The Strange Case of 133P/Elst-Pizarro: A Comet among the Asteroids

Abstract

We present a new investigation of the comet-asteroid transition object 133P/(7968) Elst-Pizarro. We find mean optical colors (BÀV = 0.69 AE 0.02, VÀR = 0.42 AE 0.03, RÀI = 0.27 AE 0.03) and a phase-darkening coefficient ( = 0.044 AE 0.007 mag deg À1 ) that are comparable both to other comet nuclei and to 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 a d $ 10 m released with low ejection velocities (v g % 1.5 m s À1 ) over a period of activity lasting at least 5 months in 2002. The doublepeaked light curve of the nucleus indicates an aspherical shape (axis ratio a/b ! 1.45 AE 0.07) and rapid rotation (period P rot = 3.471 AE 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 nongravitational forces due to asymmetric 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.

The Main Belt Comets and ice in the Solar System

“…Ejection velocities for dust emission from many MBCs tend to be relatively low compared to ejection velocities for classical comets, yet they are often comparable to the escape velocities of the corresponding nuclei (cf. Hsieh et al 2004). This competition between size preferences may be a highly consequential effect, meaning that there may be an 'ideal' size or size range for detectable MBCs where they are large enough to produce observable amounts of gas or dust emission but small enough that ejected dust can actually escape into space to become observable in the first place.…”

Rotational Properties of the Extraordinary Multi-tailed Asteroid P/2013 P5

“…As a result, comets contain a large amount of volatile material. Hsieh et al (2004) states that observationally, comets have a gravitationally unbound atmosphere called a coma and an accompanying tail, while asteroids do not. This is usually the result of near-surface ice sublimation where the comet is turned on and off as it goes through perihelion and aphelion, closest approach to the sun and furthest approach from the sun.…”

“…When it was initially studied in 1996, the tail was deemed the result of a chance impact, and thereby predicted to extinguish. However, this conclusion was disproved when the comet-like activity was observed again by Hsieh et al in 2004. Hsieh et al (2004 studied the asteroid for several months and observed changes in the structure and brightness of the dust tail.…”

Main-Belt Comets as Tracers of Ice in the Inner Solar System

“…However, observations of renewed activity in and 2007(Hsieh et al 2004Lowry & Fitzsimmons 2005) rendered that explanation highly implausible. While recurrent activity (with intervening periods of inactivity) is commonly observed for classical comets where sublimation drives activity, it would be highly improbable for impacts to occur on the same object on three separate occasions in just 11 years, each time at a similar location in the object's orbit, when impacts are not seen to occur at anywhere near that frequency or regularity on other main-belt asteroids.…”

“…All of the currently known MBCs were discovered with telescopes smaller than 2 m in size, except for 176P which was discovered by the 8 m Gemini North telescope. Main-belt asteroids with sufficient ice on their surfaces to drive cometary activity are unexpected, and so it has been hypothesized that MBCs may have instead preserved ice in subsurface layers over the age of the solar system, and that that ice has only recently become exposed, perhaps via excavation from an impact by another asteroid (Hsieh et al 2004;Capria et al 2012).…”