the inner heliosphere where the largest particles aren't really affected. This phenomenon isn't constant, but varies on an 11-year (most generally, 22-year because the polarity of the solar dipole flips) cycle (related to sunspots) - the result is a focusing/defocusing of the population of interstellar grains in the inner solar system. The Ulysses data, which spans several years, shows this effect for time periods before Stardust was launched. The timing of the Stardust interstellar collection was unfortunate in that it occurred at the most severe point of this defocusing. Landgraf understood this and thus estimated that the "total" flux of interstellar particles would be reduced by a factor of 3: 0.00015/3 = 0.00005 per square meter per second. This is where the original estimate of the total number of particles on the detector comes from (the collection period was shortened from 290 days to 180 days during the mission, however). So that gives us ~70 particles greater than 0.6 micrometers in diameter, about ~20 of these should be greater than 1 micron across (based on the observed relationship for particle size versus abundance from the Ulysses measurements).

I saw several areas for an improvement on this estimation. Most obviously, now that the collection has happened we have the benefit of hindsight. The Advanced Composition Explorer (ACE) took precise measurements of the interplanetary magnetic field during the Stardust collection, and their data is freely available to anybody from their website. Our sun is rather unpredictable, the "11-year" solar cycle can vary from 9 to 14 years, and the activity of the sun can vary significantly from one cycle to the next. It looks like the "space weather" conditions when Stardust collected interstellar dust particles were more severe than predicted by the Landgraf 1999 paper, so maybe the very rough estimate of the factor of three was larger. However, there is good news: as I mentioned before, the smallest particles are most affected by the Lorentz force - so I think the ~10 or so largest particles were not expelled from the inner solar system and made it to our collector. We just may see fewer of the smaller particles than we were originally expecting. Also, the Ulysses spacecraft made measurements from 3.2 - 5.4 AU (AU = mean sun-earth distance) whereas Stardust collected particles at ~2 AU. Also, most of the Ulysses collections happened out of solar system ecliptic plane. Galileo, in the ecliptic plane, definitely saw dust outside 2.8 AU. At 2 AU, the Galileo interstellar flux is uncertain. So, previous confirmed collections happened at different places in the inner heliosphere, and with the solar wind pushing charged charged dust grains around, this could significantly affect (for better or worse) the flux at Stardust. What is needed then, is a numerical computer program to predict what happens to charged dust streaming through the solar system, using the ACE magnetic field data for the time period of Stardust collection. This has been modeled before, but there are certain parameters that I'd like to vary to see the effect it has on our number of collected particles. For instance, it has been assumed that the particles are spherical, which would give you the lowest possible charge for a particle of given mass - any contours in the surface would give you more charge. So that makes these particles more susceptible to the Lorentz force. How much worse? I have written the computer code (in MATLAB) but numerically integrating even a relatively simple F=MA motion equation for a statistically significant number of particles is computationally intensive (and maybe MATLAB isn't the optimal language to use). But once finalized, I will count the number of particles my numerical model gives for a simulated Ulysses and Galileo collection, and for the Stardust collection, then from the Ulysses numbers we can scale the Stardust simulated counts
to get an expected number of particles of a given mass that impacted the detector. Then I can vary the mass to charge ratio and see how that changes things, as well as some other parameters that I won't go into right now. This way I can get a handle on the uncertainties associated with our expected number of particles - something that was missing from the original estimate. This number "45" should have been phrased as "between 15 and 100" or something like that. Neglecting uncertainties is a big pet peeve for me.

So, in summary, I think that it is not surprising that we are not seeing many smaller interstellar tracks at this point because it is possible that these particles were preferentially excluded from the inner solar system, an effect that was likely stronger than previously estimated. For the large particles, the statistics of small numbers tells us that it is not greatly unusual that we have not found a real interstellar dust grain yet given the fraction of the collector that has been scanned. When I finish the simulation, I will have a firmer grasp on just how the populations of particles varies with particle mass at the 2 AU collection distance (in particular, at what mass does the spectrum of particles falls off). But I think the largest particles are unaffected - and maybe this will help guide our strategy with the scanning of the rest of the detector, perhaps we should be looking for the biggest particles and not for the smaller ones. It's actually really amazing when you think about it: the fact that the Stardust@home volunteers aren't finding as many small interstellar particle tracks is telling us something about the magnetohydrodynamic conditions of interplanetary space at 2 AU. Yes, science is cool.
Ryan Ogliore
Postdoctoral Scholar, Space Sciences Laboratory
University of California, Berkeley

jsmaje
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Post by jsmaje »

Thanks for that Ryan. It has been a continuing frustration that such scientific feedback as yours has been lacking in this project of late, and it would be good to hear more from you, Zack and others on a regular basis about such relevant technical matters. Meanwhile it will take me some time to digest all the detail!

Having a science research background myself (concerning the behaviour of brain cells), I'm perfectly aware of how reality can confound one's fondest expectations, which is precisely what makes the scientific enterprise so interesting and productive. I also applaud your candidness about the personal, practical and perhaps ultimately unresolvable uncertainties involved.

Even if we never eventually find any IS particles, it's good to know that we may have contributed something to understanding the 'magnetohydrodynamic conditions of interplanetary space at 2 AU' - another brick in the wall of rational knowlege rather than superstition :wink:.

John

Nikita
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Post by Nikita »

With more simplicity, I'd just like to say that this is the way science goes. You win some big and you win some small, but good science always teaches us something...although it may not be what we want!
Frustrating, but good news for the future research!
From dust we come

pier
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ISdust simulation

Post by pier »

hi everybody,

I'm reading with interest the informations from Ryan and the comments from the collegues.

I whish to add another consideration. In the collector there are true tracks already found with true particles in their very bottom. May be these particles are not IS particles, but it is not so sure as pointed out by Andrew.

These particles impacted the collector at high speed, were found at 2 UA from our planet in a couple of years of elapsed time by a spacecraft started from our planet: in my opinion this is, in any case, a remarkable result.

Pier

Mitchell Criswell III
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interstellar dust simulations

Post by Mitchell Criswell III »

Thanks Ryan, for your illuminating report on the current estimate of interstellar particles the Stardust spacecraft may have collected.

In your modeling of particles that may have made it to the 2 AU distance is there any indication of how their trajectories may have been altered by the solar wind magnetic field? It is my understanding that the particle flux measured by Ulysses and Galileo radiated from the direction of Ophiuchus and this guided the direction of deployment for the Stardust collector. If during the time of collection the majority of particles were being deflected by the solar wind magnetic field, could not the more massive particles that reached 2 AU have been deflected so as to strike the collector at an angle to the perpendicular? In other words, might a percentage of the so called high angle tracks (HATs) that we have found be the result of interstellar particles?

Hope you can take a moment to reply.

Best regards,
Mitch

P.S. It occurs to me that Dr. Ogliore might not be monitoring this forum. Would Bryan Mendez or someone else at Berkeley pass this along. Thanks.

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