Update from Heidelberg:

We’ve had a successful time at Heidelberg so far. In the first week we shot aerogel tiles with 1/2 micron sized spheres of latex imbued with sulfur and coated with a conductive polymer. This yielded the first successful artificial tracks with known impactor size and velocities in the range of 5-30 km/s. We still have to analyze them at the synchrotron, but preliminary tracks look like:

You will notice there are both tiny crater like pits, as well as darker and more conventional looking tracks. What this means is still up in the air, but we suspect the larger dark tracks consist of multiple latex spheres bunched together in blobs before impacting.

In addition, we have tracks that were created by head on collisions with the aerogel (as opposed to the angled shots above) which should more accurately reflect the look of interstellar tracks as seen in the Stardust@Home microscope. We haven’t been able to analyze them yet, but if we have trouble finding them, we know who to ask…

While latex is a good analogue for interstellar particles, it does have a couple shortcomings. First, it’s density is around 1 gram/cm^3, which is 1/2 – 1/3 of what we expect for interstellar particles. This means that the tracks may not look entirely the same. Why don’t we shoot minerals such as we expect to hit? Well, for two reasons. First, what if interstellar particles don’t look like we expect? Second, we can’t … until now (more in a bit).

The other problem with latex is that it is made almost entirely out of carbon, hydrogen and oxygen. This makes it very hard to find in aerogel which has carbon, and oxygen in it. (Hydrogen doesn’t show up well, and besides, in a lab, there is lots of hydrogen about in the form of water.)

However, these latex spheres are special. They are imbued with sulfur. This actually kills two birds with one stone. First, we can see sulfur in some of our instruments, thus making the impactor more visible. Second, if you were following the scientific results from the cometary side, you may remember that we initially thought the comet was lacking in sulfur, but then discovered that it wasn’t all showing up due to the track and particle characteristics. There is even some evidence that sulfur may “evaporate” into the surrounding material during the heat of the impact. So, now we get to see how the sulfur behaves when it slams into aerogel at hypervelocities!

Nevertheless, we wanted something heavier. So we followed the next week with iron particles. These usually shoot well, but they have a downside: they are now several times heavier than what we expect to hit, and the particles themselves very in size from nanometers up to microns. So iron shot makes it much more difficult for us to analyze track morphologies. Nevertheless, because our instruments are so excellent at seeing iron, the variation in particle size shouldn’t really matter — it just requires more elbow grease (or is that forehead grease). That leaves the fact they are heavier. Well, now we have something 3x lighter, and something 3x heavier than an interstellar grain. If they’re not too different, then we can assume interstellar tracks look the same. If they are, we can assume interstellar tracks look like something in between. And if a heavy or light particle happened to hit the collector, then we know what it looks like too!

Unfortunately, we only got half of the iron shots, because in contrast to past experience, our iron source had a bad work ethic and went on vacation towards the end of the week, giving us only slow particles at a few km/s. This was just the luck of the draw. All the same, we got at least 1 of everything we wanted, so we’re happy. Two of each sample was greedy anyway. 🙂

In the third week, we started out with a special “bonus” mixture. Aluminum mixed with PMMA (polymer) beads. Aluminum is a wonderful material for us because it weighs approximately what we expect interstellar grains to weigh, and it shows up brightly in our instruments. Perfect you say! Yes, well, there is a problem. Aluminum has an awful history of sticking to itself, and then clogging the accelerator (resulting in no tracks). For this reason, we mixed it with PMMA as sometimes mixtures jog the material and avert sticking. We don’t really care about the PMMA, but if we get a few shots of it, that’s a bonus too. To our delight, we got very fast tracks on the first day — over 100 particles at 50-60 km/s!!! This is practically 3x the speed we expect from interstellar particles so this is a real coup d’etat! The next morning I came in grinning ear to ear, expecting to get all the other velocities we wanted but alas, it wasn’t meant to be — the accelerator was clogged. So much for the PMMA.

The downside of clogging the accelerator is it takes several days to reload — and we’re now on our last week of the trip. So, we got busy and replaced the sample. It is now loading (vacuum is pumping) and hopefully, by tomorrow afternoon we should be able to resume shooting. Wish us luck!

What are we shooting now? More aluminum? Nope. Because Mario Trieloff at the University of Heidelberg and Frank Postberg at the Max Planck Institute have pulled a rabbit out of their hat. We can shoot orthopyroxene — a mineral more or less identical to something we might expect in an interstellar dust grain. WOW! This wasn’t possible just a few weeks ago. They have developed a new technique for preparing minerals for firing just within the last few weeks, and now, for the first time, we’re going to get mineral shots into aerogel at tens of km/s. Oooh! I’m wringing my hands and licking my chops.