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Projects: Stardust@homeThe Delicate Art of Extracting "Picokeystones"by Amir Alexander
All around the world these days, Dusters are training their Virtual Microscopes on the new high resolution movies, hoping to detect the telltale signs of the impact of an interstellar dust grain. In Berkeley, California, where Stardust@home is headquartered, Andrew Westphal and his team are putting the finishing touches on a smooth and successful transition to Phase 2 of the project. Increasingly, however, they are also looking ahead, preparing for the day they will be called upon to extract miniscule interstellar dust particles from the Stardust aerogel collector. Their goal is to develop a method to safely cut out any possible track from the aerogel with pinpoint precision, without damaging the potential particle or the collector tile that surrounds it. In early September Zack Gainsforth traveled down to the Johnson Space Center in Houston for a new round of tests of the particle extraction method. In this technique, the collector tray is fixed in place and a glass needle at the end of a long arm is used to cut through the aerogel and extract a tiny section of it – a “keystone.” The arm is anchored to a “micromanipulator” at the side of the tray, which controls the needle’s every miniscule movement. A version of this system is already being used by scientists to extract the larger and more numerous particles collected by Stardust during its flyby of comet Wild-2. There is, however, an important difference: in order to extract the cometary particles, individual aerogel tiles were separated from the collector before the keystones were cut out of them; in contrast, for extracting the interstellar dust particles, the entire collector tray, containing all 130 tiles, will remain bound together as a single unit. This means that the arm connecting the glass needle to the micromanipulator at the side of the tray is much longer for the interstellar dust collector than it is for the cometary particles. As we shall see, this seemingly unimportant difference has serious consequences for the design of the keystone-cutting apparatus.
This was Zack’s second visit to the JSC cosmic dust lab. He had already been there to test the extraction system back in March, but his results at the time indicated that the system for extracting the keystones still needed to be fine-tuned. Now he was back to try the apparatus one more time in the same facility where the extractions will ultimately take place. To avoid damaging the Stardust collector trays with their precious content, Zack made use of the “flight spare” aerogel collector tray – an identical twin of the Stardust collector, which was manufactured before liftoff but did not fly in space. Only after the technique is proven to be effective and fully reliable will it be used on the actual Stardust collector. The results of the tests were once more a mixed bag. Using the micromanipulator, Zack succeeded extracting a “picokeystone” – a section of the aerogel with the size and shape that will be used to cut out actual particle tracks in the Stardust collector. He did it, furthermore, at the edge of the collector furthest away from the micromanipulator, a challenge that puts the entire cutting and extraction apparatus to the ultimate test. When the glass needle nevertheless succeeded in cutting out a clean-looking picokeystone, it demonstrated that the method the Stardust@home team is working on is feasible, and can eventually be used to remove the actual particle tracks from the aerogel collector.
But problems arose as well. The arm controlling the needle, Zack found, tends to vibrate ever so slightly. This causes the needle to deviate from its programmed course, and cut out a keystone whose shape and dimension are not as precise as would be required for the delicate job of extracting interstellar dust grains. With these results in hand Zack returned to Berkeley, and the Stardust@home crew set about resolving the problems. To minimize vibrations originating in the arm itself team member Dave Frank created an arm composed entirely of epoxy. Epoxy does not vibrate as steel does, and this in itself should increase the precision of the segments cut from the aerogel. The new arm is also much lighter than the older steel arm, and therefore puts less strain on the micromanipulator and is easier to control. To further increase the precision of the micromanipulator’s operations the team also installed new operating software that will slow down and smooth out its motions.
In addition to vibrations caused by the apparatus itself, there are also those caused by external factors. Any passing truck or slammed door might generate sufficient vibration to spoil the clean accurate cut of the glass needle. The problem is especially severe because the cosmic dust lab at JSC is located on the second floor of the building. “The best place to be for such precision work is in the basement” explained Westphal. “The higher you go the greater the potential for shaking.” To resolve this problem Westphal and his team plan to place the entire apparatus on a “vibration isolation table” which will insulate it from externally-induced vibrations. Finally, Westphal and his colleagues are hard at work building a failsafe mechanism that will rapidly shut down the extraction process in the case of a catastrophic failure, protecting the collector from damage. Will all this be enough? The stardust@home team members hope so, and in any case they will find out soon. On Monday, October 8, Andrew and Dave are heading down to Houston for another round of cutting out aerogel keystones, this time using the new and improved apparatus. Stay tuned! The Interstellar Dust adventure is only just beginning. |
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