  We study the efficiency of forming large bodies, starting from a sea of
equal-sized planetesimals. This is likely one of the earlier steps of planet
formation and relevant for the formation of the asteroid belt, the Kuiper belt
and extra-solar debris disks. Here we consider the case that the seed
planetesimals do not collide frequently enough for dynamical collisional to be
important (the collisionless limit), using a newly constructed conglomeration
code, and by carefully comparing numerical results with analytical scalings. In
the absence of collisional cooling, as large bodies grow by accreting small
bodies, the velocity dispersion of the small bodies ($u$) is increasingly
excited. Growth passes from the well-known run-away stage (when $u$ is higher
than the big bodies' hill velocity) to the newly discovered trans-hill stage
(when $u$ and big bodies both grow, but $u$ remains at the big bodies' hill
velocity). We find, concurring with the analytical understandings developed in
Lithwick (2014), as well as previous numerical studies, that a size spectrum
$dn/dR \propto R^{-4}$ results, and that the formation efficiency, defined as
mass fraction in bodies much larger than the initial size, is $\sim {\mathrm{ a
\,\, few}}\times R_\odot/a$, or $\sim 10^{-3}$ at the distance of the Kuiper
belt. We argue that this extreme inefficiency invalidates the conventional
conglomeration model for the formation of both our Kuiper belt and extra-solar
debris disks. New theories, possibly involving direct gravitational collapse,
or strong collisional cooling of small planetesimals, are required.
