Where do you need to impact an asteroid to effectively deflect its orbit?

Recently, NASA’s Double Asteroid Redirection Test (DART) spacecraft crashed into a 170m Dimorphos asteroid at 6.6km/s, as the first in-orbit demonstration of asteroid deflection by kinetic impact. The DART spacecraft was to touch the center of Dimorphos almost frontally. Ground-based telescopes have now confirmed that the impact succeeded in altering Dimorphos’ orbit period by 32 minutes, far more than expected.

But, where should we impact an asteroid to most effectively deflect its orbit? Just towards the center of the asteroid? These issues still need to be fully investigated.

In a new article published in the Orientation, Control and Dynamics Journal, researchers from Tsinghua University have proposed an optimal kinetic impact geometry to improve the effective magnitude of kinetic impact deflection, which should promote our understanding of how to fully utilize a kinetic impactor and get the best results. They found that a well-designed off-center impact seemed to increase deflection efficiency by more than 50% compared to the central impact used by the DART mission.

Their work shows a surprising result: regardless of the target asteroid’s aspect or impact location/direction, the asteroid’s velocity change is always located on a single hodograph, which is not determined than by the material properties of the asteroid. They call it the Delta-v hodograph.

“The Delta-v hodograph is actually a distortion of an ideal spherical surface. And determining a realistic profile requires quantifying the effect of angles of impact on the efficiency of momentum transfer, which can be calculated from hydrodynamic simulations of oblique impacts,” the researchers explained. .

With numerical simulations, the unique hodograph is obtained and applied to optimize the location or direction of impact, taking into account both the orbital geometry and the shapes of the asteroid. For near-spherical Bennu-like asteroids, the desired momentum is sensitive to impact locations, and the optimal impact geometry deviates from the normal central impact towards the forward side in some orbital geometries, especially for larger orbital geometry angles.

Effective deflection is increased by up to 50% when hitting the optimal location compared to a single center impact.

For elongated Itokawa-type asteroids, the direction of impact determines the Delta-v distribution defined on the hodograph and, therefore, the optimal rotational phase at impact. A preferable direction may even increase the final deflection distance by 100% over the value of an unsuitable direction.

“This work presents a simple and innovative application of the Delta-v hodograph to the design of optimal geometry in the kinetic deviations of PHAs, as a link between hyper-velocity impact and long-term orbital dynamics”, the researchers said. “The proposed optimization method should be considered in the overall trajectory design of future asteroid deflection missions, where the expected deflection distance is determined not only by the orbital geometry but also by the impact geometry. .”