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Projects: Stardust@homeSearch for Interstellar Dust Enters New PhaseBy Amir Alexander
Without much fanfare, “Phase 2” of Stardust@home was launched on Friday, August 10, 2007, marking a new chapter in the ongoing search for interstellar dust particles brought to Earth by the spacecraft Stardust. With the launch of this new stage of the project, “dusters” (as project volunteers are called) are now able to search for interstellar dust particles at an unprecedented level of sensitivity. Even the subtlest tracks and particles, which may have eluded detection in the first phase of the project, now stand an excellent chance of being found out and investigated by ever-vigilant dusters. Phase 2 begins almost exactly a year after the launch of the original Stardust@home on August 1, 2006, and a great deal has been accomplished during this time. At the Johnson Space Center in Houston, where the Stardust collector tray is housed, an automated microscope has finished scanning close to 40% of the surface of the collector. Images of the aerogel from all this area have been sent to Stardust@home headquarters in Berkeley, where they were turned into scannable movies. Dusters around the world then poured over these movies with the aid of the Virtual Microscope, searching for tracks left by miniscule interstellar dust particles impacting Stardust’s aerogel tiles. Altogether, 24,000 dusters have so far searched about 40 million(!) movies for interstellar dust grains, meaning that each Stardust@home movie has been viewed more than one hundred times on average. But the most exciting accomplishment of the project’s first phase is this: as of now, the dusters have identified 50 different locations in the aerogel that might contain actual interstellar dust particles. Finding such “candidates” was precisely what Stardust@home was designed to accomplish, and the project team couldn’t be happier with the results. “This really works!” exclaimed Stardust@home director Andrew Westphal of U.C. Berkeley, thrilled at seeing the first tangible results from years of work and preparation. Thanks to the skill and dedication of the dusters, Westphal explained, and to the fact that so many different eyes had scanned each of the movies, the project was able to detect some “very subtle things” in the collector. These appear to be tunnels bored by impacting dust particles, indicating that the grains themselves might be found buried within the aerogel at the very end of the track. Until the tracks are extracted from the collector tray and examined directly, however, the Stardust@home team won’t know for sure which of the 50 candidates is the “real thing” – an interstellar dust particle brought to Earth from interplanetary space. Every one of the 50 candidate tracks was singled out by multiple dusters who thought they saw something and singled out the movie for further study. Westphal and his Berkeley team members, Zack Gainsforth, Anna Butterworth, Nicole Kelley, Dave Frank, and Rastika Prasad, then took a close look at the movie themselves. If they also agreed that the feature could potentially be a particle track, then they classified it as a candidate that needs to be extracted and examined directly. Some of the candidates will undoubtedly prove to be false alarms, mere impurities and irregularities in the aerogel itself. But if theoretical predictions hold true, Westphal and his colleagues expect that about 40% of the total, or around 20 candidates, will turn out to be actual interstellar dust particles. Westphal is enormously impressed with the skill shown by the dusters in pinpointing the locations in the aerogel most likely to harbor interstellar dust particles. “When we first suggested this project,” he said, “we encountered a great deal of skepticism from the field.” Many it seems thought that involving the general public was more of a public relations stunt than serious scientific research. But in March of this year Westphal presented the project at the Lunar and Planetary Science Conference in Houston. He explained how the use of calibration movies enabled the Stardust@home team to evaluate the reliability of each duster, and the sensitivity of the project as a whole. Finally he showed how even the smallest tracks are detected by 70% of dusters, and tracks with a diameter of 5 microns or more are detected by almost 100% of users. This, according to Westphal convinced even the skeptics. “The calibration movies impressed everyone,” he said, showing that Stardust@home is not only reliable, but also an extremely sensitive tool for detecting interstellar dust grains. Westphal and his crew learned a great deal from the success of the first phase of the project, and this experience now serves as the basis for the new Phase 2. In place of the traditional aerogel movies, dusters are now being sent a new kind of aerogel movie to investigate. The new movies do not require scanning new images of the collector, since they are composed of the same images used in Phase 1 movies. But whereas each image previously scanned by the virtual microscope covered a field of around 500 microns square, the new movies have twice the resolution, covering a field of around 250 microns squared. This means that each old style movie is divided into four new high resolution movies. This is possible, explained Westphal, because the resolution of the images taken of the aerogel was actually much higher than was displayed in the standard movies. Much of the image data, he said. was lost in the compression process required for transmitting the movies. But if the movies present a smaller field, then the resolution is dramatically improved, and that is what the Strardust@home team has prepared for Phase 2. By sending new high resolution images to dusters for review, the sensitivity of the search for the elusive particles will be doubled. Before Stardust@home transitioned from the standard movies to the high resolution ones it needed a short break. For a few weeks during June and July the Virtual Microscope was shut down and no movies were sent out to dusters. The score and ranking of each duster were finalized, and saved as the final result of “Stage 1” of Stardust@home. When the Virtual Microscope went back online on August 10, 2007, all scores once again began at “0”, and the competition for the top ranking began anew. But those individuals who had already pinpointed one or more of the fifty interstellar dust candidates during “phase 1” enjoy a very special privilege. First of all, the person who was first to discover a particle will have the exclusive right to name it. If the candidate turns out to be a real interstellar dust particle, then as long as the grain is in existence, studied by scientists, stored, or even displayed to the public, it will always be known by the name given to if by the first duster who discovered it. Second of all, since each grain was found multiple times by different dusters, those who were not lucky enough to be first will also be credited. Their dedication, hard work, and (equally important!) sharp vision will be honored by a permanent list on the Stardust@home website. It will detail which dusters discovered each and every one of the interstellar dust candidates found in Stardust’s aerogel collector.
Along with his team Westphal is now hard at work perfecting the method that will be used to extract tracks from the Stardust collector tray. Several options for doing this were considered: one possibility was to remove tiles containing candidates from the collector in order to make it easier to handle the aerogel while extracting the tracks. In the end this seemingly attractive option was rejected as the risk to the integrity of the tiles and the samples contained in them was deemed too great. The tiles, it was decided, will remain within the tray as the extraction takes place. This solution, however, created some problems of its own. The extraction technique developed by Westphal’s team involves cutting out a minute keystone (or “picokeystone”) of aerogel containing the candidate track. This is done with a robotically controlled needle that cuts through the aerogel point by point until the minute keystone is separated from the tile. The problem is that, as previously practiced, this procedure places the “micromanipulator” controlling the needle directly above the surface of the aerogel. This creates a serious contamination risk for the aerogel, which would catch any debris falling or leaking from the micromanipulator.
To get around this difficulty the Stardust@home team decided to place the micromanipulator at the side of the collector tray rather than above it. In this arrangement the needle is controlled by means of a specially designed robotic arm that stretches from the micromanipulator, positioned alongside the collector tray, and onto the precise location on the aerogel surface where the keystone is to be cut out. The stardust@home team has developed the special mountings necessary for placing the manipulator in the correct relationship to the tray, and are now putting in place automated safety features to make sure the aerogel is not damaged if the instrument malfunctions. Much of the technique proposed for extracting interstellar dust candidates from the aerogel is already being used to extract cometary particles from the opposite side of the Stardust tray. These were captured by the spacecraft as it flew within of the core of comet Wild 2 in the dramatic encounter of January 2, 2004. These cometary grains, however, are much larger than the interstellar dust particles and there are many more of them. As a result, the extraction of these particles does not require the same degree of flawless precision as the extraction of the minute and rare interstellar grains. The lessons learned from the extraction of the cometary particles will certainly be valuable in cutting out the aerogel “picokeystones.” But Westphal and his team were looking for even more hands-on experience before sending a needle to cut out microscopic particle tracks from the collector tray. This is possible because although Stardust carried two aerogel collectors on its long journey through the solar system, three collectors were actually built. The spare collector is identical to the ones that flew in space in all respects except the fact that it never left Earth and therefore contains no precious particles. Westphal and his team of researchers are now practicing their craft on this pristine collector, refining their method of extracting picokeystones. This way, when they ultimately apply their method to the interstellar dust tray, they will already be expert at carving out the candidate tracks.
All this, however, will take time. For now, with close to 40% of the work completed, the scanning of the Stardust@home collector at the Johnson Space Center has been put on hold. The laboratory at the Johnson Space Center where the scanning takes place is normally designated as the “Cosmic Dust Laboratory” and is used to study dust particles gathered in Earth’s upper atmosphere. This activity has been suspended for the past year as the laboratory space has been converted to a de facto Stardust@home lab, and now the cosmic dust researchers want their space back to pursue their own studies. They have it now for several months, and within a few weeks the lab will be transformed once more: the aerogel collector will be returned, and Stardust@home scanning will resume. And so the search continues for the miniscule interstellar dust grains gathered by Stardust during its seven-year Space Odyssey. It is made possible by the dedication of thousands of volunteer dusters from around the world, who are devoting both time and effort to finding these miniscule particles. Thanks to their hard work, scientists will soon be able to study directly these pure and uncontaminated grains, which are the building blocks of our universe. |
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