Characteristics of cometary dust tracks in Stardust aerogel and laboratory calibrations

Abstract

Abstract-The cometary tray of the NASA Stardust spacecraft's aerogel collector was examined to study the dust captured during the 2004 flyby of comet 81P/Wild 2. An optical scan of the entire collector surface revealed 256 impact features in the aerogel (width >100 μm). Twenty aerogel blocks (out of a total of 132) were removed from the collector tray for a higher resolution optical scan and 186 tracks were observed (track length >50 μm and width >8 μm). The impact features were classified into three types based on their morphology. Laboratory calibrations were conducted that reproduced all three types. This work suggests that the cometary dust consisted of some cohesive, relatively strong particles as well as particles with a more friable or low cohesion matrix containing smaller strong grains. The calibrations also permitted a particle size distribution to be estimated for the cometary dust. We estimate that approximately 1200 particles bigger than 1 μm struck the aerogel. The cumulative size distribution of the captured particles was obtained and compared with observations made by active dust detectors during the encounter. At large sizes (>20 μm) all measures of the dust are compatible, but at micrometer scales and smaller discrepancies exist between the various measurement systems that may reflect structure in the dust flux (streams, clusters etc.) along with some possible instrument effects.

Bulbous tracks arising from hypervelocity capture in aerogel

“…Type C tracks-these have bulbous cavities without a slender terminus. A detailed discussion of the frequency and morphology of each type is given in Burchell et al (2008). Here we note that type A comprised 65%, B 33%, and C 2% of all tracks.…”

“…In the laboratory, projectiles with very different cohesive properties, composition and grain sizes reproduce the range of track morphologies as distinct classes (Hörz et al 1998;Burchell et al 2008). The Stardust aerogels contain the full range of these morphologies and their gradual transitions.…”

“…The type A tracks are considered to be due to relatively strong, well condensed grains and in this paper are not considered further. Instead we focus on the type B (and type C) tracks which comprise the majority of the largest impact features in the aerogel (Burchell et al 2008). Four tracks showing the range of morphologies for type B and C tracks are shown in Fig.…”

“…Upon return, the initial inspection of the aerogel revealed a wide variety in track size and shapes ). These were analyzed and classified in detail by Burchell et al (2008). In this initial survey and photographic documentation of tracks, each feature was labeled as follows: C2xxx-Ty, where C2 indicates it is the cometary (rather than interstellar) collector tray, xxx refers to the number and specific tray location of the aerogel tile, and y is the yth track found in that tile.…”

Java application for the superposition T-matrix code to study the optical properties of cosmic dust aggregates

“…It has also been suggested that interstellar dust grains may consist primarily of such aggregate structures [6], [7], with a mixture of various chemical compositions and vacuum. It is now well accepted from the in situ measurements of comets and Stardust-returned samples of Comet Wild 2 that cometary dust consists of a mixture of compact particles and aggregates [8], [9], [10], [11], [12], etc.…”