  We present a study of the newly-discovered main-belt comet P/2008 R1
(Garradd), an object with the dynamical characteristics of an asteroid and the
physical characteristics of a comet. Photometry sets a limit to the effective
radius of the nucleus at r_e < 0.7 km (red geometric albedo 0.05 assumed). The
coma shows a secular fading in our data caused by the escape of dust particles
from the near-nucleus environment. The optical reflection spectrum is a nearly
neutral continuum devoid of gaseous emission lines, from which we derive a
limit to the cyanide (CN) radical production rate of Q_CN <1.4e23/s and infer a
mass loss rate <1.5 kg/s at the time of our observations. Unlike the
first-reported main-belt comets, P/2008 R1 is not dynamically stable. The
nearby 8:3 mean-motion resonance with Jupiter induces dynamical instability on
timescales 20 to 30 Myr. Hence, we conclude that P/2008 R1 has recently arrived
from a more stable source elsewhere. The high Tisserand parameter of the orbit
(in fact, with T_J = 3.216 it is the highest of any comet) points to a source
in the asteroid belt itself, instead of in the Kuiper belt (putative source of
the Jupiter family comets). We infer that P/2008 R1 is an icy body from the
outer asteroid belt in which sublimation has been triggered by rising
temperatures resulting from a decreasing perihelion distance.
