Impacts in Aerogel at Low Temperatures

Introduction: The use of aerogel as a capture medium for particles traveling at high speeds has been long established (see [1] for a recent review). Aerogel collectors have several times been deployed in Low Earth Orbit. Recently, the Stardust space mission [2] has returned to Earth (Jan. 2006) carrying aerogel which was used to collect an estimated 2800±500 freshly emitted cometary dust grains (size > 15 μm) from comet P/Wild-2 at 1.86 AU [3]. The ambient temperature of the aerogel at this distance from the Sun will be less than in previous uses of the material in space and also lower than that found in previous laboratory experiments. Accordingly, we have carried out a series of light gas gun impacts of small particles into aerogel at a range of aerogel temperatures. This is to determine if any significant effects are found as a function of the aerogel ambient temperature. Experimental Method: The two stage light gas gun of the University of Kent was used in this work [4]. The projectiles were small glass beads, pre-sorted to have a diameter of 50 ± 1 μm. Clouds of several projectiles were fired in each shot. The average speed of the shots was 6.4±0.2 km s, close to the 6.1 km s at which Stardust collected cometary grains. The density of the aerogel used was 60 kg m, slightly higher than the Stardust aerogel density. The aerogel was placed in a metal container, with good thermal contact between the walls and the aerogel. The walls were steel blocks 1 cm wide. Before each shot the aerogel was loaded into the target holder and placed in a freezer for 3 to 5 days (the longer period was for the lower temperature work). The aerogel temperature was measured by a sensor pushed into the aerogel and was read out electronically during the shots. Thus all temperatures quoted are of the aerogel at the moment of the shot. A range of temperatures from 175 to 293 K was used and 5 shots were performed in this range. Multiple impacts were obtained at each temperature. Results: The tracks found in each shot were measured with an optical microscope. The entrance hole diameter, track length and diameter of the captured particle were measured for each aerogel sample. The number of measurements of each type is given in Table 1. In some cases, tracks overlaid each other when observed through the microscope, so at 260.5 K 2 tracks were not measured although the entrance holes were. Similarly, at 175.5 K it was not possible to observe the captured particle at the end of one track. The measured values at each temperature were then averaged and the error shown on the mean value is the standard deviation of the data about the mean.