Capture of hypervelocity particles in aerogel: in ground laboratory and low earth orbit

We have investigated in the laboratory the capture in aerogel (density 92.5 +/- 0.5 kg m(-3)) of small particles travelling at (5.1 +/- 0.2) km s(-1). The particles used were soda glass spheres and irregularly shaped olivine and iron particles, with mean diameters in the range 75-355 microns. We have measured the impact site for each particle, characterised by the mean diameter of the entrance hole in the aerogel, the minimum and maximum radii of the damaged region in the surface of the aerogel around the entrance hole, the length of the track in the aerogel caused by passage of the particle into the aerogel's interior, and the diameter of the captured particle (if seen) found near the end of the track. For each type of particle we establish relationships between the observed parameters and the pre-impact particle size. We find that the processes resulting in the observed surface features and the capture of the particles in the interior of the aerogel are different. We also find that the particle shape (spherical/irregular) does not unduly influence penetration depths in the aerogel. We have studied the effects of non-normal incidence on the observed impact features and find that the angle of incidence can be reconstructed to within +/-2 degrees. We compare the laboratory obtained data with that measured for four particles captured in a sample of aerogel flown in a Low Earth Orbit on board the EuReCa spacecraft. The density of one of the particles is predicted to be (1776 +/- 346) km m(-3). Using the ability to reconstruct impact direction the probable nature of the particles is shown to be micrometeoroids with retrograde trajectory.