What has been happening…..

We want to share with you some new developments in the search for interstellar dust that we are cautiously excited about. We want to say from the beginning that we don’t understand everything yet, and that this may lead to a dead end. This is a bit like detective work – we’re sorting through the clues, and there is always a stage of confusion before things become clear. It is important to follow every lead. This is often how science works. The confusing parts (we hope) become the exciting parts, because we learn from what we don’t know. We’re sharing our confusion and our excitement with you, even though the story is far from complete.

First, a quick status report on the project. Last year we focused on extracting candidates, so we were not able to do any additional scanning. We have good scan data on 247 cm² of the collector – about 30% of the scannable area. 30 extractions have been done since the beginning of the Interstellar Preliminary Examination. 16 of these were interstellar candidates. The extraction process has evolved somewhat during the first year of the ISPE, but not in a substantial way. All candidates are now mounted between silicon nitride windows by default. 14 of these have been analyzed at synchrotrons all over the world. 12 do not appear to be consistent in their composition with interstellar dust. Two are sufficiently interesting that we want to do further study on them. One was lost due to a pre-existing fracture in the keystone. Unfortunate as this is, it emphasizes the need for a cautious approach to sample prep and extractions. Rather than rush, it is better to move carefully through the extraction phase in order to minimize sample loss and damage. One additional candidate is still under analysis.

Based on the early estimates of the number of interstellar dust particles that we are likely to have collected, we have about ten times more candidates than we expect to have interstellar dust particles – in other words, we expect to extract and analyze about 10 candidates for every real interstellar dust particle that we find. We are refining this estimate now – so far it appears that the number of interstellar dust particles collected was likely lower than the original estimate. If this turns out to be correct, this means we will have more work to do than we initially anticipated to find the interstellar dust particles. We have done no extractions since last Fall, since the Cosmic Dust Lab (CDL) has been switched over to doing work on Interplanetary Dust Particles (IDPs). We expect to resume extractions when CDL is switched back over to interstellar dust work, nominally at the end of May. We also expect that some foils will be extracted so that the foils subteam can begin work on searching for impact craters of interstellar dust.

Now to the recent developments that we’re cautiously excited about. Stardust@home dusters have found 29 so-called “high-angle” tracks. These are tracks of particles that didn’t come straight into the collector, but entered at a significant angle – typically about 45 degrees. I want to emphasize that these were identified by Stardust@home dusters, not by us. The fact that these were found by our amateur colleagues is a tremendous demonstration of the value of the Stardust@home approach. Some of these high-angle tracks were discovered in the first days of the project. We had not thought of these as being likely interstellar candidates, for three reasons. First, the projectiles came in at a large zenith angle with respect to the aerogel surface. This was not expected for interstellar dust, because the interstellar dust collector was intentionally oriented during the exposures so that the interstellar dust would come more or less straight into the detector. Second, the azimuth angles were such that the tracks seemed to be coming not from space, but from the solar panels of the spacecraft. Third, the tracks did not appear to us to be high-velocity tracks, such as those seen on the cometary side of the collector, where particles were captured at 6 km/sec. High-velocity tracks, from projectiles at >>1 km/sec, make carrot-shaped tracks in aerogel. The tops are wide because the projectile produces an energetic shock wave in the aerogel that expands dramatically and blows out a relatively large cavity in the aerogel. As the projectile slows, the shock weakens and eventually disappears when the projectile drops below the speed of sound. The result is a carrot-shaped track. But the high-angle tracks were thin and smooth, which in our experience looked more like slow. (<< 1 km/sec) projectiles. Finally, we extracted five of these tracks and examined them using synchrotron x-ray fluorescence microscopy (SXRF) and by scanning transmission x-ray microscopy (STXM). We found that three of them contained significant amounts of cerium, which is a rare element and is not expected in any significant quantities in any primitive extraterrestrial material. All of these lines of evidence led us to conclude that the high-angle tracks were highly likely to be ejecta from impacts on the aft solar panels of the spacecraft. But not so fast. Three developments have led us to reconsider seven of these 29 tracks.

First, we recently did some experiments with our colleagues Frank Postberg, Mario Treiloff, Ralf Srama, Sebastian Bugiel, and Eberhard Grün at the University of Heidelberg. They have cleverly adapted a particle accelerator, a van de Graaf accelerator, to accelerate small dust particles. For very small dust particles, they can achieve speeds up to 100 km/sec! This is the same accelerator that produced the calibration track in the phase I Stardust@home images. The dust accelerator works as follows. A bowl of the dust that you want to shoot is placed at the high-voltage end of a long evacuated pipe. The voltage is about 2 million volts. A sharp needle sticks up from the bowl. A high frequency voltage is applied to the bowl, which occasionally causes one of the dust particles to jump out of the bowl onto the tip of the needle, where it acquires a large charge and is accelerated down the 2 megavolt potential into the target. (Who thought of that??) However, it gets even more clever. Because this is a stochastic (random) process, the dust particles acquire wildly varying charges, so the accelerated dust comes out with a wide range of speeds. But to get unambiguous results, we need to have the particles coming out in only a narrow range of speeds, say 15-16 km/sec. So they use three detectors located at different positions along the flight path of the dust. These detectors can detect the passage of the dust particle, and by measuring the time delay between the passage of the dust grain past the three stations, they can measure the speed of the dust grain. If the speed isn’t right, the particle is rejected using a pair of electrostatic plates near the end of the pipe, to deflect the dust grain away from the target. Of course, all of this happens so fast that no person actually makes the decision – it is all done by electronics.

Using the dust accelerator, Zack Gainsforth and our German colleagues shot small particles of latex, aluminum, a polymer called PMMA, orthopyroxene, and iron, into aerogel and aluminum foils at speeds from 10-20 km/sec. The latex particles produced very short tracks in the aerogel, which was not a surprise. However, the iron and the orthopyroxene particles appeared to produce very long, thin tracks, reminiscent of the high-angle tracks in the Stardust collector. The reason that we emphasize the word appeared is that there was a complication in the experiment. Sometimes the dust source in the accelerator “spits” — that is, it produces rare bursts of particles, that can confuse the triggering system and allow slow, large particles to get through while the gate is open to allow a legitimate high-velocity particle through. We saw evidence of these in some of the shots, but not in the shots of the orthopyroxene or of the iron. In the iron shots, we saw about as many particles in the aerogel as we expected based on the number of times that the filter triggered. So there are two questions: (1) if these particles were actually slow, where are the fast ones that should also be in the target aerogels? (2) if the tracks we see are made by particles that are truly traveling fast, why do they make whisker-shaped tracks instead of carrot-shaped ones? We still don’t know the answer to either of these questions. We’re doing some additional shots in the next months with a new, improved filter that we expect to do better at rejecting “spits”.

Iron:

Orthopyroxene:

Second, Ryan Ogliore in our group at SSL has been modeling the trajectories of interstellar dust particles in the solar system. He has found that the largest interstellar dust particles can come into the detector at a substantial angle. This is because the interstellar collector was oriented to collect dust with β = 1. What is β? β is a ratio — it is the ratio of the outward force that sunlight exerts on dust particles to the inward gravitational force exerted by the sun. For particles much larger than 1 micron (like the Earth!) β is essentially zero. But for small particles, around the wavelength of light (a few hundred nanometers) β can be 1 or even greater, depending on its size, shape and density. Large particles from interstellar space travel along hyperbolic orbits that curve inward toward the sun as they go through the solar system. Small particles with β<1 also travel along hyperbolic orbits, but ones that curve outward from the sun. Particles with β exactly equal to one just travel in straight lines through the solar system. Because it was not known what the average value of β was for the interstellar dust, the spacecraft controllers oriented the spacecraft and the collector to track the β = 1 particles. When you do the simulation, you find that the largest particles (those with small β) would come into the collector by flying between the solar panels of the spacecraft, and so in the collector you would expect them to produce tracks that have an orientation of "north", or, if you think about the hands of a clock, about 12 o'clock. There is a complication in that the Sample Return Capsule lid, during some phases of the collections, could actually block particles with small β and keep them from reaching the collector.

Third, intriguingly, among these 29 tracks, there are seven that have an azimuth between 11:00 and 12:00. We have assumed that these are ejecta from an impact or impacts on the SRC lid, based on their similarity to the others that really do appear to be secondary ejecta based on the Ce content. And in fact, they all do point back to the SRC lid, but only for some of the collection times. The point is that the collector was on a hinged “wrist” and was constantly being rotated to track the β=1 dust stream. For other collection times, these tracks appear to point over the lid and into space. We don’t know when each track was collected, so the situation is ambiguous: for some collection times these events point into the small-β interstellar dust stream, for others they point into the SRC lid. Obviously, this is a critical question, and is not one that is resolvable just by looking at the track trajectories. We have to do more. We have to extract and analyze them.

It turns out that we have extracted one of these “midnight” tracks. This is sample, I1004,1,2, VM number VM number 862370V1. Last year we examined it using beamline 11.0.2 at the Advanced Light Source. This is the most powerful Scanning Transmission X-ray Microscope (STXM) in the world. We have been working with the beamline scientist, Tolek Tyliszczak, to analyze interstellar candidates on 11.0.2. Unfortunately, it turns out that we had inadvertently analyzed not the track but a feature that was very similar to the track but turned out to be a machining artifact left over from the “keystone” extraction of the track from the collector. We didn’t find any detectable elements in it, which in retrospect is no surprise. So recently George Flynn and Steve Sutton reexamined this track on beamline 2-ID-D at the Advanced Photon Source at Argonne National Laboratory. They did find detectable elements. They are still analyzing the data from this track, so stay tuned. We will also re-examine this track on 11.0.2 at the end of May.

Even if I1004,1,2 turns out to be secondary ejecta, there are still six more tracks that could turn out to be interstellar. When the Cosmic Dust Lab re-opens in May, the first order of business will be to very carefully re-measure the trajectories of these particles, then extract them for analysis.

It’s possible that you have collectively identified real bona-fide interstellar dust particles in the Stardust collectors, the first contemporary particles from outside the Solar System ever identified. But it’s also possible that we are following a false lead. We’re still not sure, so we’re trying to gather as much evidence about the particles as we can. I expect we’ll know more in the weeks and months ahead.

I want to express again my gratitude to you. Without your tremendous efforts, we would not have found these tracks. If it turns out that any one of these is likely to be interstellar dust, after our big celebration, we will redo the calibrations for Stardust@home to include these high-angle tracks. Meanwhile, keep up the fantastic work!

Andrew and the Stardust@home team