  Main belt asteroid (300163) 2006 VW139 (later designated P/2006 VW139) was
discovered to exhibit comet-like activity by the Pan-STARRS1 survey telescope
using automated point-spread-function analyses performed by PS1's Moving Object
Processing System. Deep follow-up observations show both a short (\sim 10")
antisolar dust tail and a longer (\sim 60") dust trail aligned with the
object's orbit plane, similar to the morphology observed for another main-belt
comet, P/2010 R2 (La Sagra), and other well-established comets, implying the
action of a long-lived, sublimation-driven emission event. Photometry showing
the brightness of the near-nucleus coma remaining constant over \sim 30 days
provides further evidence for this object's cometary nature, suggesting it is
in fact a main-belt comet, and not a disrupted asteroid. A spectroscopic search
for CN emission was unsuccessful, though we find an upper limit CN production
rate of Q_CN < 1.3x10^24 mol/s, from which we infer a water production rate of
Q_H2O < 10^26 mol/s. We also find an approximately linear optical spectral
slope of 7.2%/1000A, similar to other cometary dust comae. Numerical
simulations indicate that P/2006 VW139 is dynamically stable for > 100 Myr,
while a search for a potential asteroid family around the object reveals a
cluster of 24 asteroids within a cutoff distance of 68 m/s. At 70 m/s, this
cluster merges with the Themis family, suggesting that it could be similar to
the Beagle family to which another main-belt comet, 133P/Elst-Pizarro, belongs.
