NASA’s Double Asteroid Redirection Test (DART) spacecraft will crash into asteroid Dimorphos on September 26, executing the first asteroid deflection test that has been years in the planning.
Dimorphos, 150 meters wide, is the “moon” of a binary asteroid system, orbiting the larger companion asteroid, Didymos (800 meters). The thrust of the ~600 kg spacecraft, traveling at ~6 km/s, will provide a small change in velocity to Dimorphos, which will be detectable from ground-based telescopes as a change in the orbital period of the asteroid system .
As part of this mission, Lawrence Livermore National Laboratory (LLNL) researchers have been bringing multiphysics simulation expertise to this planetary defense technology demonstration mission since 2014, developing new methods to simulate the range of potential targets of asteroids and model the DART spacecraft with greater fidelity.
A new paper in the Planetary Science Journal, “Spacecraft Geometry Effects on Kinetic Impactor Missions,” led by LLNL’s Mike Owen, explores the consequences of including realistic spacecraft geometries in multiphysics simulations.
Previously, most impact modelers considered idealized shapes for the DART spacecraft, such as a sphere, cube, or disc. The use of detailed computer-aided design (CAD) models provided by spacecraft engineers was not a readily available capability for many impact codes. Owen worked to streamline the process in Spheral, an LLNL-based Adaptive Soft Particle Hydrodynamics (ASPH) code for which he created and serves as lead developer. Collaborators from the United States and internationally also worked to implement CAD-based DART geometries, providing code comparisons for both detailed and simplified spacecraft geometries as part of the study.
“Over the years, many researchers have worked hard to study how kinetic impactors like DART might perform if we were to deflect an asteroid, using both numerical models and laboratory experiments,” Owen said. “Almost all of this research focuses on the effects of how different properties of the asteroid itself might affect the outcome, but of all the unknowns in these scenarios, probably the factor we know the most about is the spacecraft itself, which is generally approximated using a simple solid geometry such as a solid cube or sphere”.
Owen said that now that a full-scale live experiment is underway on the DART mission, it makes sense to look at how important the actual geometry of the spacecraft that was launched could be, especially given the difference that the spacecraft has compared to typical simplifications.
“These realistic models are very difficult to set up and run, and we had to develop new capabilities in our modeling tools to be able to address this problem,” he added.
The same DART spacecraft impact can result in very different craters on Dimorphos depending on the material of the asteroid. The crater on the left is the result if Dimorphos is composed of strong rocky material, while the much larger crater shown on the right could occur if Dimorphos is composed of much weaker rubble-like material. Image provided by Mike Owen/LLNL.
The geometry of the DART spacecraft, consisting of a central body the size of a vending machine (1.8 x 1.9 x 2.3 m) and two 8.5 m solar panels, creates a ” footprint’ much larger than a solid aluminum sphere of the same mass. . This affects the cratering process and ultimately the momentum imparted to the asteroid, reducing it by 25%. While this is a measurable effect, uncertainties in asteroid target properties can produce even larger changes in deflection effectiveness.
However, modeling the full CAD geometry typically requires a finer resolution and can be computationally expensive. Owen also explored cylinders of different thicknesses and three-sphere approaches to the problem, to find a “half termite” that was easier to simulate but also behaved more like the real DART spacecraft. A three-sphere model was able to explain most of the effect of using the full spacecraft geometry. This three-sphere simplification allows many more models of DART’s impact to be run accurately, across different codes and users.
“While it may seem intuitive that an idealized spherical representation of DART would overestimate the deflection, quantifying this effect was important to understand the limitations of previous approaches,” said Megan Bruck Syal, LLNL’s Planetary Defense Project Manager. “Conducting this study was an essential component of the preparation for the DART experiment and has redefined best practices for both LLNL and other impact modeling groups.”
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