Hypervelocity Particle Capture Material

The Stardust Mission used silica aerogel as the hypervelocity particle capture and return material since silica aerogel was the most reproducible to manufacture and the most thoroughly evaluated type of aerogel at the time [1]. However, the Stardust Science Team recognized that the use of silica aerogel, while expedient, introduced limits to mission’s eventual science return. This is due to the fact that planetary geochemists generally measure and report element/silicon ratios in the analyses of terrestrial and extraterrestrial samples. During the particle capture process, material from the Stardust aerogel, i.e., silica, will accrete onto the particles, thus obscuring portions of the chemical analyses that can be performed on the samples. By using nonsilicate aerogels to complement the silica aerogel being employed in a sample capture instrument, the full spectrum of chemical analyses available can be performed without interference by any specific element from the aerogel. Aerogel is used as a hypervelocity capture and return medium because it can decelerate hypervelocity particles while incurring minimal damage to the particle [2]. This is due in large part to the fact that the network that makes up the aerogel is composed of filaments that are nanometer in size, while the particles are micron sized, and are comparatively widely spaced. Thus, aerogels can be extremely low in density, < 0.100 grams/cc, and are frequently more than 99% air by volume. Methods for producing non-silicate aerogels have been reported in the scientific literature for many years, with carbon aerogel being the most extensively produced and thoroughly tested [3-6]. Although the production of non-silicate oxide based aerogels has been reported, there has been little in the way of results that verify the production of reasonably large monoliths of non-silicate aerogel and the structural robustness of these materials. To be used as the capture medium in a sample capture and return instrument, non-silicate aerogel must be produced as reasonably large monoliths (25 to 250 cc’s), they must be mechanically robust to survive launch loads, and they must have a network that is suitable for efficient hypervelocity particle capture. Based on methods outlined in previously published articles monoliths of carbon, alumina, titania, germania, zirconia, niobia, tin oxide, and hafnia have been produced for this study. Table 1 lists the types of non-silicate aerogel produced for this study, the atomic number of the metal (or carbon) in each, and the lowest density of each type produced thus far. The atomic number is listed since the chemical analysis conducted on these aerogels after impact testing will be x-ray fluorescence, and for the higher atomic number elements the various xray wavelengths begin to interfere. The density of the cometary Stardust aerogel gradates from 10 mg/cc to 50 mg/cc in each piece, which was determined to be the optimal density range for efficient hypervelocity particle capture. Thus, sample densities in approximately this range are desirable.