ACTIVITY IN GEMINID PARENT (3200) PHAETHON

The asteroid (3200) Phaethon is widely recognized as the parent of the Geminid meteoroid stream. However, it has never shown evidence for ongoing mass loss or for any form of comet-like activity that would indicate the continued replenishment of the stream. Following an alert by Battams & Watson, 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 similar to 2.5 x 10(8)a(1) kg, where a(1) is the particle radius in millimeters. Assuming a(1) = 1, this is approximately 10(-4) of the Geminid stream mass and to replenish the stream in steady state within its estimated similar to 10(3) yr lifetime would require similar to 10 events like the one observed, per orbit. Alternatively, ongoing 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 the possibility that dust is ejected through gas drag from sublimated ice unlikely. 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).