As an attractive collector medium for hypervelocity particles, combined with outstanding physical properties and suitable compositional characteristics, silica aerogel has been proved as an ideal capture media for space debris, interplanetary dust and shock-loaded fragments (highly transparent, low-density, highly porous, etc.), which can capture the hypervelocity particles efficiently and nondestructively. On the one hand, silica aerogel capture cells had been deployed in space missions and many researches focusing on the captured particles and track had been done. On the other hand, numerous efforts of modeling and experimental approaches were concluded to estimate the conditions of impact in aerogel. Hörz classified the diverse aerogel track shapes into three broad types (A, B, and C), In addition to straight path, track curvature was exhibited at the result of ground experiment in the available reports. The gently curving carrot-shaped -tracks observed in aerogel was assigned as being due to irregular shape of projectile. That is closer to the actual situation that many space debris have irregular shapes. However, till now, there is almost no complete understanding of the impact behaviors and typical track morphology of irregular particles. Thus, it is worthwhile to analyze track morphology of hypervelocity irregular grains in silica aerogel.
In the paper published on High Power Laser Science and Engineering, Vol. 9, Issue 2, 2021 (Ai Du, Yi Ma, Mingfang Liu, et al. Morphology analysis of tracks in the aerogels impacted by hypervelocity irregular particles[J]. High Power Laser Science and Engineering, 2021, 9(2): 02000e14), a series of density SiO2 aerogel samples were obtained via one-step and two-step sol-gel route and ethanol supercritical dried method.
The irregularly shaped A12O3 grains with two different sizes, provided by Nation Space Science Center (NSSC), the Chinese Academic of Sciences (CAS), named as Particles-100 and Particles-200, are used as projectiles to simulate space debris. The tracks penetrated by projectiles were measured by optical imagery, and the track morphology and the residues were carefully observed by video measuring machine, laser scanning confocal microscopy and optical coherence tomography. The classification and analysis of poses of residues had been studied in order to improve the understanding of the impact behaviors of irregular particles. The impact laboratory experiments were conducted by the plasma gun (PG) of plasma dynamic accelerator (PDA) of NSSC, CAS. The schematic diagram of the experiment is shown in Fig. 1. The impact velocity covered by the two data sets were about 2.3 km/s and 7 km/s.
Fig. 1 Schematic diagram of projectile flight in the plasma gun.
Eventually, the impact experimental data of irregular grains shows that there is no clear function between the length of the track, the size of the residual particles, and the impact velocity. In addition to the basic straight carrot-like track, VMM can be used to observe the process of penetration left by irregular projectile, where other changes happened, such as curve, rotation, etc. According to the tracks and status of residues, tracks are classified 4 types to explain the possibility of discretization data about penetration length to some extent. The trajectory classification after impact is shown in Fig. 2.
Fig. 2 The simplified diagram classified from impact tracks (The impact direction is from the left)
The track could be mainly divided into 4 types as follows:
1) the Near-sphere residues with the relatively smooth track;
2) polyhedron residues changed from ablation and reduction of irregular projectile;
3) streamlined/ wedge-shaped residues rest on the end of curve track with the tip forward at the stop status;
4) residues with rotational behavior during penetration process in the terminal of the track curvature with the largest cross-section.
Compared with the launch of the regular projectile, these uncertain behaviors affect the trend of data points profoundly during the penetration. The authors hope that morphology analysis of particle and track would be helpful to understand the penetration by irregular grains. In addition, OCT was trying to obtain the detail of the track in a rapid, easy, and nondestructive manner, which will be a potential track observation.