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Collisions between cm-sized protoplanetary aggregates in microgravity

Akbar Whizin
Asteroid Population Characterization
Delivered As: 
Abstract Text: 

In the early protoplanetary disk the earliest coalescing cm-sized bodies are weakly bound and can shatter easily at the highest possible relative collision velocities in the disk at 1 AU. At these sizes the self-gravity of cm-sized aggregates is almost non-existent and electrostatic surface forces such as van der Waals-type forces play a critical role in holding loosely bound rubble-piles together during their early formation. Our goal is to further understand the mechanical, material, and collisional properties of cm-sized aggregates, in order to determine the outcomes of their collisions at low-velocities. The collisional outcomes can be determined by a set of definable collision parameters and experimental constraints on these parameters will improve disk formation models of planetesimals. We have carried out a series of microgravity laboratory experiments where we collide small silica aggregates into each other in order to determine under what conditions collisional growth can occur. In our free-fall chamber we collide two aggregates together at collision velocities ranging from 50 to 170 cm/s for 3 cm aggregates with pressures ~1 mbar. Centimeter-sized aggregates made up of mm-sized particles are weakly bound and require internal cohesion to avoid fragmentation supplied by adding 0 - 0.1 g of a well-mixed liquid adhesive to simulate surface forces and bonds of the particles. We measure the compressive strengths of the aggregates using an apparatus we built, find their coefficients of restitution (CoR), and determine their bouncing and fragmentation thresholds over the range of velocities and internal strengths. The stronger the aggregate is the more resistant to disruption it becomes and varying the strengths we see outcomes such as, bouncing, erosion (mass-loss), and fragmentation (loss of 50% or more mass). We find the CoR of the aggregates have a mean value of 0.1 with no dependence on velocity or strength. Collisions with high impact parameter (glancing) often resulted in increased post-collision rotation implying a high tangential component of the CoR. Impact velocities above ~2 m/s resulted in fragmentation of our aggregates, higher than the ~1 m/s threshold for dust aggregates of roughly the same size.

Dante S. Lauretta
SSERVI Identifier: 

Recognizing that science and human exploration are mutually enabling, NASA created the Solar System Exploration Research Virtual Institute (SSERVI) to address basic and applied scientific questions fundamental to understanding the Moon, Near Earth Asteroids, the Martian moons Phobos and Deimos, and the near space environments of these target bodies. As a virtual institute, SSERVI funds investigators at a broad range of domestic institutions, bringing them together along with international partners via virtual technology to enable new scientific efforts."