Dynamical Evolution of Simulated Particles Ejected From Asteroid Bennu

Abstract

In early 2019, the OSIRIS‐REx spacecraft discovered small particles being ejected from the surface of the near‐Earth asteroid Bennu.sww Although they were seen to be ejected at slow speeds, on the order of tens of cm/s, a number of particles were surprisingly seen to orbit for multiple revolutions and days, which requires a dynamical mechanism to quickly and substantially modify the orbit to prevent re‐impact upon their first periapse passage. This paper demonstrates that, based on simulations constrained by the conditions of the observed events, the combined effects of gravity, solar radiation pressure, and thermal radiation pressure from Bennu can produce many sustained orbits for ejected particles. Furthermore, the simulated populations exhibit two interesting phenomena that could play an important role in the geophysical evolution of bodies such as Bennu. First, small particles (<1 cm radius) are preferentially removed from the system, which could lead to a deficit of such particles on the surface. Second, re‐impacting particles preferentially land near or on the equatorial bulge of Bennu. Over time, this can lead to crater in‐filling and growth of the equatorial radius without requiring landslides.

Heterogeneous mass distribution of the rubble-pile asteroid (101955) Bennu

“…Many of the ejected particles were placed into temporary orbits about Bennu before reimpacting the surface or escaping from the asteroid. These dynamics were due to the combined interactions between the gravitational attractions and nongravitational forces acting on the particles dominated by direct and reflected SRP (28). These particles were observed by the spacecraft navigation cameras and their orbits fit by combining current knowledge of the spacecraft location with observation geometry.…”

Particle Ejection Contributions to the Rotational Acceleration and Orbit Evolution of Asteroid (101955) Bennu

“…Their ejection speeds range from near 0 to above 3 m/s. Across this range of speeds, we find very different final outcomes, which are studied in detail in McMahon et al (). These can be classified as immediate reimpact, eventual reimpact, eventual escape, or immediate escape.…”

“…Ignoring these competing effects for the moment and assuming that most of this population reaccumulates on Bennu, a random reimpacting of the surface would suggest that the upper tens of centimeters of Bennu's surface would be homogenized in 10 Myr. However, if there is an equatorial preference for particle reaccumulation, as described by McMahon et al (), then we would expect that tens of centimeters of material from the midlatitudes and poles would be ballistically redistributed to the equator (which occupies roughly half of Bennu's total surface area), suggesting an accumulation of small particles ( 5 cm) in these regions. There is little direct visual evidence for such a latitudinal size sorting of particles on Bennu (Walsh et al, ); thus, other geologic processes such as radial size sorting or impact excavation may have dominated over this process and depleted the equator of this fine layer of particles.…”

“…In the case of mass movement, this could potentially either increase or decrease. As it is expected that particles preferably reimpact and are trapped in the equatorial region (McMahon et al, ), this effect should tend to increase and hence decrease the spin rate assuming a fixed angular momentum. The overall change in due to a particle migration is where is the horizontal radius, as measured from the spin axis.…”

“…Let us assume that the particles are uniformly ejected from the asteroid surface and deposited onto the equator, which is seen as the most likely region for impact based on detailed numerical propagation of particles (McMahon et al, ). In the following, we just use a simple spherical moment of inertia to develop the main, qualitative effects of such a migration.…”