  We report the discovery of a comet-like tail on asteroid (3200) Phaethon when
imaged at optical wavelengths near perihelion. In both 2009 and 2012, the tail
appears >=350" (2.5x10^8 m) in length and extends approximately in the
projected anti-solar direction. We interpret the tail as being caused by dust
particles accelerated by solar radiation pressure. The sudden appearance and
the morphology of the tail indicate that the dust particles are small, with an
effective radius ~1 micrometer and a combined mass ~3x10^5 kg. These particles
are likely products of thermal fracture and/or desiccation cracking under the
very high surface temperatures (~1000 K) experienced by Phaethon at perihelion.
The existence of the tail confirms earlier inferences about activity in this
body based on the detection of anomalous brightening. Phaethon, the presumed
source of the Geminid meteoroids, is still active.
