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Microgravity Experiments on Accretion
in the Protoplanetary Disk

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


Observations of mass transfer in COLLIDE and PRIME were made during flight-based experimentation, which allowed us to make our observations for extended periods of time in a microgravity environment as well as in a vacuum. To achieve a microgravity environment in ground-based trials, we used a laboratory drop tower: a 3.7-m tall apparatus from which we could drop an experimental payload. The drop tower allows for ~0.7 seconds of a microgravity environment in free fall during which we could conduct experiments. This setup does not provide sufficient time to observe the full extent of an impact and rebound collision event at the low impact speeds necessary for mass transfer. Therefore, we designed an experimental chamber to simulate only the rebound of a projectile. Figure 3 provides a graphic representation of the experimental apparatus. We suspended a marble from a spring and let it rest in a bed of granular material prior to dropping it in the drop tower. We carried out our experiments using JSC-1 lunar regolith simulant (McKay et al. 1994) and quartz sand (Table 1).

Table 1: Granular Materials

Figure 3: Schematic of the experimental apparatus, consisting of
a marble suspended by a spring in a bed of granular material.
The spring pendulum system is encased in a polycarbonate box
with a wooden frame and an attached video camera.

In a 1-g environment, the weight of the marble caused the spring to stretch away from equilibrium while the marble remained in contact with the bed of granular material. When the payload was in free fall, the spring retracted, lifting the marble out of the bed of granular material. This setup effectively simulated the rebound of a collision similar to those observed in flight-based experiments. In prior flight-based experiments, collisions caused the projectile to compress the granular material at the point of contact before it rebounded. To control for this variable, spring lengths were chosen such that a minimal amount of weight was supported by the granular material in 1-g. The weight of the marble supported by the granular material in 1-g is given in Table 2. The granular material was poured gently into the container before each trial, and the marble was placed onto the top layer as lightly as possible.

We attached a GoPro Hero 3+ camera to the apparatus as shown in Figure 3, which provided video at 240 frames per second with a 420p resolution for each trial. We tracked the frames from these videos using the program ImageJ (Rasband 1997-2016), an open source image processing package, to measure the rebound acceleration of the spring system. To enhance the visual contrast between the marble and granular material for tracking purposes, we coated the steel marble with white Teflon spray during the JSC-1 trials, and we tinted the quartz marble with black ink for the quartz sand trials. We tracked the motion of the marble in each video multiple times to confirm consistency in our measurements. We were able to qualitatively determine the presence or absence of mass transfer in each trial.

In addition, we varied the acceleration of the marble away from the target surface by using marbles (1.9 cm diameter) with variable masses and springs with different spring constants and lengths as shown in Tables 2 and 3.

Table 2: Marble Materials and Masses

Table 3: Spring Constants and Lengths

We chose combinations of spring constants and marble masses to allow for sufficiently slow rebound velocities and thereby maximize the opportunity for mass transfer to occur. We performed calculations with the equations of elastic potential energy and kinetic energy to determine these parameters. In equations (1-4), PE is potential energy, k is spring constant, x is the displacement of the spring from equilibrium, KE is kinetic energy, m is marble mass, v is velocity, and Etotal is total energy:

The velocity of the marble after the spring has returned to its equilibrium position is given by:

As soon as the apparatus is dropped, the marble begins accelerating away from the surface of the granular material with an initial speed of zero. In the longduration microgravity experiments of COLLIDE-3 and PRIME-3 (Brisset et al. 2016), there was sufficient time for the marble to strike the target and rebound at lower speed, potentially carrying some accreted target material with it. The spring mechanism used in this experiment results in the marble accelerating away from the target surface instead of moving away at a constant rebound velocity. Therefore, these laboratory-based results cannot be directly compared to long-duration flight experiments. Instead of characterizing the rebound event by the marble velocity, we measure the acceleration of the marble away from the surface. We then characterize the acceleration of the rebounding projectiles in the flight-based experiments using a simple impulse approximation. To measure the acceleration of the marble, we first track the change in the marble's position. From the change in the marble's position over time we can measure the velocity as a function of time and calculate the acceleration from the rate of change of the velocity. We measured accelerations between approximately 1 and 6 m/s2. For comparison, we estimate the acceleration of the projectile in COLLIDE (Figure 1) to be 1.3 m/s2 by simply taking the rebound velocity minus the impact velocity divided by the time of contact with the target material.

Results >>