  The size distribution of asteroids and Kuiper belt objects in the solar
system is difficult to reconcile with a bottom-up formation scenario due to the
observed scarcity of objects smaller than $\sim$100 km in size. Instead,
planetesimals appear to form top-down, with large $100-1000$ km bodies forming
from the rapid gravitational collapse of dense clumps of small solid particles.
In this paper we investigate the conditions under which solid particles can
form dense clumps in a protoplanetary disk. We use a hydrodynamic code to model
the interaction between solid particles and the gas inside a shearing box
inside the disk, considering particle sizes from sub-millimeter-sized
chondrules to meter-sized rocks. We find that particles down to millimeter
sizes can form dense particle clouds through the run-away convergence of radial
drift known as the streaming instability. We make a map of the range of
conditions (strength of turbulence, particle mass-loading, disk mass, and
distance to the star) which are prone to producing dense particle clumps.
Finally, we estimate the distribution of collision speeds between mm-sized
particles. We calculate the rate of sticking collisions and obtain a robust
upper limit on the particle growth timescale of $\sim$$10^5$ years. This means
that mm-sized chondrule aggregates can grow on a timescale much smaller than
the disk accretion timescale ($\sim$$10^6 - 10^7$ years). Our results suggest a
pathway from the mm-sized grains found in primitive meteorites to fully formed
asteroids. We speculate that asteroids may form from a positive feedback loop
in which coagualation leads to particle clumping driven by the streaming
instability. This clumping, in turn reduces collision speeds and enhances
coagulation.} Future simulations should model coagulation and the streaming
instability together to explore this feedback loop further.
