  In an effort to better understand the early stages of planet formation, we
have developed a 1.5U payload that flew on the International Space Station
(ISS) in the NanoRacks NanoLab facility between September 2014 and March 2016.
This payload, named NanoRocks, ran a particle collision experiment under
long-term microgravity conditions. The objectives of the experiment were (a) to
observe collisions between mm-sized particles at relative velocities of
$<$1~cm/s, and (b) to study the formation and disruption of particle clusters
for different particle types and collision velocities. Four types of particles
were used: mm-sized acrylic, glass, and copper beads, and 0.75 mm-sized JSC-1
lunar regolith simulant grains. The particles were placed in sample cells
carved out of an aluminum tray. This tray was attached to one side of the
payload casing with three springs. Every 60~s, the tray was agitated and the
resulting collisions between the particles in the sample cells were recorded by
the experiment camera.
  During the 18 months the payload stayed on ISS, we obtained 158 videos, thus
recording a great number of collisions. The average particle velocities in the
sample cells after each shaking event were around 1 cm/s. After shaking
stopped, the inter-particle collisions damped the particle kinetic energy in
less than 20~s, reducing the average particle velocity to below 1 mm/s, and
eventually slowing them to below our detection threshold. As the particle
velocity decreased, we observed the transition from bouncing to sticking
collisions. We recorded the formation of particle clusters at the end of each
experiment run. This paper describes the design and performance of the
NanoRocks ISS payload.
