# Stardust Interstellar Preliminary Examination I: Identification of tracks in aerogel

Andrew J. Westphal, David Anderson, Anna L. Butterworth, David R. Frank, Robert Lettieri, William Marchant, Joshua Von Korff, Daniel Zevin, Augusto Ardizzone, Antonella Campanile, et al.

► To cite this version:

Andrew J. Westphal, David Anderson, Anna L. Butterworth, David R. Frank, Robert Lettieri, et al. Stardust Interstellar Preliminary Examination I: Identification of tracks in aerogel. Meteoritics and Planetary Science, 2014, 49 (9), pp.1509–1521. 10.1111/maps.12168. hal-02351161

HAL Id: hal-02351161

https://univ-lyon1.hal.science/hal-02351161v1

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<figure><img src="image_1.png" /></figure>

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Stardust Interstellar Preliminary Examination I: Identification of tracks in aerogel

Andrew J. WESTPHAL<fnref n="1" />*, David ANDERSON<fnref n="1" />, Anna L. BUTTERWORTH<fnref n="1" />, David R. FRANK<fnref n="2" />,
Robert LETTIERI<fnref n="1" />, William MARCHANT<fnref n="1" />, Joshua VON KORFF<fnref n="1" />, Daniel ZEVIN<fnref n="1" />,

Augusto ARDIZZONE<fnref n="3" />, Antonella CAMPANILE<fnref n="4" />, Michael CAPRARO<fnref n="5" />, Kevin COURTNEY<fnref n="6" />,
Mitchell N. CRISWELL III<fnref n="7" />, Dixon CRUMPLER<fnref n="8" />, Robert CWIK<fnref n="9" />, Fred Jacob GRAY<fnref n="10" />,

Bruce HUDSON<fnref n="11" />, Guy IMADA<fnref n="12" />, Joel KARR<fnref n="13" />, Lily Lau Wan WAH<fnref n="14" />, Michele MAZZUCATO<fnref n="15" />,
Pier Giorgio MOTTA<fnref n="15" />, Carlo RIGAMONTI<fnref n="16" />, Ronald C. SPENCER<fnref n="17" />,

Stephens B. WOODROUGH<fnref n="18" />, Irene Cimmino SANTONI<fnref n="19" />, Gerry SPERRY<fnref n="20" />, Jean-Noel TERRY<fnref n="21" />,

Naomi WORDSWORTH<fnref n="22" />, Tom YAHNKE Sr.<fnref n="23" />, Carlton ALLEN<fnref n="24" />, Asna ANSARI<fnref n="25" />, Saša BAJT<fnref n="26" />,

Ron K. BASTIEN<fnref n="2" />, Nabil BASSIM²⁷, Hans A. BECHTEL²⁸, Janet BORG²⁹, Frank E. BRENKER³⁰,

John BRIDGES³¹, Donald E. BROWNLEE³², Mark BURCHELL³³, Manfred BURGHAMMER³⁴,

Hitesh CHANGELA³⁵, Peter CLOETENS³⁴, Andrew M. DAVIS³⁶, Ryan DOLL³⁷, Christine FLOSS³⁷,

George FLYNN³⁸, Zack GAINSFORTH<fnref n="1" />, Eberhard GRÜN³⁹, Philipp R. HECK⁴⁰, Jon K. HILLIER⁴¹,

Peter HOPPE⁴², Joachim HUTH⁴², Brit HVIDE<fnref n="25" />, Anton KEARSLEY⁴³, Ashley J. KING⁴⁴,

Barry LAI⁴⁵, Jan LEITNER⁴², Laurence LEMELLE⁴⁶, Hugues LEROUX⁴⁷, Ariel LEONARD³⁷,

Larry R. NITTLER⁴⁸, Ryan OGLIORE⁴⁹, Wei Ja ONG³⁷, Frank POSTBERG⁴¹, Mark C. PRICE³³,

Scott A. SANDFORD⁵⁰, Juan-Angel Sans TRESSERAS³⁴, Sylvia SCHMITZ³⁰, Tom SCHOONJANS⁵¹,

Geert SILVERSMIT⁵¹, Alexandre S. SIMIONOVICI⁵², Vicente A. SOLÉ³⁴, Ralf SRAMA⁵³,

Thomas STEPHAN³⁶, Veerle J. STERKEN⁵⁴, Julien STODOLNA<fnref n="1" />, Rhonda M. STROUD²⁷,

Steven SUTTON⁴⁵, Mario TRIELOFF⁴¹, Peter TSOU⁵⁵, Akira TSUCHIYAMA⁵⁶,

Tolek TYLISZCZAK²⁸, Bart VEKEMANS⁵¹, Laszlo VINCZE⁵¹, and Michael E. ZOLENSKY<fnref n="24" />

<footnote n="1">¹Space Sciences Laboratory, U.C. Berkeley, Berkeley, California, USA</footnote>

<footnote n="2">²ESCG, NASA JSC, Houston, Texas, USA</footnote>

<footnote n="3">³Red Team, via Simone Cuccia, 45, Palermo, Italy</footnote>

<footnote n="4">⁴Red Team, Reggio Emilia, Italy</footnote>

<footnote n="5">⁵Red Team, 13998 Kingswood, Riverview, Michigan, USA</footnote>

<footnote n="6">⁶Red Team, 918 Toni Marie Ct., Ballwin, Missouri, USA</footnote>

<footnote n="7">⁷Red Team, Dog Star Observatory, P.O. Box 528, Pearce, Arizona, USA</footnote>

<footnote n="8">⁸Red Team, Durham, North Carolina, USA</footnote>

<footnote n="9">⁹Red Team, Rosewood Circle, Silver City, New Mexico, USA</footnote>

<footnote n="10">¹⁰Red Team, Hampton, South Carolina, USA</footnote>

<footnote n="11">¹¹Red Team, Ontario, Canada</footnote>

<footnote n="12">¹²Red Team, Brookings, Oregon, USA</footnote>

<footnote n="13">¹³Red Team, 9812 N Lydia Ave, Kansas City, Missouri, USA</footnote>

<footnote n="14">¹⁴Red Team, 93 Yishun St 81, Tower 8 #10-07, Orchid Park Condo, Singapore</footnote>

<footnote n="15">¹⁵Red Team, Italy</footnote>

<footnote n="16">¹⁶Red Team, Strada della Rovere 3/z, Moncalieri, Italy</footnote>

<footnote n="17">¹⁷Red Team, Leominster, Massachusetts, USA</footnote>

<footnote n="18">¹⁸Red Team, 100 Beach Drive, Suite 1801-03, St. Petersburg, Florida, USA</footnote>

<footnote n="19">¹⁹Red Team, 40 Oak Drive, Upper Saddle River, New Jersey, USA</footnote>

<footnote n="20">²⁰Red Team, Tacoma, Washington, USA</footnote>

<footnote n="21">²¹Red Team, Tarentaise, France</footnote>

<footnote n="22">²²Red Team, South Buckinghamshire, UK</footnote>

<footnote n="23">²³Red Team, 5231 Jamieson Ave 1-S., Louis, Missouri, USA</footnote>

<footnote n="24">²⁴ARES, NASA JSC, Houston, Texas, USA</footnote>

<footnote n="25">²⁵Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, Chicago, Illinois, USA</footnote>

<footnote n="26">²⁶DESY, Hamburg, Germany</footnote>
<sup>27</sup>Materials Science and Technology Division, Naval Research Laboratory, Washington, District of Columbia, USA

<sup>28</sup>Advanced Light Source, Lawrence Berkeley Laboratory, Berkeley, California, USA

<sup>29</sup>IAS Orsay, Orsay, France

<sup>30</sup>Geoscience Institute, Goethe University Frankfurt, Frankfurt, Germany

<sup>31</sup>Space Research Centre, University of Leicester, Leicester, UK

<sup>32</sup>Department of Astronomy, University of Washington, Seattle, Washington, USA

<sup>33</sup>University of Kent, Canterbury, Kent, UK

<sup>34</sup>European Synchrotron Radiation Facility, Grenoble, France

<sup>35</sup>George Washington University

<sup>36</sup>University of Chicago, Chicago, Illinois, USA

<sup>37</sup>Washington University, St. Louis, Missouri, USA

<sup>38</sup>SUNY Plattsburgh, Plattsburgh, New York, USA

<sup>39</sup>Max-Planck-Institut für Kernphysik, Heidelberg, Germany

<sup>40</sup>Field Museum of Natural History, Chicago, Illinois, USA

<sup>41</sup>Institut für Geowissenschaften, University of Heidelberg, Heidelberg, Germany

<sup>42</sup>Max-Planck-Institut für Chemie, Mainz, Germany

<sup>43</sup>Natural History Museum, London, UK

<sup>44</sup>The University of Chicago and Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, Chicago, Illinois, USA

<sup>45</sup>Advanced Photon Source, Argonne National Laboratory, Chicago, Illinois, USA

<sup>46</sup>Ecole Normale Superieure de Lyon, Lyon, France

<sup>47</sup>University Lille 1, France

<sup>48</sup>Carnegie Institution of Washington, Washington, District of Columbia, USA

<sup>49</sup>University of Hawai'i at Manoa, Honolulu, Hawai'i, USA

<sup>50</sup>NASA Ames Research Center, Moffett Field, California, USA

<sup>51</sup>University of Ghent, Ghent, Belgium

<sup>52</sup>Institut des Sciences de la Terre, Observatoire des Sciences de l'Univers de Grenoble, Grenoble, France

<sup>53</sup>IRS, University Stuttgart, Stuttgart, Germany

<sup>54</sup>IRS, University Stuttgart, IGEP, TU Braunschweig, Braunschweig, Germany and MPIK, Heidelberg, Germany

<sup>55</sup>Jet Propulsion Laboratory, Pasadena, California, USA

<sup>56</sup>Osaka University, Osaka, Japan

*Corresponding author. E-mail: westphal@ssl.berkeley.edu

(Received 05 December 2012; revision accepted 14 June 2013)

**Abstract**—Here, we report the identification of 69 tracks in approximately 250 cm² of aerogel collectors of the Stardust Interstellar Dust Collector. We identified these tracks through Stardust@home, a distributed internet-based virtual microscope and search engine, in which > 30,000 amateur scientists collectively performed >9 × 10⁷ searches on approximately 10⁶ fields of view. Using calibration images, we measured individual detection efficiency, and found that the individual detection efficiency for tracks > 2.5 µm in diameter was >0.6, and was >0.75 for tracks >3 µm in diameter. Because most fields of view were searched >30 times, these results could be combined to yield a theoretical detection efficiency near unity. The initial expectation was that interstellar dust would be captured at very high speed. The actual tracks discovered in the Stardust collector, however, were due to low-speed impacts, and were morphologically strongly distinct from the calibration images. As a result, the detection efficiency of these tracks was lower than detection efficiency of calibrations presented in training, testing, and ongoing calibration. Nevertheless, as calibration images based on low-speed impacts were added later in the project, detection efficiencies for low-speed tracks rose dramatically. We conclude that a massively distributed, calibrated search, with amateur collaborators, is an effective approach to the challenging problem of identification of tracks of hypervelocity projectiles captured in aerogel.
# INTRODUCTION

The primary mission of the NASA Discovery-class mission Stardust was to return a sample of cometary material from the coma of Jupiter-family comet 81P/Wild 2 (Tsou et al. 2003). But Stardust was effectively two missions in one spacecraft—the second mission was to return a sample of contemporary interstellar dust, and, to that end, the spacecraft carried a tray of aerogel and aluminum foil that was exposed to the interstellar dust stream during two periods before the encounter with the comet. After the successful recovery of the collector in 2006, NASA initiated a preliminary examination (PE) of the Stardust interstellar collector. This was the fourth PE that NASA has conducted on returned extraterrestrial samples, after Apollo, Genesis, and the Stardust cometary dust collection.

The first order of business, before any interstellar dust candidates could be extracted from the collector and analyzed, was simply to identify them. The challenge is straightforward: to identify tracks of approximately 1 µm particles, it was necessary to search at sufficiently high magnification (≤0.5 µm pixel⁻¹) in an optical microscope. A simple estimate quickly showed that of order one million fields of view would have to be searched. Before launch, Landgraf et al. (1999) predicted that approximately 50 interstellar dust particles would be captured in the collector, so only one field of view in approximately 20,000 would contain a track. A search of this magnitude was beyond the capability of any professional research group. This collaboration, therefore, includes more than 30,000 amateur scientists. “Citizen Science” is enjoying a new vogue with the advent of the internet, but, in fact, there is a long tradition of highly productive participation of amateurs in astronomy. This approach has some parallels with Operation Moonwatch at the dawn of the space age (McCray 2008). So far, we have identified 69 tracks in the aerogel collectors, including 22 that appear to be consistent in their trajectories with an origin in the interstellar dust stream (Frank et al. 2013; Sterken et al. 2014) or as secondaries from impacts on the Sample Return Capsule, which was in the field of view of the collector.

# METHODS

## The Stardust Mission

The Stardust spacecraft was launched on 7 Feb 1999 21:31 UTC. The interstellar collector was exposed for two periods to the interstellar dust stream: from 22 Feb 2000 to 1 May 2000 and from 5 Aug 2002 to 9

Dec 2002 (Tsou et al. 2003). The heliocentric distance of the spacecraft was 2.1–2.2 AU during the first exposure, and 2.2–2.6 AU for the second exposure. The total exposure time was 195 days. The collector tracked the interstellar dust stream such that particles with $β = 1$ would have been captured at normal incidence. Here, $β$ is the dimensionless ratio of the force due to radiation pressure from sunlight to the gravitational force. The interstellar dust stream was assumed to originate from ecliptic latitude +8° ecliptic longitude +259°. The spacecraft attitude was maintained in a deadband of ±15° in all three axes during the exposures, with brief excursions for communication and navigation.

## Stardust Interstellar Dust Collector

The Stardust interstellar collector comprised 132 aerogel tiles, approximately 283 aluminum foils, and the aluminum collector frame itself. One hundred thirty aerogel tiles presented a rectangular face measuring 20 × 40 mm to space, and two tiles were polygonal in shape and somewhat smaller in area. All interstellar tiles were 10 mm thick, and had an average density of approximately 26 mg cm⁻³ (Butterworth et al. 2014). The total area of the aerogel portion of the collector was approximately 1039 cm².

## Image Data Collection

We used an automated microscope to collect digital imagery of aerogel tiles in the Stardust Interstellar Collector. All scanning was carried out in the Cosmic Dust Laboratory at the Johnson Space Center in Houston. We used a customized Leica Metalloplan microscope with an automated stage (Technical Instruments) controlled by computer (Apple Mac Mini) through a stage controller (Compumotor 6K4). The stage positioning accuracy was <2 µm. We used a 1024 × 768 monochromatic CCD camera (Flea, Point Grey Research) mounted on the camera port of the microscope, and a 10× objective. Image data were collected at 15 Hz, using the Astro IIDC image acquisition package (Outcast Software) running on a desktop computer (Apple Mac Mini). The field of view was 480 × 360 µm, so the spatial resolution, as defined by the size of a pixel projected onto the tray, was 0.47 µm pixel⁻¹. This is comparable to the intrinsic diffraction limit of the microscope.

We scanned one tile at a time. We wrote programs in MATLAB to generate Unix scripts for making semiautomated altitude maps of each aerogel tile and for fully automated scanning. First, we used a semiautomated script and manual focusing to measure
<figure><img src="image_2.png" /><figcaption>Fig. 1. A screen shot of the Stardust@home virtual microscope.</figcaption></figure>

the height of the aerogel tile over a 10 × 20 grid. This step was required because the aerogel tiles exhibit smooth, but large-amplitude, topography. We then used this altitude map to generate a script for fully automated scanning, and used a spline interpolation between measured altitudes to define the surface altitude for each field of view. The outer 1–2 mm of each tile was typically heavily fractured, so these areas were not scanned.

In each field of view, the scanning system collected a QuickTime stack, called a “focus stack,” consisting of approximately 50 frames acquired during a slow slew of the microscope’s vertical axis through approximately 200 µm. The scan started approximately 50 µm above the interpolated aerogel surface and ended approximately 150 µm below the interpolated surface. Because of slight variations in the timing of the beginning of recording the focus stack, these positions varied by several micrometers. Each stack was stored on an external disk, with a filename that included the coordinates of the stack; 200–300 Mb of data storage

was required for each tile. We collected approximately 250,000 focus stacks.

## Stardust@Home Distributed Search

We searched for candidate interstellar dust impacts in the image data using a massively distributed, internet-based search tool that we called Stardust@home (S@H). Two of us, Dave Anderson and Josh Von Korff, designed and wrote a virtual microscope (VM) that ran natively in html and Java on common internet-connected Web browsers. In Fig. 1, we show the Stardust@home Virtual Microscope as it appeared to volunteers.

We prepared the raw focus stacks for internet-based searching by splitting them into individual frames, and compressing each frame in compressed jpeg format. We did this processing automatically using Applescript and the image processing capability of QuickTime Professional. After this processing, we uploaded the image data to the Amazon S3 “cloud.” These images
were then available for downloading to the VM as described below.

Through one of the online forums on the Stardust@home website, those of us among the Stardust@home volunteers named ourselves “dusters.” We adopt the same nomenclature here. To qualify to become a duster, a volunteer was required first to go through a training session on the Stardust@home website, and, after training, to pass an online test with at least eight correct responses to ten test images. After passing this qualification test, we were invited to register as Stardust@home dusters. As of 23 Jan 2012, 30,649 people passed the test and registered as S@H dusters.

We searched image data as follows. The client browser initiated the search by sending a request to the S@H server for a VM webpage. The VM generated a new webpage automatically by choosing a focus stack randomly from a list of available focus stacks. Approximately 43 frames of image data were loaded onto the client browser, but only one was displayed at a time. We slewed through the stack of images by moving the computer cursor along a focus bar located adjacent to the image window. By moving the cursor, we focused up and down through the stack of images, thus simulating what one would see if turning the focus knob on a real microscope and looking at the same field of view.

We then responded to the focus stack in one of three ways. If no candidate track was identified, we pressed a “no track” button, and this response was recorded in the database by the server, along with supporting information (volunteer identification, stack identification, and time). If a candidate was found, we clicked on the location of the deepest feature in the candidate track that was visible in the focus stack. We then were asked to confirm the identification before the positive response and the coordinates of the click were recorded in the database, along with supporting information, such as a timestamp. Finally, if no surface could be identified, or if the topography within the field of view prevented an adequate search, we responded by clicking a “bad focus” button. This response was also be recorded by the server along with supporting information. In all cases, the VM running on the client browser would then automatically request and serve up the next focus stack for searching.

We measured detection efficiencies and rate of false positives (the equivalent of noise rate in an electronic detector) using images with known characteristics—so-called calibration stacks. Half of the calibration stacks were focus stacks from the general data set into which the image of a 12 µm diameter track was dubbed. The track image was randomly rotated through 2π and

scaled in diameter and independently in depth. The scale factor in diameter was chosen randomly in the range 0.2–2.0. The other half of the calibration stacks were focus stacks from the Stardust Interstellar data set, which we had carefully examined and determined to be blank. The types and numbers of calibration focus scans were varied through the various phases of the S@H project (see the next section). Any given search had a 20% probability of being a calibration stack. During scanning on the VM, we knew in general that some stacks were calibration stacks, but the VM presented no information that would allow us to know whether any specific stack was a calibration stack or not.

The use of calibration stacks had two serendipitous advantages. First, we used the responses to calibration stacks to automatically generate an individual, real-time score for each of us. The score simply consisted of the number of correct responses to calibration stacks less the number of incorrect responses. Although we did not use this score in any data analysis, we found that maintaining a score was highly motivating for many of us. Second, we found that the frequent presentation of calibrations helped to maintain attention. Attention is difficult to maintain in lengthy searches for rare events.

## Stardust@Home Phases

### Phase I

Phase I was the initial phase of Stardust@home. Calibrations were of two types: known blanks, which we had searched for candidate tracks, and positive stacks, in which the image of a single track was digitally dubbed. The track was rotated randomly through 2π and scaled in size both in depth and in projection, between 0.2 and 2.0 in magnification.

### Phase II

Here, we processed and presented “high-res” stacks. Because stacks were compressed for uploading to Stardust@home, the compression resulted in some degradation of image quality. We therefore reprocessed the raw image data by dividing each field of view into quarters and recompressing them. We uploaded these “high-res” images to Stardust@home. These images were presented with a corrected scalebar on the Stardust@home viewer.

### Phase III

In Phase III, we added new calibration stacks, based on the image of “whisker-like” feature in the Stardust aerogel, and upgraded the training and testing procedure to include this. We re-examined all stacks during Phase III.
<table><caption>Table 1. Statistics of each phase of Stardust@home.</caption><thead><tr><th>Phase</th><th>Launch date</th><th>End date</th><th>New dusters</th><th>Calibration stacks used</th><th>Total searched</th><th>Calibrations searched</th><th>Searches per duster</th></tr></thead><tbody><tr><td>I</td><td>28 Jul 2006</td><td>30 Jul 2007</td><td>24176</td><td>2421</td><td>46.7 × 10<sup>6</sup></td><td>11.7 × 10<sup>6</sup></td><td>35</td></tr><tr><td>II</td><td>07 Aug 2007</td><td>09 Mar 2010</td><td>4396</td><td>3021</td><td>25.2 × 10<sup>6</sup></td><td>4.2 × 10<sup>6</sup></td><td>40</td></tr><tr><td>III</td><td>10 Mar 2010</td><td>04 Jul 2011</td><td>1683</td><td>1879</td><td>15.6 × 10<sup>6</sup></td><td>2.2 × 10<sup>6</sup></td><td>48</td></tr><tr><td>IV</td><td>12 Jul 2011</td><td>16 Jan 2011</td><td>458</td><td>17580</td><td>1.8 × 10<sup>6</sup></td><td>0.2 × 10<sup>6</sup></td><td>101</td></tr><tr><td>All phases</td><td>28 Jul 2006</td><td>>23 Jan 2012</td><td>30714</td><td></td><td>89.4 × 10<sup>6</sup></td><td>17.3 × 10<sup>6</sup></td><td>38</td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>Fig. 2. Integral distribution of search statistics for focus stacks. The abscissa is the number of times that a stack was searched, and the ordinate is the number of stacks, which were searched at least that number of times. The steps in the statistics are due to large additions of image data to the database at discrete times.</figcaption></figure>

## Phase IV

In Phase IV, we entirely replaced the calibration stack set with three different types of calibrations. The first type was similar to the calibrations in Phase I, but included both analog tracks and real tracks identified in the Stardust@home collector. The second type consisted of real, undubbed stacks in which tracks had been identified, but in four different configurations: as collected, rotated 180°, mirrored around the vertical axis, and mirrored around the vertical axis and rotated 180°. Finally, we included undubbed stacks of analog tracks, processed the same way. We re-examined all stacks during Phase IV.

## Stardust@Home Statistics

In Table 1, we show the statistics for each Stardust@home phase.

In Fig. 2, we show the distribution of search statistics for each focus stack, as of 23 January 2012.

Calibration data from Phases I and II (Fig. 3) indicate an ensemble-wide average individual detection

<figure><img src="image_4.png" /><figcaption>Fig. 3. Detection efficiency for calibration stacks during S@H Phases I (upper curve) and II (lower curve), based on $15.6 \times 10^6$ responses.</figcaption></figure>

efficiency of ≥60% for hypervelocity tracks with diameter >2.5 µm, and ≥90% for hypervelocity tracks with diameter >5 µm. We emphasize that the individual detection efficiency is different from the overall detection efficiency, because of the large multiplicity of individual searches.

## α-List

To efficiently identify candidate tracks, we employed two levels of selection. An intermediate list called the α-list consisted of preliminary candidates, and a final list of confirmed, unambiguous tracks was called the β-list. Candidates were selected for the α-list and β-list as follows.

For each field of view, we define $\xi$ as the ratio of the number of positive identifications to the total number of searches conducted on that field of view. Periodically, we selected approximately 1000 stacks with the largest fraction $\xi$ of positive responses from dusters to be promoted to the α-list, which corresponded to a promotion threshold value of $\xi = 0.2$ or smaller. By comparison, Phase I and II calibration data from hypervelocity tracks show a detection efficiency of ≥0.75 for
<figure><img src="image_5.png" /><figcaption>Fig. 4. Integral distribution of ξ for all stacks in phases I–IV. Red line: Stacks that were searched at least 30 times. Blue line: Stacks that were searched at least 50 times. The vertical line at ξ = 0.75 corresponds to the measured detection efficiency of tracks with diameter 3.0 µm, and the vertical line at ξ = 0.62 corresponds to this threshold adjusted for finite statistics with an average search multiplicity of 50. In practice, we examined all tracks with a cut far lower than this, with ξ > 0.15.</figcaption></figure>

tracks >3 µm in diameter, so this threshold is highly
conservative. In Fig. 4, we show the integral
distributions of ξ for all stacks in phases I through IV,
which had been searched ≥50 times, and ≥30 times. The
effective search areas for these data sets are 225 cm²
and 246 cm², respectively. Because, for any given field
of view, the statistics are relatively small, statistical
fluctuations in the number of positive identifications are
expected. To compensate, one must lower the threshold
to be sure to catch all of the positive detections. To
illustrate, we imagine a data set consisting of stacks that
have been searched 50 times, with a uniform detection
efficiency of 0.75. Then, each stack would have
collected, on average, 37.5 positive identifications.
However, the distribution of positive identifications is
approximately gaussian with a width of √37.5 ~ 6.1, so,
to be sure to identify 84% (1σ) of the actual tracks, one
would set the threshold at (37.5 - 6.1)/50 = 0.62 For
high-statistics stacks, those searched at least 50 times,
12 stacks fit this criterion. In practice, and to be highly
conservative, at the end of phase IV, we individually
reviewed all stacks with ξ ≥ 0.15, 2256 focus stacks. For
stacks that had been searched 30 times or more, we
selected those with ξ > 0.38, which corresponds to 2473
stacks.

During Phase II, we invited the top-scoring dusters
to join a so-called Red Team. The Red Team members,
who were the most experienced dusters in the project,

were given the system privileges to be able to promote
any candidates, whom they thought interesting, into the
α-list. The α-list, therefore, included three sets of
candidates: those with ξ > 0.15 and N_search > 50, those
with ξ > 0.38 and N_search > 50, and those who were
promoted by the Red Team. The Red Team members
were also able to rate, on a scale from 1 to 10, any of
the stacks on the α-list.

One of us (AJW) reviewed all entries on the α-list, to identify bona fide tracks and candidate features that had the potential to be impacts, but were ambiguous. In this way, 69 tracks have now been identified, and included in the β-list of confirmed tracks. There are an additional 14 features that are ambiguous to varying extents, and may require extraction for definitive identification.

RESULTS

In Table 2, we show the list of candidate tracks that have been evaluated by the Berkeley group and confirmed as unambiguous tracks.

DISCUSSION

While the measured sensitivity for dust detection of
calibrations was >0.62 over the range of calibration
track diameters (>2.5 µm, Fig. 2), the measured
efficiency for detection of actual tracks was generally
considerably lower (Table 2). Almost all tracks found
by Stardust@home had sizes near the detection
threshold (approximately 2 µm), and differed strongly
in appearance from the dubbed images in the original
calibration stacks. The track image that was used in
Phases I and II was due to a >15 km s⁻¹ track
generated by the Heidelberg Dust Accelerator (HDA)
before the Stardust launch, so it was bulbous in
appearance, in accordance with our initial expectations
of the interstellar dust capture speed. But the tracks
identified in the actual collector were morphologically
similar to particles captured at low speed
(<< 10 km s⁻¹) observed in Heidelberg experiments
(Postberg et al. 2014), and thus exhibited very small
track diameter/length ratios (discussed further in Frank
et al. [2013]). In Phase III, we included a very small
linear feature in the calibration stacks, but this was
different in appearance from the bona fide tracks. (The
linear feature was later analyzed by synchrotron and
determined to be a submicron fiber.) However, in
Phase IV, we used calibration stacks of three types:
stacks dubbed with randomly rotated and magnified
images of previously identified bona fide tracks; actual
stacks that include bona fide tracks, that we presented
with four possible orientations (unchanged, reflection
<table><caption>Table 2. List of 69 tracks identified by the Stardust@home project. The columns are: stack number; azimuth angle of the particle trajectory in degrees, measured counterclockwise from the midnight (sunward) direction; the detection efficiency ξ and the number of searches for each of phases I through IV; the cumulative detection efficiency ξ and the number of searches; the first Stardust@home duster to identify the track; the Stardust@home Red Team members who identified and promoted the track to the α-list.</caption><thead><tr><th>ID</th><th>Φ</th><th>N<sub>I</sub></th><th>ξ<sub>I</sub></th><th>N<sub>II</sub></th><th>ξ<sub>II</sub></th><th>N<sub>III</sub></th><th>ξ<sub>III</sub></th><th>N<sub>IV</sub></th><th>ξ<sub>IV</sub></th><th>N<sub>total</sub></th><th>ξ<sub>total</sub></th><th>Duster discoverer</th><th>Red Team promoter(s)</th></tr></thead><tbody><tr><td>225116V1</td><td>52</td><td>216</td><td>0.13</td><td>0</td><td>—</td><td>7</td><td>0.14</td><td>6</td><td>1</td><td>229</td><td>0.16</td><td>H Leithead</td><td>Fred J. Gray</td></tr><tr><td>237326V1</td><td>49</td><td>464</td><td>0.25</td><td>60</td><td>0.22</td><td>9</td><td>0.22</td><td>0</td><td>—</td><td>533</td><td>0.25</td><td>Pete Clearwater</td><td>Fred J. Gray</td></tr><tr><td>6065723V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Jacopo Falchi Picchinesi</td></tr><tr><td>6544999V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>305392V1</td><td>96</td><td>453</td><td>0.22</td><td>105</td><td>0.57</td><td>27</td><td>0.37</td><td>8</td><td>1</td><td>593</td><td>0.3</td><td>Ajay Chopra</td><td>SSL</td></tr><tr><td>2574341V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Fred J. Gray</td></tr><tr><td>5406521V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>8823383V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>433711V1</td><td>47</td><td>482</td><td>0.21</td><td>1773</td><td>0.02</td><td>1029</td><td>0.01</td><td>0</td><td>—</td><td>3284</td><td>0.05</td><td>Michael Kopkow</td><td>SSL</td></tr><tr><td>7270647V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>6634692V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>451142V1</td><td>53</td><td>160</td><td>0.22</td><td>35</td><td>0.37</td><td>6</td><td>0.33</td><td>10</td><td>0.6</td><td>211</td><td>0.27</td><td>Devon Trevor Pollard</td><td>Augusto Ardizzone</td></tr><tr><td>8505527V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>730481V1</td><td>42</td><td>473</td><td>0.19</td><td>2</td><td>0</td><td>28</td><td>0.54</td><td>0</td><td>—</td><td>503</td><td>0.21</td><td>Vance Thompson</td><td>Fred J. Gray</td></tr><tr><td>4065507V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Lily Lau Wan Wah</td></tr><tr><td>862370V1</td><td>350</td><td>494</td><td>0.28</td><td>3</td><td>0</td><td>17</td><td>0.41</td><td>0</td><td>—</td><td>514</td><td>0.28</td><td>Stephan Wagner</td><td>Augusto Ardizzone</td></tr><tr><td>4009893V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>1203103V1</td><td>351</td><td>305</td><td>0.04</td><td>33</td><td>0.06</td><td>12</td><td>0.17</td><td>0</td><td>—</td><td>350</td><td>0.05</td><td>Naomi Wordsworth</td><td>Fred J. Gray</td></tr><tr><td>2625717V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Antonella Campanile</td></tr><tr><td>1303955V1</td><td>355</td><td>503</td><td>0.04</td><td>0</td><td>—</td><td>0</td><td>—</td><td>10</td><td>0.8</td><td>513</td><td>0.05</td><td>Bruce A Hudson</td><td>Fred J. Gray</td></tr><tr><td>3964782V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>1728279V1</td><td>86</td><td>694</td><td>0.22</td><td>52</td><td>0.46</td><td>21</td><td>0.24</td><td>0</td><td>—</td><td>767</td><td>0.24</td><td>Elizabeth Wiggins</td><td>Mitchell Criswell</td></tr><tr><td>4566810V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Fred J. Gray</td></tr><tr><td>6406644V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>285812V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>2205922V1</td><td>339</td><td>796</td><td>0.07</td><td>89</td><td>0.13</td><td>13</td><td>0.31</td><td>5</td><td>1</td><td>903</td><td>0.08</td><td>Robert Coursey</td><td>Fred J. Gray</td></tr><tr><td>2220548V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Antonella Campanile</td></tr><tr><td>3303601V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Gerry L. Sperry</td></tr><tr><td>5802607V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>9276534V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>3059039V1</td><td>58</td><td>859</td><td>0.16</td><td>125</td><td>0.07</td><td>30</td><td>0.07</td><td>0</td><td>—</td><td>1014</td><td>0.14</td><td>Jim Hinkey</td><td>Fred J. Gray</td></tr><tr><td>8153179V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>2965266V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>5202109V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>5492352V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>9040053V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>3914098V1</td><td>358</td><td>124</td><td>0.04</td><td>25</td><td>0.04</td><td>10</td><td>0.3</td><td>6</td><td>0.67</td><td>165</td><td>0.08</td><td>Auggie Nanz</td><td>Fred J. Gray</td></tr><tr><td>3721744V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Augusto Ardizzone</td></tr></tbody></table>
<table><caption>Table 2. *Continued.* List of 69 tracks identified by the Stardust@home project. The columns are: stack number; azimuth angle of the particle trajectory in degrees, measured counterclockwise from the midnight (sunward) direction; the detection efficiency ξ, and the number of searches for each of phases I through IV; the cumulative detection efficiency ξ, and the number of searches; the first Stardust@home duster to identify the track; the Stardust@home Red Team members who identified and promoted the track to the α-list.</caption><thead><tr><th>ID</th><th>Φ</th><th>N<sub>I</sub></th><th>ξ<sub>I</sub></th><th>N<sub>II</sub></th><th>ξ<sub>II</sub></th><th>N<sub>III</sub></th><th>ξ<sub>III</sub></th><th>N<sub>IV</sub></th><th>ξ<sub>IV</sub></th><th>N<sub>total</sub></th><th>ξ<sub>total</sub></th><th>Duster discoverer</th><th>Red Team promoter(s)</th></tr></thead><tbody><tr><td>6355541V1</td><td>58</td><td>984</td><td>0.16</td><td>5</td><td>0</td><td>64</td><td>0.23</td><td>0</td><td>—</td><td>1053</td><td>0.17</td><td>Michael Hershberg</td><td>Lily Lau Wan Wah<br>Michael A. Capraro</td></tr><tr><td>4720866V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>5159391V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>6150862V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>7387315V1</td><td>48</td><td>0</td><td>—</td><td>77</td><td>0.3</td><td>21</td><td>0.43</td><td>0</td><td>—</td><td>98</td><td>—</td><td>Gerald Bell</td><td>Michael A. Capraro</td></tr><tr><td>2717134V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>6839286V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>7703161V1</td><td>61</td><td>399</td><td>0.28</td><td>36</td><td>0.56</td><td>17</td><td>0.59</td><td>0</td><td>—</td><td>452</td><td>0.31</td><td>John Rote</td><td>SSL</td></tr><tr><td>8130472V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>8894764V1</td><td>356</td><td>182</td><td>0.11</td><td>0</td><td>—</td><td>4</td><td>0.75</td><td>3</td><td>1</td><td>189</td><td>0.14</td><td>Robert Kurt Myers</td><td>Joe Karr</td></tr><tr><td>9982449V1</td><td>30</td><td>472</td><td>0.21</td><td>5</td><td>0.4</td><td>26</td><td>0.35</td><td>0</td><td>—</td><td>503</td><td>0.22</td><td>Barry Weeg</td><td>SSL</td></tr><tr><td>3997562V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>9987464V1</td><td>48</td><td>257</td><td>0.09</td><td>0</td><td>—</td><td>0</td><td>—</td><td>4</td><td>0.75</td><td>261</td><td>0.1</td><td>Bruce A Hudson</td><td>Fred J. Gray</td></tr><tr><td>1779983V1</td><td>35</td><td>383</td><td>0.06</td><td>2</td><td>1</td><td>17</td><td>0.41</td><td>0</td><td>—</td><td>402</td><td>0.08</td><td>Paul M. Campbell</td><td>Michael A. Capraro</td></tr><tr><td>6133043V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>6389213V1</td><td>302</td><td>318</td><td>0.07</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>318</td><td>0.07</td><td>Pamela J. Copples</td><td>SSL</td></tr><tr><td>6721021V1</td><td>29</td><td>514</td><td>0.26</td><td>4</td><td>0.5</td><td>20</td><td>0.15</td><td>0</td><td>—</td><td>538</td><td>0.26</td><td>Fred Bruenjes</td><td>SSL</td></tr><tr><td>9925512V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Will Marchant</td></tr><tr><td>6842109V1</td><td>60</td><td>369</td><td>0.07</td><td>0</td><td>—</td><td>21</td><td>0.38</td><td>0</td><td>—</td><td>390</td><td>0.08</td><td>David Moses</td><td>SSL</td></tr><tr><td>8729246V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>7559378V1</td><td>56</td><td>299</td><td>0.06</td><td>0</td><td>—</td><td>0</td><td>—</td><td>5</td><td>0.8</td><td>304</td><td>0.07</td><td>Mieke Abels</td><td>SSL</td></tr><tr><td>7663035V1</td><td>3</td><td>263</td><td>0.02</td><td>0</td><td>—</td><td>0</td><td>—</td><td>6</td><td>0.83</td><td>269</td><td>0.04</td><td>Bruce A Hudson</td><td>Fred J. Gray</td></tr><tr><td>7933874V1</td><td>44</td><td>423</td><td>0.03</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>423</td><td>0.03</td><td>Dipali Vasadia</td><td>SSL</td></tr><tr><td>5103315V1</td><td>302</td><td>299</td><td>0.04</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>299</td><td>0.04</td><td>John Lokken</td><td>SSL</td></tr><tr><td>1078907V1</td><td>66</td><td>0</td><td>—</td><td>2</td><td>0</td><td>24</td><td>0.21</td><td>0</td><td>—</td><td>26</td><td>—</td><td>Patryk Kleman</td><td>Lily Lau Wan Wah</td></tr><tr><td>2344842V1</td><td>57</td><td>345</td><td>0.15</td><td>31</td><td>0.58</td><td>16</td><td>0.56</td><td>12</td><td>1</td><td>404</td><td>0.22</td><td>Daniel Lichtenwald</td><td>Mitchell Criswell</td></tr><tr><td>2595293V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Carlo Rigamonti</td></tr><tr><td>5256120V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Lily Lau Wan Wah</td></tr><tr><td>3160814V1</td><td>356</td><td>0</td><td>—</td><td>0</td><td>—</td><td>18</td><td>0.11</td><td>3</td><td>0</td><td>21</td><td>—</td><td>Patrick Fougeray</td><td>SSL</td></tr><tr><td>4359211V1</td><td>14</td><td>0</td><td>—</td><td>0</td><td>—</td><td>13</td><td>0.54</td><td>1</td><td>1</td><td>14</td><td>—</td><td>Kevin A Courtney</td><td>SSL</td></tr><tr><td>4969429V1</td><td>51</td><td>0</td><td>—</td><td>19</td><td>0.37</td><td>16</td><td>0.19</td><td>0</td><td>—</td><td>35</td><td>—</td><td>Michael A. Capraro</td><td>Michael A. Capraro</td></tr><tr><td>5848045V1</td><td>3</td><td>0</td><td>—</td><td>26</td><td>0.23</td><td>13</td><td>0.31</td><td>4</td><td>0.75</td><td>43</td><td>—</td><td>Steven E. Miles</td><td>Lily Lau Wan Wah</td></tr><tr><td>3689408V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>Daniel Zevin</td></tr><tr><td>6125350V1</td><td>7</td><td>483</td><td>0.18</td><td>5</td><td>0</td><td>23</td><td>0.26</td><td>0</td><td>—</td><td>511</td><td>0.18</td><td>Gerald Fisher</td><td>SSL</td></tr><tr><td>7810698V1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>6366986V1</td><td>51</td><td>0</td><td>—</td><td>33</td><td>0.42</td><td>5</td><td>0.6</td><td>0</td><td>—</td><td>38</td><td>—</td><td>Robert L Cwik</td><td>Mitchell Criswell</td></tr><tr><td>6474626V1</td><td>352</td><td>0</td><td>—</td><td>2</td><td>0</td><td>15</td><td>0.13</td><td>0</td><td>—</td><td>17</td><td>—</td><td>Augusto Ardizzone</td><td>SSL</td></tr></tbody></table>
<table><caption>Table 2. Continued. List of 69 tracks identified by the Stardust@home project. The columns are: stack number; azimuth angle of the particle trajectory in degrees, measured counterclockwise from the midnight (sunward) direction; the detection efficiency ξ, and the number of searches for each of phases I through IV; the cumulative detection efficiency ξ, and the number of searches; the first Stardust@home duster to identify the track; the Stardust@home Red Team members who identified and promoted the track to the α-list.</caption><thead><tr><th>ID</th><th>Φ</th><th>N<sub>I</sub></th><th>ξ<sub>I</sub></th><th>N<sub>II</sub></th><th>ξ<sub>II</sub></th><th>N<sub>III</sub></th><th>ξ<sub>III</sub></th><th>N<sub>IV</sub></th><th>ξ<sub>IV</sub></th><th>N<sub>total</sub></th><th>ξ<sub>total</sub></th><th>Duster discoverer</th><th>Red Team promoter(s)</th></tr></thead><tbody><tr><td>6522857V1</td><td>44</td><td>129</td><td>0.13</td><td>30</td><td>0.27</td><td>11</td><td>0.36</td><td>3</td><td>0.67</td><td>173</td><td>0.18</td><td>Michael Paperin</td><td>SSL</td></tr><tr><td>2933051V1</td><td>58</td><td>392</td><td>0.1</td><td>5</td><td>0.2</td><td>27</td><td>0.15</td><td>0</td><td>—</td><td>424</td><td>0.1</td><td>Mike Bunch</td><td>Mitchell Criswell<br>Michael A. Capraro</td></tr><tr><td>5088094V1</td><td>57</td><td>0</td><td>—</td><td>29</td><td>0.07</td><td>5</td><td>0</td><td>0</td><td>—</td><td>34</td><td>—</td><td>Michael A. Capraro</td><td>Michael A. Capraro</td></tr><tr><td>1306378V1</td><td>58</td><td>0</td><td>—</td><td>1</td><td>0</td><td>17</td><td>0.06</td><td>0</td><td>—</td><td>18</td><td>—</td><td>Joseph Moschetti</td><td>Fred J. Gray</td></tr><tr><td>6535376V1</td><td>57</td><td>0</td><td>—</td><td>29</td><td>0.07</td><td>5</td><td>0</td><td>0</td><td>—</td><td>18</td><td>—</td><td>Michael A. Capraro</td><td>Augusto Ardizzone</td></tr><tr><td>5599106V1</td><td>358</td><td>0</td><td>—</td><td>1</td><td>0</td><td>17</td><td>0.06</td><td>0</td><td>—</td><td>18</td><td>—</td><td>Michael A. Capraro</td><td>Augusto Ardizzone</td></tr><tr><td>1218276V1</td><td>26</td><td>0</td><td>—</td><td>0</td><td>—</td><td>5</td><td>0.4</td><td>6</td><td>0.33</td><td>11</td><td>—</td><td>Michael A. Capraro</td><td>Augusto Ardizzone</td></tr><tr><td>5300933V1</td><td>55</td><td>145</td><td>0</td><td>0</td><td>—</td><td>8</td><td>0.75</td><td>4</td><td>0.75</td><td>157</td><td>0.06</td><td>Michael A. Capraro</td><td>Augusto Ardizzone</td></tr><tr><td>2599361V1</td><td>0</td><td>0</td><td>—</td><td>64</td><td>0.02</td><td>16</td><td>0.31</td><td>0</td><td>—</td><td>80</td><td>—</td><td>Ilya Alexeev</td><td>Augusto Ardizzone</td></tr><tr><td>3365250V1</td><td>9</td><td>0</td><td>—</td><td>31</td><td>0.03</td><td>7</td><td>0.14</td><td>0</td><td>—</td><td>38</td><td>—</td><td>Charles Tabor</td><td>Michael A. Capraro</td></tr><tr><td>5484077V1</td><td>9</td><td>0</td><td>—</td><td>31</td><td>0.03</td><td>7</td><td>0.14</td><td>0</td><td>—</td><td>38</td><td>—</td><td>Gerry L. Sperry</td><td>Augusto Ardizzone</td></tr><tr><td>8250577V1</td><td>315</td><td>0</td><td>—</td><td>0</td><td>—</td><td>6</td><td>0.5</td><td>7</td><td>1</td><td>13</td><td>—</td><td>Tom Yahnke Sr.</td><td>Augusto Ardizzone</td></tr><tr><td>5198758V1</td><td>55</td><td>0</td><td>—</td><td>0</td><td>—</td><td>1038</td><td>0.36</td><td>0</td><td>—</td><td>1038</td><td>—</td><td>Augusto Ardizzone</td><td>Augusto Ardizzone</td></tr><tr><td>9219038V1</td><td>55</td><td>0</td><td>—</td><td>0</td><td>—</td><td>984</td><td>0.6</td><td>0</td><td>—</td><td>984</td><td>—</td><td>Michael A. Capraro</td><td>Augusto Ardizzone</td></tr><tr><td>661377V1</td><td>10</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>8</td><td>0.38</td><td>8</td><td>—</td><td>Michael A. Capraro</td><td>Augusto Ardizzone</td></tr><tr><td>1506030V1</td><td>0</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>8</td><td>0.63</td><td>8</td><td>—</td><td>Martin St-pierre</td><td>Augusto Ardizzone</td></tr><tr><td>2923270V1</td><td>73</td><td>82</td><td>0.17</td><td>0</td><td>—</td><td>7</td><td>0</td><td>7</td><td>0.29</td><td>96</td><td>0.17</td><td>Koorosh Mofazzali</td><td>SSL</td></tr><tr><td>4563629V1</td><td>351</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>5</td><td>1</td><td>5</td><td>—</td><td>Antonella Campanile</td><td>Antonella Campanile</td></tr><tr><td>6279005V1</td><td>335</td><td>179</td><td>0</td><td>0</td><td>—</td><td>6</td><td>—</td><td>6</td><td>0.33</td><td>191</td><td>0.01</td><td>Antonella Campanile</td><td>Antonella Campanile</td></tr><tr><td>8454485V1</td><td>27</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>7</td><td>0.57</td><td>7</td><td>—</td><td>Michael A. Capraro</td><td>Michael A. Capraro</td></tr><tr><td>8806280V1</td><td>50</td><td>0</td><td>—</td><td>0</td><td>—</td><td>7</td><td>0.29</td><td>6</td><td>1</td><td>13</td><td>—</td><td>Richard Koschier</td><td>Daniel Zevin</td></tr><tr><td>31269V1</td><td>72</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>6</td><td>0.67</td><td>6</td><td>—</td><td>Ivan Dimov</td><td>Carlo Rigamonti</td></tr><tr><td>9732478V1</td><td>158</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>6</td><td>0.83</td><td>6</td><td>—</td><td>Carlo Rigamonti</td><td>Carlo Rigamonti</td></tr></tbody></table>
<table><caption>Table 2. Continued. List of 69 tracks identified by the Stardust@home project. The columns are: stack number; azimuth angle of the particle trajectory in degrees, measured counterclockwise from the midnight (sunward) direction; the detection efficiency ξ, and the number of searches for each of phases I through IV; the cumulative detection efficiency ξ, and the number of searches; the first Stardust@home duster to identify the track; the Stardust@home Red Team members who identified and promoted the track to the α-list.</caption><thead><tr><th>ID</th><th>Φ</th><th>N<sub>I</sub></th><th>ξ<sub>I</sub></th><th>N<sub>II</sub></th><th>ξ<sub>II</sub></th><th>N<sub>III</sub></th><th>ξ<sub>III</sub></th><th>N<sub>IV</sub></th><th>ξ<sub>IV</sub></th><th>N<sub>IV</sub></th><th>ξ<sub>IV</sub></th><th>N<sub>total</sub></th><th>ξ<sub>total</sub></th><th>Duster discoverer</th><th>Red Team promoter(s)</th></tr></thead><tbody><tr><td>2715058V1</td><td>4</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>11</td><td>0.91</td><td>11</td><td>—</td><td>—</td><td>Ronald C. Spencer</td><td>Michael A. Capraro</td></tr><tr><td>6493751V1</td><td>100</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>6</td><td>0.17</td><td>6</td><td>—</td><td>—</td><td>Martin St-pierre</td><td>Michael A. Capraro</td></tr><tr><td>19284V1</td><td>350</td><td>0</td><td>—</td><td>0</td><td>—</td><td>3</td><td>0</td><td>6</td><td>0.67</td><td>9</td><td>—</td><td>—</td><td>Tom Yahnke Sr.</td><td>SSL</td></tr><tr><td>535699V1</td><td>310</td><td>0</td><td>—</td><td>0</td><td>—</td><td>1</td><td>0</td><td>9</td><td>0.56</td><td>10</td><td>—</td><td>—</td><td>Tom Yahnke Sr.</td><td>SSL</td></tr><tr><td>709134V1</td><td>347</td><td>179</td><td>0.01</td><td>0</td><td>—</td><td>7</td><td>0</td><td>6</td><td>0.67</td><td>192</td><td>0.03</td><td>Cereal Killer</td><td>SSL</td></tr><tr><td>2293539V1</td><td>101</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>11</td><td>0.55</td><td>11</td><td>—</td><td>—</td><td>Charles Tabor</td><td>SSL</td></tr><tr><td>3726006V1</td><td>38</td><td>158</td><td>0.11</td><td>0</td><td>—</td><td>0</td><td>—</td><td>8</td><td>0.75</td><td>166</td><td>0.14</td><td>—</td><td>Paul Walboom</td><td>SSL</td></tr><tr><td>4028216V1</td><td>0</td><td>184</td><td>0.01</td><td>0</td><td>—</td><td>9</td><td>0.11</td><td>5</td><td>1</td><td>198</td><td>0.04</td><td>—</td><td>Bruce Hull</td><td>SSL</td></tr><tr><td>6031444V1</td><td>0</td><td>0</td><td>—</td><td>23</td><td>0.09</td><td>6</td><td>0.5</td><td>0</td><td>—</td><td>29</td><td>—</td><td>—</td><td>Jeremy Beckett</td><td>SSL</td></tr><tr><td>7510686V1</td><td>339</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>7</td><td>0.57</td><td>7</td><td>—</td><td>—</td><td>Vuillemot Christophe</td><td>SSL</td></tr><tr><td>9751354V1</td><td>37</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>8</td><td>0.63</td><td>8</td><td>—</td><td>—</td><td>Antonella Gastaldi</td><td>SSL</td></tr><tr><td>9659008V1</td><td>158</td><td>0</td><td>—</td><td>0</td><td>—</td><td>0</td><td>—</td><td>8</td><td>0.63</td><td>8</td><td>—</td><td>—</td><td>Maurizio Mandarino</td><td>Tom Yahnke Sr.</td></tr><tr><td>5958593V1</td><td>25</td><td>181</td><td>0.01</td><td>0</td><td>—</td><td>4</td><td>0</td><td>5</td><td>0.6</td><td>190</td><td>0.02</td><td>—</td><td>Daniel Smith</td><td>SSL</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>Fig. 5. Example images of tracks discovered by Stardust@home. The largest image, of track 451142V1, includes the entire field of view of the Virtual Microscope, and shows the scalebar. The other examples have been masked to show only the track, but all are shown at the same scale.</figcaption></figure>

about the x- and y-axes, rotation by 180°); and stacks
dubbed with images of tracks generated from
submicron projectiles at the HDA (Postberg et al.
2014). The use of these calibrations resulted
in dramatically higher ensemble-wide sensitivity
in identification of real tracks (see, e.g., ξ<sub>IV</sub> values in
Table 2), although the statistics are smaller because of
duster attrition over >5 years of operation. We
conclude that detection efficiency improves
dramatically when calibration images are presented
that closely match the actual search targets.

A total of 69 unambiguous tracks were identified by
Stardust@home. In Fig. 5, we show examples of the 69
tracks that were identified. Of these tracks, 21 were
within 20° of the “midnight” direction (Westphal et al.
2014), which is consistent with an origin either in the
interstellar dust stream, or with an origin as secondary
ejecta from impacts on the Sample Return Capsule
department. This is discussed in more detail in the companion
paper by Frank et al. (2013).

We conclude that a massively distributed,
calibrated search, with amateur collaborators, is an
effective approach to the challenging problem of
identification of tracks of hypervelocity projectiles
captured in aerogel. Because the actual tracks in the
Stardust collector were morphologically strongly

distinct from expectations, detection efficiency of actual
tracks was lower than detection efficiency of
calibrations presented in training, testing, and ongoing
calibration. Presentation of actual tracks as calibrations
resulted in very high detection efficiency for new tracks
in Phase IV. A more precise measurement of overall
detection efficiency for these small tracks will require
the accumulation of better search statistics in Phase V
and beyond.

Acknowledgments—We thank Sean Brennan and Giles Graham for thoughtful comments, and John Bradley for editorial handling. The ISPE consortium gratefully acknowledges the NASA Discovery Program for Stardust, the fourth NASA Discovery mission. AJW, ALB, ZG, RL, DZ, WM, and JVK were supported by NASA grant NNX09AC36G. We thank Steve Boggs for astrophysical soft X-ray spectra. RMS, HCG, and NDB were supported by NASA grant NNH11AQ61I. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Contract No. DE-AC02-98CH10886.

*Editorial Handling*—Dr. John Bradley

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