Deep Impact? On 15 February 2013, the Russian district of Chelyabinsk, with a population of more than 1 million, suffered the impact and atmospheric explosion of a 20-meter-wide asteroid—the largest impact on Earth by an asteroid since 1908. Popova et al. (p. 1069 , published online 7 November; see the Perspective by Chapman ) provide a comprehensive description of this event and of the body that caused it, including detailed information on the asteroid orbit and atmospheric trajectory, damage assessment, and meteorite recovery and characterization.

Abstract-The Novato L6 chondrite fragmental breccia fell in California on 17 October 2012, and was recovered after the Cameras for Allsky Meteor Surveillance (CAMS) project determined the meteor's trajectory between 95 and 46 km altitude. The final fragmentation from 42 to 22 km altitude was exceptionally well documented by digital photographs. The first sample was recovered before rain hit the area. First results from a consortium study of the meteorite's characterization, cosmogenic and radiogenic nuclides, origin, and conditions of the fall are presented. Some meteorites did not retain fusion crust and show evidence of spallation. Before entry, the meteoroid was 35 AE 5 cm in diameter (mass 80 AE 35 kg) with a cosmic-ray exposure age of 9 AE 1 Ma, if it had a one-stage exposure history. A two-stage exposure history is more likely, with lower shielding in the last few Ma. Thermoluminescence data suggest a collision event within the last $ 0.1 Ma. Novato probably belonged to the class of shocked L chondrites that have a common shock age of 470 Ma, based on the U,Th-He age of 420 AE 220 Ma. The measured orbits of Novato, Jesenice, and Innisfree are consistent with a proposed origin of these shocked L chondrites in the Gefion asteroid family, perhaps directly via the 5:2 mean-motion resonance with Jupiter. Novato experienced a stronger compaction than did other L6 chondrites of shockstage S4. Despite this, a freshly broken surface shows a wide range of organic compounds.

A model of the Oort cloud has been developed by accounting for planetary, stellar and Galactic perturbations using numerical symplectic integrations covering 4.5 Gyr. The model is consistent with the broad dynamical characteristics of the observed cometary populations injected from the Oort cloud into different regions of the Solar system. We show that the majority of observed high-eccentricity trans-Neptunian objects, Centaurs and short-period comets have visited the Oort cloud (a > 1000 au) during their dynamical history. Assuming from observations that the near-parabolic flux from the Oort cloud with absolute magnitudes H 10 < 7, perihelion distances q < 5 au and a > 10 4 au is approximately 1 comet per year, our calculations imply a present Oort cloud population of ∼5 × 10 11 comets with H 10 < 10.9. Roughly half this number have a > 10 4 au. The number of comets reaching the planetary region from the Oort cloud (a > 1000 au) is more than an order of magnitude higher per unit perihelion distance immediately beyond Neptune than in the observable zone q < 5 au. Similarly, the new-comet flux from the Oort cloud per unit perihelion distance is a few tens of times higher in the near-Neptune region than in the observable zone. The present number of high-eccentricity trans-Neptunian objects (q > 30 au and 60 < a < 1000 au) originating from the Oort cloud is in the approximate range 1-3 × 10 10 , depending on details of the initial model. A substantial fraction of these have a > 200 au and/or q > 40 au, and they are found mostly to originate from initial orbits with 25 < q < 36 au. Similarly, the number of Centaurs produced from the Oort cloud, where we define Centaurs to have 5 < q < 28 au and a < 1000 au, is smaller by a factor of 20-30. About 90 per cent of these Centaurs have a > 60 au. Objects that have visited the Oort cloud represent a substantial fraction of the Jupiter-family comet population, achieving short-period orbits by a process of gradual dynamical transfer, including a Centaur stage, from the outer Solar system to near-Earth space. A similar mechanism produces Halley-type comets, in addition to the well-known diffusion process operating at small perihelion distances.

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