We present a new investigation of the comet-asteroid transition
object 133P/(7968) Elst-Pizarro. We find mean optical colors
(B - V = 0.69 ± 0.02;
V - R = 0.42 ± 0.03; R - I = 0.27 ± 0.03) and a phase darkening coefficient
(b = 0.044 ± 0.007 mag deg-1) that are comparable both to other
comet nuclei and C-type asteroids. As in 1996, when this object's
comet-like activity was first noted, data from 2002 show a long, narrow
dust trail in the projected orbit of the object. Observations over
several months reveal changes in the structure and brightness of this
trail, showing that it is actively generated over long periods of time.
Finson-Probstein modeling is used to constrain the parameters of the dust
trail. We find optically dominant dust particle sizes of ad ~ 10 µm
released with low ejection velocities (vg 1.5 m s-1) over a
period of activity lasting at least 5 months in 2002. The double-peaked
lightcurve of the nucleus indicates an aspherical shape (axis ratio
a/b 1.45 ± 0.07) and rapid rotation (period
Prot= 3.471 ± 0.001 hr). The practical identification of
133P/Elst-Pizarro as a comet (i.e. a mass-losing body) is not in doubt,
but the origin of the mass loss is unclear. The 1996 trail has been
previously explained as debris released by a chance impact, but our
discovery of recurrent activity renders this interpretation implausible.
We consider two hypotheses for the activity in 133P/Elst-Pizarro. The
ejection of particles is naturally explained if the object is a barely
active Jupiter family comet that has evolved into an asteroid-like orbit,
perhaps under the prolonged action of non-gravitational forces due to
asymmetrical mass loss. In this case, the orbital similarity to the
Themis family must be considered coincidental. Alternatively,
133P/Elst-Pizarro could be a true member of the Themis family on which
buried ice has been recently excavated by impact.
Subject headings: comets: individual (133P/Elst-Pizarro)
— comets: general — minor
planets, asteroids — solar system: general
1.5 m s-1) over a
period of activity lasting at least 5 months in 2002. The double-peaked
lightcurve of the nucleus indicates an aspherical shape (axis ratio
a/b 
1.45 ± 0.07) and rapid rotation (period
Prot= 3.471 ± 0.001 hr). The practical identification of
133P/Elst-Pizarro as a comet (i.e. a mass-losing body) is not in doubt,
but the origin of the mass loss is unclear. The 1996 trail has been
previously explained as debris released by a chance impact, but our
discovery of recurrent activity renders this interpretation implausible.
We consider two hypotheses for the activity in 133P/Elst-Pizarro. The
ejection of particles is naturally explained if the object is a barely
active Jupiter family comet that has evolved into an asteroid-like orbit,
perhaps under the prolonged action of non-gravitational forces due to
asymmetrical mass loss. In this case, the orbital similarity to the
Themis family must be considered coincidental. Alternatively,
133P/Elst-Pizarro could be a true member of the Themis family on which
buried ice has been recently excavated by impact.