Designer Asteriods: Investigating the dependence of the catastrophic disruption threshold on target interior properties

Introduction:

In collisional physics, a target body is said to be ‘catastrophically disrupted’ when the largest fragment is equal to half of the original target mass [e.g., Benz & Asphaug 1999]. The specific energy, Q, required to result in that disruption is known as Q*_D. Q*_D is known to be sensitive to the interior properties and strength of an object [Holsapple & Housen 1999, Raducan et al. 2022, Ballouz et al. 2026]. Here, we present results of high-speed impacts at the catastrophic disruption threshold on “designer asteroids”: impact targets with well characterized strength and interior properties. 

Background:

Knowledge of Q*_D is important for planetary defense as it sets a limit at which an earth-bound asteroid may be mitigated through impulsive techniques, such as kinetic impacts [e.g., Daly et al. 2023]. Specifically, kinetic impacts that result in catastrophic disruption may do more harm than good as the resulting fragments may still be sufficiently large as to remain a hazard to Earth. In such a scenario, other mitigation options may become more favorable, such as slow-pull/push mitigation through gravity tractor or ion-beam deflection [OWL 2022].

Methodology:

In this work, we created target materials out of a variety of dental plasters, each with distinct strengths (12 to 117 MPa), densities (1.1 to 2 kg/m³), and porosities (15 to 51%). The target categories included 1) homogeneous targets made of a single type of dental plaster, and 2) “rubble pile” targets made up of a matrix of dental plaster binding loose granular material.

Targets that were 4” and 6” were prepared for impact experiments at NASA’s Ames Vertical Gun Range (AVGR). Projectile sizes of ⅛” to ¼” were launched at the targets at a 15° angle from the horizontal at speeds between 1.72 and 4.65 km/s.

Results:

Figure 1 show images of one our disrupted targets, a rubble pile composed of aquarium gravel with a ‘mounting plaster’ matrix. This 4” target disrupted after a 2.4 km/s impact of ¼” diameter Pyrex projectile.

Figure 1: 4" Rubble Pile (Mounting Plaster with Aquarium Gravel) after disruption from 2.4  km/s impact. Not pictured: additional smaller fragments.

Table 1 summarizes this and the additional experimental outcomes that resulted in disruption: 2 homogeneous mounting plaster targets, one 4” and one 6”, and another 4” rubble pile target with different rubble material.

Table 1: Disrupted Target/Shot Parameters. Largest fragment mass is less than half of the original mass of the target.

Discussion:

Q for impacts were calculated using Equation 1:

Figure 2 shows this Q as a function of target mass for the various targets we impacted, regardless of whether they were disrupted or deflected. Triangular data points with <0.5 fragment mass fraction are those that were decidedly ‘disrupted.’ The disrupted target in each target type typically had the highest Q values. An exception is target RP3, a mounting plaster and lava rock rubble pile mixture. For this same target, an impact with a higher Q value, but a very similar mass did not disrupt. We continue to work towards understanding why this was the case. Plausible culprits that we are exploring include small differences in the internal structure of the target, impact angle relative to the surface normal, and impact velocity.

Figure 2: Q of select AVGR Experimental Data is for the target categories in which disruption was observed. MP = mounting plaster, RP1 = rubble pile made of MP matrix and aquarium gravel, RP3 = rubble pile made of MP matrix and lava rocks.

In Figure 3, we present preliminary analysis to estimate Q*_D for three types of designer asteroids. These cases had sufficient data to determine Q*_D = 1040 ± 208 J/kg (MP), 1006 ± 116 J/kg (RP1), and 2200 ± 413 J/kg (RP3). The scatter in data for a given target type is likely due to the variation in impact speeds (a factor of ~3) across the different cases presented here, as Q*_D is known to depend on impact velocity [e.g., Holsapple & Housen 1999]. The larger value of Q*_D of RP3 (by a factor of ~2) may be due to the large porosity of the embedded lava rocks which increases the resistance of the target to disruption as the interior is able to compact in response to an impact.

Figure 3: Q of select AVGR Experimental Data is for the target categories in which disruption was observed, and sufficient data was collected to estimate Q*_D.

To gain a better understanding of our measurement, we intend to fully characterize the interior structure and strengths of our rubble pile targets, as well as other homogenous targets that were not readily disrupted. We have already measured the density and porosity of the matrix plasters used in our target.  In many instances, we have made x-ray CT scans of target samples before impact, and the fragments generated after. These remain to be analyzed in detail. We have also begun analyzing efforts to characterize the static and dynamic strength of these targets. Through these measurements, we anticipate to get a better understanding of what factor contribute to the disruption of our rubble pile targets. Our findings will be critical for developing the right sort of strategies to deflect asteroids, without disrupting them involuntarily.

Acknowledgements: This project was funded by NASA Grant 80NSSC21K1139 to A. Stickle under the NASA Yearly Opportunities for Research in Planetary Defense (YORPD). We thank Freddy Perez, Chuck Cornelison, and JP Weins at the NASA Ames Vertical Gun Range for supporting these experiments.

References: Benz, W., & Aphaug, E. (1999) Icarus 142, 5-20. Housen, H. & Holsapple, H. (1999) Icarus, Volume 142, Issue 1, pp. 21-33. Raducan, S. D., et al. (2022) Astronomy & Astrophysics, Volume 665, id.L10, 12 pp. Ballouz, R.-L., et al. (2026) The Planetary Science Journal, Volume 7, Issue 4, id.87, 30 pp. Daly, R.T., et al. (2023) Nature 616, 443. National Academies of Sciences, Engineering, and Medicine. (2022). Origins, worlds, and life: a decadal strategy for planetary science and astrobiology 2023-2032.