  Porosity is a key characteristic of the regolith on the surface of small
bodies. The porosity of the regolith on the surface of asteroids is changed by
applied pressure, and the relationship between pressure and porosity depends on
the particle properties of the regolith. We performed compression measurements
on samples of different materials, particle size distributions, and shapes to
examine the relationship between particle size and pressure required for
compression. We used a centrifuge and a compression testing machine for the
experiments. The applied pressure for the centrifuge and the compression
testing machine experiments ranged from 10^2 to 5x10^3 Pa and from 10^4 to
5x10^6 Pa, respectively. The initial porosity before compression was generally
higher for samples with smaller particles and narrower particle size
distributions. A sample compressed more easily when it consisted of smaller
particles, probably due to smaller frictional forces between particles. We
estimated the porosity of granular asteroids based on the results of our
experiments. The estimated porosity at the center of a homogeneous asteroid
with a radius of 50 km is 0.43. This porosity is typical of the porosity of
asteroids of similar size.
