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Microgravity Experiments on Accretion
in the Protoplanetary Disk

By: Addison Brown and Stephanie Jarmak | Mentor: Dr. Joshua Colwell

Abstract

We present the results of an experimental investigation of low-energy collisions between cm-scale and smaller particles in the protoplanetary disk to better understand conditions conducive to the growth of planetesimals, the km-sized building blocks of planets. The COLLIDE (Collisions Into Dust Experiment) and PRIME (Physics of Regolith Impacts in Microgravity Experiment) programs involve cm-scale projectiles impacting a target bed of unconsolidated granular material in microgravity environments on Space Shuttle missions and parabolic airplane flights, respectively. In these experiments, a portion of the target material adhered onto the impactor in some lowvelocity impacts (<40 cm/s). Such impact speeds are too slow to study in a normal gravity environment (1 g, where g = 9.8 m/s2 is the acceleration due to gravity on the surface of the Earth). Factors including ambient air pressure, impactor mass and composition, impact velocity, and particle composition may affect the occurrence of mass transfer. We designed and built an apparatus that made use of a laboratory drop tower to obtain a brief (0.7 s) microgravity environment during free fall at standard atmospheric pressure. The laboratory drop tower does not provide enough time to study impact and rebound of a projectile at impact speeds of less than 1 m/s, so we designed our apparatus to simulate only the rebound portion of a collisional event. We suspended a marble from a spring with the marble resting on a bed of granular material in 1-g. During free fall, the spring pulls the marble away from the granular material at a low acceleration, simulating rebound of a projectile. Because the spring continually accelerates the sphere away from the granular material, the rebounds are characterized by an acceleration rather than a rebound velocity. We obtained rebound accelerations from ~1-6 m/s2, and we observed mass transfer at accelerations below 4 m/s2. Mass transfer events such as these indicate one potential growth mechanism for the building blocks of planetesimals.

KEYWORDS: mass transfer; microgravity; low-velocity; collision; acceleration

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