In the absence of a firm link between individual meteorites and their asteroidal parent bodies, asteroids are typically characterized only by their light reflection properties, and grouped accordingly into classes. On 6 October 2008, a small asteroid was discovered with a flat reflectance spectrum in the 554-995 nm wavelength range, and designated 2008 TC(3) (refs 4-6). It subsequently hit the Earth. Because it exploded at 37 km altitude, no macroscopic fragments were expected to survive. Here we report that a dedicated search along the approach trajectory recovered 47 meteorites, fragments of a single body named Almahata Sitta, with a total mass of 3.95 kg. Analysis of one of these meteorites shows it to be an achondrite, a polymict ureilite, anomalous in its class: ultra-fine-grained and porous, with large carbonaceous grains. The combined asteroid and meteorite reflectance spectra identify the asteroid as F class, now firmly linked to dark carbon-rich anomalous ureilites, a material so fragile it was not previously represented in meteorite collections.

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.

Comets are icy bodies that sublimate and become active when close to the Sun. They are believed to originate in two cold reservoirs beyond the orbit of Neptune: the Kuiper Belt (equilibrium temperatures of approximately 40 kelvin) and the Oort Cloud (approximately 10 kelvin). We present optical data showing the existence of a population of comets originating in a third reservoir: the main asteroid belt. The main-belt comets are unlike the Kuiper Belt and Oort Cloud comets in that they likely formed where they currently reside and may be collisionally activated. The existence of the main-belt comets lends new support to the idea that main-belt objects could be a major source of terrestrial water.

Some asteroids eject dust, producing transient, comet-like comae and tails; these are the active asteroids. The causes of activity in this newly-identified population are many and varied. They include impact ejection and disruption, rotational instabilities, electrostatic repulsion, radiation pressure sweeping, dehydration stresses and thermal fracture, in addition to the sublimation of asteroidal ice. These processes were either unsuspected or thought to lie beyond the realm of observation before the discovery of asteroid activity. Scientific interest in the active asteroids lies in their promise to open new avenues into the direct study of asteroid destruction, the production of interplanetary debris, the abundance of asteroid ice and the origin of terrestrial planet volatiles.

Comet 133P/Elst–Pizarro is the first known and currently best‐characterized member of the main‐belt comets, a recently identified class of objects that exhibit cometary activity but which are dynamically indistinguishable from main‐belt asteroids. We report here on the results of a multiyear monitoring campaign from 2003 to 2008, and present observations of the return of activity in 2007. We find a pattern of activity consistent with seasonal activity modulation. Additionally, recomputation of phase function parameters using data in which 133P was inactive yields new IAU parameters of HR= 15.49 ± 0.05 mag and GR= 0.04 ± 0.05, and linear parameters of mR(1, 1, 0) = 15.80 ± 0.05 mag and β= 0.041 ± 0.005 mag deg−1. The comparison between predicted magnitudes using these new parameters and the comet's actual brightnesses during its 2002 and 2007 active periods reveals the presence of unresolved coma during both episodes, of the order of ∼0.20 of the nucleus cross‐section in 2002 and ∼0.25 in 2007. Multifilter observations during 133P's 2007 active outburst yield mean nucleus colours of B−V= 0.65 ± 0.03, V−R= 0.36 ± 0.01 and R−I= 0.32 ± 0.01, with no indication of significant rotational variation, and similar colours for the trail. Finally, while 133P's trail appears shorter and weaker in 2007 than in 2002, other measures of activity strength such as dust velocity and coma contamination of nucleus photometry are found to remain approximately constant. We attribute changes in trail strength to the timing of observations and projection effects, thus finding no evidence of any substantial decrease in the activity strength between 2002 and 2007.