Stardust Interstellar Preliminary Examination I: Identification of tracks in aerogel

Abstract

Abstract-Here, we report the identification of 69 tracks in approximately 250 cm 2 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 9 10 7 searches on approximately 10 6 fields of view. Using calibration images, we measured individual detection efficiency, and found that the individual detection efficiency for tracks > 2.5 lm in diameter was >0.6, and was >0.75 for tracks >3 lm 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 lowspeed 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. 1510 A. J. Westphal et al.

Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft

“…Modeling indicates that very few IDP impacts on the SIDC are expected to coincide with the "midnight" direction where interstellar impacts occur (10,19) and we observed no tracks in the angular range where IDPs should have their maximum flux, indicating that the IDP background is small. Based on the observed angular distribution of captured particles, and model trajectories, the statistical likelihood of an interplanetary origin for all three interstellar dust candidates in aerogel is <0.03% (10,20) The ecliptic longitude of the interstellar dust radiant that best fits the observed trajectories of the three candidates in aerogel is somewhat larger than anticipated (9,18,20) based on observations from Ulysses and Galileo, but this may indicate a real long-term radiant shift which is consistent with the long-term increase in radiant longitude in neutral helium (30 (32), who predicted a total collected particle count of 120 (80 < 2µm and 40 > 2µm diameter) interstellar particles and 20 IDPs. With the conservative assumption that all of the interplanetary dust is < 2 µm, this equates to 100 small particles (80 interstellar and 20 interplanetary), of which 20% should be interplanetary.…”

“…The latter estimate assumes an uncorrelated origin for the impacting particles. A correlated origin as secondary ejecta from micrometeroid impacts on the sample capsule or solar cell array can be discounted because in the ejecta of such impacts, spacecraft material is expected to dominate over impactor material by approximately two orders of magnitude (9,20), and to have lower impact velocity and shallower impact depth than observed for interstellar candidate craters (33). This is inconsistent with the observed low ratio of target/projectile material in the impacts, even accounting for the low statistics (34), and the observed interstellar candidate crater morphologies.…”

“…All but two were identified through the Stardust@home project (9,10), in which volunteers searched online for tracks in digital micrographs of the aerogel collector. We extracted a subset of these tracks in volumes of aerogel, called "picokeystones" (10,23), and mounted them between 70 nm-thick Si 3 N 4 membranes to protect from loss and contamination.…”

“…The identification of these seven impacts is the result of a massively-distributed, volunteerbased, search of optical micrographs of the aerogel collectors, manual and automated searches of scanning electron micrographs of aluminum foils, extensive coordinated sample analyses, laboratory hypervelocity impact experiments, and numerical modeling of ISD propagation in the heliosphere. These are described in detail in a series of papers (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) published contemporaneously with this article; further supplementary details can be found on Science online (21).…”

“…The low density of the silica aerogel enables capture of hypervelocity particles with mild deceleration as compared with other capture media, to limit the capture alteration effects, while simultaneously recording particle trajectory in the form of a carrot-shaped track. The optical transparency of the aerogel allows for detection of tracks ≥ 2 µm in diameter (9). The Al foil is a complementary collection medium to the silica aerogel.…”