  Planetesimal accretion during planet formation is usually treated as
collisionless. Such accretion from a uniform and dynamically cold disk predicts
protoplanets with slow retrograde rotation. However, if the building blocks of
protoplanets, planetesimals, are small, of order of a meter in size, then they
are likely to collide within the protoplanet's sphere of gravitational
influence, creating a prograde accretion disk around the protoplanet. The
accretion of such a disk results in the formation of protoplanets spinning in
the prograde sense with the maximal spin rate allowed before centrifugal forces
break them apart. As a result of semi-collisional accretion, the final spin of
a planet after giant impacts is not completely random but is biased toward
prograde rotation. The eventual accretion of the remaining planetesimals in the
post giant-impact phase might again be in the semi-collisional regime and
delivers a significant amount of additional prograde angular momentum to the
terrestrial planets. We suggest that in our Solar System, semi-collisional
accretion gave rise to the preference for prograde rotation observed in the
terrestrial planets and perhaps the largest asteroids.
