Fig. 1. SEM images of irregularly shaped Al₂O₃ grains of (a) Particles-100 and (c), (d) Particles-200 (with surface holes) used as projectiles in the PG experiment. (b) Al₂O₃ grains on Mylar film.

Fig. 2. Schematic diagram of projectile flight in the PG.

Fig. 3. Contrast figures for the landing surface of the targets (a) before and (b) after impact.

Fig. 4. Observation of the track entrance: (a) attachments of Mylar near the track entrance marked by red circles: (b) near circular entrance hole of a penetration track; (c), (e), (f) irregular shape of penetration entrance hole; (d) unclear observation of entrance by VMM; (f) three-dimensional graph matching with (e).

Fig. 5. (a), (b) Morphology of entrance holes by impactors and (c), (d) part of track along the impact direction by SEM. The red arrow is marked at the point of observation.

Fig. 6. Summary of results: (a) plots of the diameter of original and captured projectiles; (b) plots of measured track lengths shown as a function of aerogel target density, all at 2.3 and 7 km/s by impacting (measurement error is 0.002 mm); (c) the aerogel track lengths () normalized to projectile diameter () against target bulk density; (d) penetration track length scaled () against density ratio ().

Fig. 7. Aerogel tracks created by impacts of Al₂O₃ obtained by VMM, the impact direction is from the right (except in (e)): (a) the near-spherical captured projectile with the relatively straight track; (b) a near-spherical particle on the terminal track; (c), (d) the irregular captured projectile with the relatively straight track; (e) the detail of track by impact at 2.3 km/s on 182 mg/cm³; (f) flat-shaped projectile residue on the curved track; (g), (h) the residual grain slightly reduced on the curved track; (i) fine features of a track in the aerogel.

Fig. 8. The simplified diagram classified from impact tracks (the impact direction is from the left).

Fig. 9. The projectile-breakage situation outside of available data of track classification (the impact direction is from the right): (a) two adjacent grains injected simultaneously into the target, the track branch is not caused by particle breakage; (b) three branches at the track tail caused by a projectile rupture; (c) larger particles and smaller ruptured grains remaining in the track tail; (d) tiny grains spread forward in the local enlargement of (c); (e) simplified diagram of the track shown in (b).

Fig. 10. Aerogel tracks created by impacts of Al₂O₃ at 7 km/s (the impact direction is from the right). (a) A track-scan photograph along the impact direction. (b) The track photograph just passing through the two spherical bubbles near the cylindrical aerogel side obtained by VMM. (c), (d) The two segments of the track of (b) revealed by OCT.