  Many small bodies in the solar system are believed to be rubble piles, a
collection of smaller elements separated by voids. We propose a model for the
structure of a self-gravitating rubble pile. Static friction prevents its
elements from sliding relative to each other. Stresses are concentrated around
points of contact between individual elements. The effective dimensionless
rigidity, $\tilde\mu_{rubble}$, is related to that of a monolithic body of
similar composition and size, $\tilde\mu$ by $\tilde \mu_{rubble} \sim \tilde
\mu^{1/2} \epsilon_Y^{-1/2}$, where $\epsilon_Y \sim 10^{-2}$ is the yield
strain. This represents a reduction in effective rigidity below the maximum
radius, $R_{max}\sim [\mu\epsilon_Y/(G\rho^2)]^{1/2}\sim 10^3\km$, at which a
rubble pile can exist. Densities derived for binary near-Earth asteroids imply
that they are rubble piles. As a consequence, their tidal evolution proceeds
$10^3$ to $10^4$ times faster than it would if they were monoliths. This
accounts for both the sizes of their semimajor axes and their small orbital
eccentricities. We show that our model for the rigidity of rubble piles is
compatible with laboratory experiment in sand.
