  The asteroid (3200) Phaethon is widely recognized as the parent of the
Geminid meteoroid stream. However, it has never shown evidence for on-going
mass loss or for any form of comet-like activity that would indicate the
continued replenishment of the stream. Following an alert by Battams and Watson
(2009), we used NASA's STEREO-A spacecraft to image Phaethon near perihelion,
in the period UT 2009 June 17 - 22, when the heliocentric distance was near
0.14 AU. The resulting photometry shows an unexpected brightening, by a factor
of two, starting UT 2009 June 20.2+/-0.2, which we interpret as an impulsive
release of dust particles from Phaethon. If the density is near 2500 kg/m^3,
then the emitted dust particles must have a combined mass of ~2.5x10^8 a1 kg,
where a1 is the particle radius in millimeters. Assuming a1 = 1, this is
approximately 10^-4 of the Geminid stream mass and to replenish the stream in
steady-state within its estimated ~1000 yr lifetime would require ~10 events
like the one observed, per orbit. Alternatively, on-going mass loss may be
unrelated to the event which produced the Phaethon-Geminid complex. An impact
origin of the dust is highly unlikely. Phaethon is too hot for water ice to
survive, rendering unlikely the possibility that dust is ejected through
gas-drag from sublimated ice. Instead, we suggest that Phaethon is essentially
a rock comet, in which the small perihelion distance leads both to the
production of dust (through thermal fracture and decomposition-cracking of
hydrated minerals) and to its ejection into interplanetary space (through
radiation pressure sweeping and other effects).
