  We describe the dynamics of a stream of equally spaced macroscopic particles
in orbit around a central body (e.g. a planet or star). A co-orbital
configuration of small bodies may be subject to gravitational instability,
which takes the system to a spreading, disordered and collisional state. We
detail the linear instability's mathematical and physical features using the
shearing sheet model and subsequently track its nonlinear evolution with local
N-body simulations. This model provides a convenient tool with which to
understand the gravitational and collisional dynamics of narrow belts, such as
Saturn's F-ring and the streams of material wrenched from tidally disrupted
bodies. In particular, we study the tendency of these systems to form
long-lived particle aggregates. Finally, we uncover an unexpected connection
between the linear dynamics of the gravitational instability and the
magnetorotational instability.
