Designer Asteroids: Hypervelocity impact experiments to track internal structure effects on deflection by kinetic impactors

Several techniques may be appropriate for deflecting a threatening asteroid on a collision course with Earth. Slow-push techniques, such as gravity tractors, require long lead times. Fast-push techniques such as nuclear standoff bursts require much less lead time but come with a host of additional issues. Kinetic impactors are an alternative fast-push technique that can be used on small-to-medium hazards with moderate warning time. The Double Asteroid Redirection Test (DART) mission demonstrated the kinetic impactor technique when it successfully changed the orbital period of Dimorphos in 2022; however, questions still remain about the efficiency of momentum transfer with this technique. Modeling work performed in support of DART suggests that properties such as material strength, porosity, crush properties, asteroid internal structure, and inherent flaw distribution, can significantly affect the deflection that will be caused by a kinetic impactor.  Here, we undertake a set of experiments to better constrain how material strength and target structure affect the outcomes of kinetic impactor asteroid deflection. In this study, we focus on hypervelocity (>1km/s) kinetic impactor results.

We constructed “designer asteroids” with varying internal structures, from coherent asteroids to rubble piles, and performed impact experiments at the Johns Hopkins Applied Physics Laboratory (APL) Impact Lab (IL) and the NASA Ames Vertical Gun Range (AVGR) to evaluate the momentum transfer efficiency following impact.  In this study, we report on impacts ranging from 1-5 km/s undertaken at the AVGR. A separate companion study focuses on the lower speed impact undertaken at the IL.  

Near-spherical targets were created using a range of well understood plaster materials of varying strengths, at three different sizes (e.g., Figure 1).  Rubble piles were created using plaster matrix material surrounding either aquarium gravel or porous pumice. All these plasters have well characterized strength properties ranging from 12 to 117 MPa. The internal structure of the targets was evaluated using X-ray Computed Tomography (XCT) scans (e.g., Figure 2), the shape and volume of the craters and resulting deformation was tracked, and a ballistic pendulum was used to track deflection. 

Thirty-seven shots were performed at the NASA AVGR into 7 different target types, with 2 different target sizes (10 and 15-cm diameter spheres). Impact speeds ranged from ~ 1 km/s to ~5 km/s.  Four separate plasters were used to create homogenous targets, with varying strength and porosity properties (Table 1). 

High-speed videos were used to measure the impact location and angle and to determine the displacement and rotation of the target following impact. We use particle and object tracking algorithms to compute the horizontal, vertical and lateral displacements, and the rotations of the pendulum to determine post-impact momentum. We also measure the ejection speed and direction of target ejecta to understand the origin of any momentum enhancement, and characterize the shape and mass of the largest individual ejecta. Finally, fragments are collected following the impact and basic fragment analysis is performed to generate fragment size frequency distributions.

Figure 1. A “family portrait” of example impact targets at three different sizes. (left) 5-cm diameter sphere, (middle) 10-cm diameter sphere, (right) 15-cm diameter sphere. The medium and large targets are “rubble pile” targets, with aquarium gravel visible just below the surface.

Table 1. Summary of target materials and number of hypervelocity experiments performed on each different target type. The strength and bulk porosity for the rubble pile targets is in the process of being measured.

The XCT scans can be used to evaluate internal structure in more detail as well as post-impact damage patterns.  In the AVGR experiments, impact-induced fracturing is seen below the craters and throughout the targets. For rubble pile targets, damage patterns are significantly affected by pre-impact structure, with fractures largely concentrated in the plaster matrix material and curving around gravel and pre-existing large voids. In rare cases, the fractures are seen through the aquarium gravel (Figure 3).

Differences in momentum enhancement are seen across velocity and target structures. We find that more porous targets generate less ejecta and the impacts likely cause less momentum change, consistent with findings in the literature. At the time of writing, other factors, like impact angle and strength differences, are still being evaluated for our hypervelocity impact experiments. Rubble pile targets behave noticeably different than homogenous targets regardless of impact speed.

Our experiments show that there are variations in experimental outcomes across the replicates of the same target type (e.g., “RP1” or “RP3”), but experimental outcomes vary more when we compare across the different target structures. Targets with higher initial porosity also tended to have deeper craters, while targets with lower initial matrix/plaster strength had larger craters overall compared to those with stronger matrix or plaster strength.  The internal rubble structure did also affect crater size and shape.

In this presentation, we will describe the initial experiments, results, and discuss internal structure effects on deflection and damage patterns in our hypervelocity impact experiments.

Figure 2. Examples of post-impact XCT images for 6 different target types.  All targets are 4” spheres. (MP2): Homogenous “mounting plaster” target, impact velocity = 0.98 km/s; (SR1) Homogenous “silky rock” target, impact velocity = 1.62 kms; (PR1) Homogenous “prima rock” target, impact velocity = 2.17 km/s; (RP1_1) “Rubble Pile 1” target, impact velocity = 1.72 km/s; (RP2_2) “Rubble Pile 2” target, impact velocity = 1.95 km/s; (RP3_1) “Rubble Pile 3” target, impact velocity = 2.12 km/s.  Black arrows show examples of areas of density variation or large voids that naturally result from the manufacturing process.  Light blue arrows highlight post-impact fracturing; green arrows highlight post-impact compression of material beneath the point of impact.

Figure 3. Example of impact-induced fracturing from a “rubble pile 2” target.  Impact velocity = 1.95 km/s. Most fractures are concentrated in the matrix and pass around, or truncate on, pre-existing gravel or large voids.  For sufficiently energetic impacts, the aquarium gravel can also become fractured (e.g., white stone, left).