Planetary and exoplanetary Astronomy Lunch Seminar (PALS) for 2023-05-01

Series: Planetary and exoplanetary Astronomy Lunch Seminar (PALS)
Date: Monday 01-May-2023
Time: 11:15-12:15
Location: ATL 1250
Speaker: Esteban Wright
Title: Ricochets and Stranding Boulders on Rubble Pile Asteroids

Abstract: Many asteroids are ‘rubble piles’, conglomerations of weakly gravitationally bounded material with a wide span of grain sizes present on their surfaces. Impacts, spin-out events, and seismic activity can produce a population of gravitational bound objects in orbit that can impact the surface at low velocity and a distribution of impact angles. First, we present low velocity, oblique impacts of spherical projectiles into several monodisperse granular substrates. With high speed video and fluorescent markers on the projectiles we track projectile trajectory and spin during impact. We estimate constraints on projectile radius, velocity, and impact angle that would allow projectiles on asteroids to ricochet or roll away from the initial impact site. We perform experiments of ricocheting impactors, with fixed impact velocity and angle, into granular media with different mean grain size. We find that hydro-static-like, drag-like and lift-like forces, used in empirical impact force laws are sensitive to mean grain size, with the lift coefficient being the most sensitive. Second, we consider how a subsurface seismic pulse affects the surface of an asteroid distant from the impact site. Using dry polydisperse gravel mixture, we film and track the ejecta launched by a single upward propagating pressure pulse. We find ejecta trajectories are independent of particle size, and collisions primarily take place upon landing, where smaller particles are scattered off larger particles, a process dubbed ‘ballistic sorting’. These projects illustrate that surface size distribution should be considered in future impact models on granular asteroid surfaces and that a single strong pulse is an additional mechanism for stranding previously buried boulders on the surface.

For further information contact PALS coordinators Quanzhi Ye at and Tad Komacek at


Special accommodations for individuals with disabilities can be made by calling (301) 405-3001. It would be appreciated if we are notified at least one week in advance.


Directions and information about parking can be found here.

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