Resurfacing of Dimorphos in the Antipodal Hemisphere of the DART impact.
Authors: Liu Po-Yen , Campo Bagatin Adriano , Benavidez Paula Gabriela , Richardson Derek Charles 
Published: September 2022

&lt;p&gt;&lt;strong&gt;Introduction&lt;/strong&gt;: Scheduled to be performed in September 2022, the DART (Double Asteroid Redirection Test, NASA) mission will be the first-ever space mission to demonstrate asteroid deflection by crashing an impactor spacecraft into a binary NEA target: the moon of (65803) Didymos, called Dimorphos. In the frame of the studies related to the EC-H2020 NEO-MAPP project, we perform numerical simulations of the collision event on Dimorphos without the presence of Didymos by using the N-body numerical code, PKDGRAV [1] with an implementation of the soft-sphere discrete element method [2], under the assumption that it is a spherical gravitational aggregate produced in the formation of the binary system [3, 4]. The very structure of the target is unknown; therefore, we model it by (a) 100550 mono-dispersed spherical particles in a close hexagonal packing (hereafter mono-CHP) configuration, with particle radii of 1.5 m; (b) multi-dispersed distribution of 100000 spherical particles in a random packing (hereafter multi-RP) configuration, with particle radii ranging from 1 to 2.5 m.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Method&lt;/strong&gt;: The real DART impact should be a hyper-speed cratering event where most impact kinetic energy goes into the shattering phase including asteroid local material damage like vaporization, melting, rock deformation, heat transfer, and so on. However, due to the fact that PKDGRAV is not able to simulate the shattering phase, we instead concentrate on the effect of the collision on the target, once the shattering phase is over. Therefore, our synthetic projectile needs to deliver the same nominal linear momentum to Dimorphos as the DART spacecraft will do, but it delivers to the target only a small fraction of kinetic energy expected to survive once the shattering phase has dissipated most of the impact kinetic energy (non-elastic collision). According to the cratering experiments [5], only a small fraction of impact energy will go into the kinetic energy of the target after the shattering phase. This residual kinetic energy is about 0.25% of the initial impact energy.&lt;/p&gt;
&lt;p&gt;For the multi-RP case, we also account for different centre and off-centre impact geometry compatible with DART nominal impact angle (20&amp;#186;) with respect to the target orbital plane, considering the possibility that the real DART spacecraft may not impact exactly toward the centre of Dimorphos.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Results&lt;/strong&gt;: From all of our cases, we find that none of the cases can transmit the impact kinetic energy to the other side of the impact point. However, particle movements in the antipodal hemisphere of the impact are possible even without impact energy reaching there because the aggregate tries to reach a new equilibrium shape after the relatively large crater and deformation were generated by the DART impact. As a result, the surface particles in the antipodal hemisphere move toward the crater, causing the particles to sink into the ground.&amp;#160;&lt;br /&gt;In the antipodal hemisphere, we found all the particles have negative vertical displacements typically around -1 m and can be up to around -2 m. The center of the surface depression is very close to a longitude and latitude of 0&amp;#176; and 20&amp;#176; because the center of the crater is at 180&amp;#176; and 20&amp;#176; longitude and latitude, which is the antipodal point.&lt;br /&gt;&lt;br /&gt;The surface particle movements have both vertical and horizontal components. We found many particles in the northern region have horizontal displacements larger than 1 m and can be up to 3.2 m. It takes tens of minutes or a few hours for the particles to settle down and reach their final locations. On the other hand, at about 3 mins after the impact, LICIACube will be able to take images on the antipodal side of the impact, with a resolution of about 1.5 m. None of the surface particles can have horizontal displacements larger than 1.5 m at that time, therefore LICIACube will be essentially taking images of the original locations of the boulders on the antipodal side of the impact.&lt;br /&gt;&lt;br /&gt;We therefore predict that by comparing the LICIACube and Hera observations, it is possible to obtain the real horizontal displacements of the boulders. The exact displacements caused by this resurfacing process depend on several internal material parameters and also on the level of deformation of Dimorphos by the real impact, and thus the observation may give information about the internal structure of Dimorphos.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;:&lt;/p&gt;
&lt;p&gt;[1] D. C. Richardson et al. (2000). Icarus, 143, 45.&lt;/p&gt;
&lt;p&gt;[2] S. R. Schwartz et al. (2012) Granular Matter 14, 363.&lt;/p&gt;
&lt;p&gt;[3] A. Campo Bagatin et al. (2001) Icarus, 149-1, 198.&amp;#160;&lt;/p&gt;
&lt;p&gt;[4] A. Campo Bagatin et al. (2018) Icarus, Volume 302, 343.&lt;/p&gt;
&lt;p&gt;[5] D. W. Walker (2013) Int. J. of Impact Engin. 56, 12.&lt;br /&gt;&lt;br /&gt;&lt;/p&gt;