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.

Context. Near-Earth asteroid (25143) Itokawa was visited by the Hayabusa spacecraft in 2005, resulting in a highly detailed shape and surface topography model. This model has led to several predictions for the expected radiative torques on this asteroid, suggesting that its spin rate should be decelerating. Aims. To detect changes in rotation rate that may be due to YORP-induced radiative torques, which in turn may be used to investigate the interior structure of the asteroid. Methods. Through an observational survey spanning 2001 to 2013 we obtained rotational lightcurve data at various times over the last five close Earth-approaches of the asteroid. We applied a polyhedron-shape-modelling technique to assess the spin-state of the asteroid and its long term evolution. We also applied a detailed thermophysical analysis to the shape model determined from the Hayabusa spacecraft. Results. We have successfully measured an acceleration in Itokawa's spin rate of dω/dt = (3.54 ± 0.38) × 10 −8 rad day −2 , equivalent to a decrease of its rotation period of ∼45 ms year −1 . From the thermophysical analysis we find that the centre-of-mass for Itokawa must be shifted by ∼21 m along the long-axis of the asteroid to reconcile the observed YORP strength with theory. Conclusions. This can be explained if Itokawa is composed of two separate bodies with very different bulk densities of 1750 ± 110 kg m −3 and 2850 ± 500 kg m −3 , and was formed from the merger of two separate bodies, either in the aftermath of a catastrophic disruption of a larger differentiated body, or from the collapse of a binary system. We therefore demonstrate that an observational measurement of radiative torques, when combined with a detailed shape model, can provide insight into the interior structure of an asteroid. Futhermore, this is the first measurement of density inhomogeneity within an asteroidal body, that reveals significant internal structure variation. A specialised spacecraft is normally required for this.

Asteroid (175706) 1996 FG3 is a binary asteroid and the baseline target for the proposed MarcoPolo‐R sample return mission. We present thermal‐infrared photometry obtained with the European Southern Observatory (ESO) Very Large Telescope using the VISIR instrument, together with optical photometry obtained with the ESO New Technology Telescope using the EFOSC2 instrument. An absolute visual magnitude HV= 17.833 ± 0.024 and phase parameter G=−0.041 ± 0.005 are derived. The near‐Earth asteroid thermal model has been fitted to the measured fluxes to derive a geometric visual albedo pv= 0.046 ± 0.014, effective diameter at the observed aspect Deff= 1.68 ± 0.25 km and beaming parameter η= 1.15 for phase angle α= 117. The advanced thermophysical model (ATPM) has been fitted to the measured fluxes to derive a more accurate effective diameter Deff= 1.71 ± 0.07 km and albedo pv= 0.044 ± 0.004. Based on the ATPM results, assuming the same albedo for primary and secondary, we derive a primary mean spherical diameter Dp= 1.69+0.18− 0.12 km, secondary diameter Ds= 0.51 ± 0.03 km and a secondary orbital semimajor axis a= 2.8+1.7−0.7 km. A low surface thermal inertia Γ= 120 ± 50 J m−2 s−1/2 K−1 was also derived, suggesting a dusty surface and raising questions as to the binary formation mechanism of this asteroid. These physical properties are used to predict a Yarkovsky drift in semimajor axis of −60+31−45 m yr−1.

Context. Comet 67P/Churyumov-Gerasimenko is the target of the European Space Agency Rosetta spacecraft rendez-vous mission. Detailed physical characteristation of the comet before arrival is important for mission planning as well as providing a test bed for ground-based observing and data-analysis methods. Aims. To conduct a long-term observational programme to characterize the physical properties of the nucleus of the comet, via ground-based optical photometry, and to combine our new data with all available nucleus data from the literature. Methods. We applied aperture photometry techniques on our imaging data and combined the extracted rotational lightcurves with data from the literature. Optical lightcurve inversion techniques were applied to constrain the spin state of the nucleus and its broad shape. We performed a detailed surface thermal analysis with the shape model and optical photometry by incorporating both into the new Advanced Thermophysical Model (ATPM), along with all available Spitzer 8-24 µm thermal-IR flux measurements from the literature. Results. A convex triangular-facet shape model was determined with axial ratios b/a = 1.239 and c/a = 0.819. These values can vary by as much as 7% in each axis and still result in a statistically significant fit to the observational data. Our best spin state solution has P sid = 12.76137 ± 0.00006 h, and a rotational pole orientated at Ecliptic coordinates λ = 78The nucleus phase darkening behaviour was measured and best characterized using the IAU HG system. Best fit parameters are: G = 0.11 ± 0.12 and H R(1,1,0) = 15.31 ± 0.07. Our shape model combined with the ATPM can satisfactorily reconcile all optical and thermal-IR data, with the fit to the Spitzer 24 µm data taken in February 2004 being exceptionally good. We derive a range of mutually-consistent physical parameters for each thermal-IR data set, including effective radius, geometric albedo, surface thermal inertia and roughness fraction. Conclusions. The overall nucleus dimensions are well constrained and strongly imply a broad nucleus shape more akin to comet 9P/Tempel 1, rather than the highly elongated or "bi-lobed" nuclei seen for comets 103P/Hartley 2 or 8P/Tuttle. The derived low thermal inertia of <15 J m −2 K −1 s −1/2 is comparable with that measured for other comets scaled to similar heliocentric distances, and implies a surface regolith finer than lunar surface material.