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Projects: Stardust@homeShifting Gears at the Johnson Space CenterStardust@home Update, February 11, 2008
Preparations towards extracting interstellar dust grain candidates from the Stardust aerogel collector have shifted into high gear. In the middle of January Statrdust@home project director Andrew Westphal and team members Zack Gainsforth and Dave Frank spent a week at the Johnson Space Center (JSC) in Houston, where the Stardust collectors are housed. Working in the cosmic dust lab – temporarily reconfigured as the Stardust@home lab – they meticulously tested the methods they developed for extracting aerogel "picokeystones" from the collectors. Early in February they were back in Houston running even more tests, and Dave Frank has stayed behind to further perfect the extraction techniques. The plan, said Westphal, is to maintain a permanent presence at JSC of at least one team member for the next few weeks – right until the actual extractions of interstellar dust candidates begin. In Houston the the Stardust@home team is working closely with two veterans of the space program, Ron Bastien and Jack Warren. "Working with Jack," Westphal said, "has special significance for us: Jack was the first person to open the Apollo sample return boxes from Apollo 11 in 1969 – the first person, aside from the Apollo astronauts, to touch rocks from the moon." "And now," Westphal added, "he is working with on extracting the first solid particles from the local interstellar medium!" For now, Westphal and his crew are conducting all their trials on the flight spare – a collector tray composed of aerogel tiles, identical to the one that flew on the Stardust, except that it stayed behind when the spacecraft embarked on its long journey. This way, even if things go very wrong during the tests, the real collector remains undamaged and its precious samples safe. Only when they are fully convinced that their procedures are safe and reliable will the Stardust team begin extracting keystones from the actual collector.
As is often the case when trying out new and untested methods, things did not go smoothly during the team's last visit to JSC. One difficulty arose during the extraction process, when minute tunnels were drilled in the keystone so that it could be grabbed and pulled out by a miniscule "picklefork." As in previous tests, the tunnels were drilled at a 63 degree angle, but this time a problem arose: the picklefork could not be inserted because it interfered with the microscope objective. Only after much tinkering did Dave finally manage to insert the fork and extract the keystone. Another problem was a minute but unmistakable shift in the positioning of the glass needle that occurred while the keystone was being cut out of the aerogel. Normally, a first pass by the cutting needle along one side of the keystone is then followed by additional passes in the exact same location, each one cutting deeper into the aerogel. This time, however, the position of the needle had shifted ever so slightly between the first two passes. Westphal and his crew were able to compensate for the error and continued cutting out the keystone, but they were not sure why the problem occurred in the first place Only after returning home to Berkeley did they find out the true cause: it was due to a small shift (only a few millionths of a meter!) in the coordinate system of the micromanipulator during power cycling. But there was good news as well. Despite the difficulties, the latest round of trials at JSC brought about significant progress towards the extraction of samples from the aerogel collector. For one thing, the team discovered a way to significantly speed up the cutting of the keystones from the collector. Up to now Westphal and his crew had kept the cutting needle moving at a relatively slow speed, in order to reduce vibrations in the long stainless steel arm that holds and controls the needle. Unfortunately, this meant that the extraction of keystones in Houston took much longer than it did in the lab in Berkeley, where keystones were cut out of single tiles rather than out of the entire collector, and no long arm was therefore needed. The recent trials at JSC demonstrated, however, that this slowing down was not always necessary. Once the cutting had proceeded to a certain point, and the needle was deeply enough embedded inside the aerogel, the needle's motion could safely be speeded up seven-fold. This is because the aerogel itself now absorbed the shock of the needle's motions, and prevented the stainless steel arm from vibrating. Zack is currently writing the software that would control the motions of the needle taking advantage of the increased cutting speed.
In addition, the team also discovered that they could save time by modifying not just the speed, bur also the "shape" of the needle's motion. In the past, the needle's movement through the aerogel was exclusively vertical; it would cut downwards through the aerogel, pull straight up into the air, and then move to an adjacent position before plunging downwards again. This time however Westphal and his team experimented with a new kind of motion: once the needle was embedded in the aerogel, instead of being pulled straight up. It was moved diagonally – upwards and sideways – through the aerogel to its next position. At this point the needle was plunged downwards again, before repeating the diagonal motion to the next position. The end result is that the cutting needle proceeds in a "sawtooth" motion through the aerogel, rather than in a series of vertical thrusts adjacent to each other. "It worked beautifully" said Westphal. Not only did the team manage to save time because of the more efficient motion, but the needle's motion through the aerogel tended to smooth out the rough edges that were sometimes created by the old "up and down" cutting motion. Finally, Westphal, Dave and Zack seem to have found an effective mounting for keystones, to hold the keystone in place once it is extracted. While this may seem like a small matter, it is in fact crucial for studying the captured particles after they had been cut out from the aerogel. As Westphal explained, the giant synchrotrons that will be used to analyse the particles are very particular about the mountings they allow, and no single mounting can be used in all of these instruments. For now, a "TEM grid" mounting seems to work well, protecting the keystone and holding it firmly in place.
Overall, during their two visits to Houston the Stardust@home crew managed to cut out three new keystones from the spare collector tray. Each extraction profited from the experience with its predecessors, allowing the team to proceed with increasing skill and confidence. The main factor slowing down the keystone extractions, Westphal explained, is the need to operate on the bulky aerogel tray as a whole. This makes the extraction of the interstellar dust particles far more challenging than cutting out the cometary particles collected by Stardust, because for those extractions the tiles are separated from the bulky tray before the keystones are cut out. For the rare and miniscule interstellar dust particles, however, it was decided that caution was the better part of valor, and the tiles should be kept in their place in the tray. That way no particles will be accidentally lost through the removal of the tiles, and crucial information derived from the particles' trajectory at the time they impacted the collector will also be preserved. The difference between the two arrangements seems minor, said Westphal, but experience shows that is not the case. "We have found that it is ten times more complicated to extract a keystone from the entire tray than it is to extract one from a single tile" he added. Did you like this article? Send
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