Speaker
Description
(See attached file for formatted layout and figures.)
We developed asteroid rubble pile simulations made of crushed basalt. Within the DART community, nearly all computations were performed with basalt as a surrogate for the Dimorphos surface material since it is viewed by many as an appropriate analog to the asteroid material. Basalt was also used by DART Investigation Team members and others in impact computations. Some of our basalt targets had the crushed basalt held in place with a binder (grout with water). Some targets did not – the crushed basalt material was packed into the box. Either a binder or packing was required since our targets hang vertically in a ballistic pendulum. The targets are large – their size is on the order of 60 x 60 x 30 cm and they exceed 200 kg in mass. Using our large 38-mm two-stage light gas gun (for the 5.35 km/s shots) and a 50-mm powder gun (for the 2 km/s shots), we launched aluminum spheres of diameters 2, 3 and 4.45 cm (Table 1).
TABLE 1. Nominal experimental impact conditions for the aluminum sphere impactor.
Projectile diameter (cm) Projectile mass (g) Impact speed (km/s) Momentum (kg m/s) Energy (kJ)
2.0 11.4 5.35 69 163
3.0 38.2 2.05 78 80
3.0 38.2 5.35 204 547
4.45 128 2.05 262 269
A total of 17 new shots into crushed basalt were performed [1]. These add to impacts performed prior to the DART spacecraft impact into Dimorphos, where our group performed impact tests into rock structures measuring momentum enhancement [2,3]. One of these earlier targets was a collection of rocks held in place by cement [2]. In particular, the measured in that test was 3.4 for an impact of a 3-cm-diameter aluminum sphere at 5.44 km/s. The crushed basalt targets had β ranging from 2.12 to 2.83 for the 5.5 km/s shots and 1.52 to 2.51 for the 2 km/s shots. Thus, these crushed basalt shots showed a lower momentum enhancement than some of our previous rock impact work where the rock was more consolidated.
Material properties were measured for the crushed basalt targets such as compressive strength of the material. The strength showed a final-target-density dependence. Work is ongoing in modeling the impacts and comparing to the experimental impact momentum enhancement β as well comparing to the DART mission result.
These experiments should play an important role in the momentum enhancement studies in general and in the DART results analysis in particular by pinning down some specific data points with a well characterized experiment at impact speeds of interest for the DART program with targets made of distributed rocks. We will present some of our related computations.
Figure 1. Impacts at 2 km/s by a 4.45 cm diameter sphere of aluminum. Left: A post-test crushed-basalt target hanging in the pendulum (Test 7). Right: roughly 8 milliseconds after impact (projectile came from right to left; Test 1).
1. “Momentum enhancement from impacts into crushed basalt at 2 and 5.5 km/s, motivated by DART,” J. D. Walker, S. Chocron, D. J. Grosch, D. D. Durda, S. Marchi, M. V. Grimm, C. Sorini, Proceedings of the 2024 Hypervelocity Impact Symposium, September 9-13, 2024, Tsukuba, Japan.
2. “Momentum enhancement from a 3 cm diameter aluminum sphere striking a small boulder assembly at 5.4 km s−1,” J. D. Walker, S. Chocron, D. J. Grosch, S. Marchi, A. M. Alexander, Planetary Science Journal 3 215, 2022. https://doi.org/10.3847/PSJ/ac854f.
3. “Momentum enhancement from 3-cm-diameter aluminum sphere impacts into iron and rock at 5 km/s,” J. D. Walker, S. Chocron, D. J. Grosch, S. Marchi, A. M. Alexander, Int. J. Impact Engng 180, 104694:1–11, 2023.