Designer asteroids: Target interior effects on momentum transfer by slow (<1km/s) kinetic impactors.

Spacecraft observations (Thomas et al., 2001; Fujiwara et al., 2006; Barnouin et al., 2019; Sugita et al., 2019; Bierhaus et al., 2023; Barnouin et al., 2024) combined with theoretical models (Zhang et al., 2022) indicate that asteroids likely possess a wide range of internal structures. In many cases, asteroids may possess some degree of internal coherence or strength, while their uppermost surfaces are likely cohesionless (Arakawa et al., 2020; Barnouin et al., 2024; Lauretta et al., 2022).

Figure 1. Example of monolithic large boulders on Itokawa, Eros and Ryugu.

The presence of large boulders (Figure 1) on many asteroids (Thomas et al., 2001; Fujiwara et al., 2006; Barnouin et al., 2019; Sugita et al., 2019) with diameters between 50 and 100 m, suggests that some asteroids may be coherent monoliths. Objects of this size would be capable of causing substantial damage in the event of an Earth impact. The rapid spin rates observed for some small asteroids (Holsapple, 2007) also support the possibility of significant internal strength. Asteroids in this size range generally fall within the strength-dominated regime of commonly used Q* disruption scaling relationships.

To develop effective planetary defense strategies against such objects, it is important to understand the likely outcomes of deflection attempts involving asteroids with plausibly strong interior structures. Here, we investigate the effects of impacts into a variety of coherent targets with differing size, strength, porosity, and internal structure. This study is one of two investigations examining momentum transfer by kinetic impactor, focused on experiments conducted at the JHUAPL Impact Lab (IL) low (<0.5 km/s) impact velocities. A complementary investigation examines hypervelocity impacts (>1.0 km/s) conducted at NASA Ames.

In the IL, we fire ¼” Alumina projectiles into a range of well understood near-spherical targets that possess diameters of 6 to 10 cm  The targets are hung from a string, like a ballistic pendulum (Figure 2). They are made of either uniform plasters of varying strength and porosity, or a mixture of these same plasters with aquarium gravel, or large porous lava rocks. The targets have well characterized porosity and strength properties; variations in their interior are understood using X-ray computed tomography (XCT) scans.

Figure 2. Example of shots at the JHUAPL Impact Laboratory (IL) used to measure momentum transfer.

During each experiment, we use two high speed (>1000f/s) cameras to measure the impact location and angle of the projectile, and directly measure the linear and angular displacement of the target as it swings following impact. The displacements provide estimates of the velocity changes experienced by the target, and its post-impact momentum. We then estimate the fraction of the momentum imparted by the projectile to the target. 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 excavated during each impact.

Our preliminary findings indicate that impact angle is a major factor controlling the effectiveness of a kinetic impact in displacing an asteroid during low-speed impacts. More normal impacts produce greater displacement and result in more efficient transfer and enhancement of projectile momentum. Target porosity appears to be the next most important factor, as its presence tends to suppress ejecta production and spallation.

Although still under investigation, variations in target strength, which differ by as much as a factor of four in our experiments, may be among the least important factors influencing momentum transfer. In contrast, internal target structure, including the presence of cavities or strength heterogeneities, can have significant effects and lead to less predictable outcomes at the slower impact velocities considered here.

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