The zodiacal cloud is a thick circumsolar disk of small debris particles produced by asteroid collisions and comets. Their relative contribution and how particles of different sizes dynamically evolve to produce the observed phenomena of light scattering, thermal emission, and meteoroid impacts are unknown. Until now, zodiacal cloud models have been phenomenological in nature, composed of ad-hoc components with properties not understood from basic physical processes. Here, we present a zodiacal cloud model based on the orbital properties and lifetimes of comets and asteroids, and on the dynamical evolution of dust after ejection. The model is quantitatively constrained by IRAS observations of thermal emission, but also qualitatively consistent with other zodiacal cloud observations, with meteor observations, with spacecraft impact experiments, and with properties of recovered micrometeorites. We find that 85-95% of the observed mid-infrared emission is produced by particles from the Jupiter-family comets (JFCs) and <10% by dust from long period comets. The JFC particles that contribute to the observed cross-section area of the zodiacal cloud are typically D ≈ 100 µm in diameter. Asteroidal dust is found to be present at <10%. We suggest that spontaneous disruptions of JFCs, rather than the usual

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.

It is only in the past ten years that advanced computing techniques and painstaking observations have enabled the successful prediction and observation of meteor storms. Spectacular displays of ''shooting stars'' are created when the Earth crosses a meteoroid stream, causing the meteoroids to light up into meteors as they enter our atmosphere. Meteor Showers and Their Parent Comets is a unique handbook for astronomers interested in observing meteor storms and outbursts. The author, a leading astronomer in the field and an active meteor storm chaser, explains how meteoroid streams originate from the decay of comets (and asteroids) and how they evolve into ever changing orbits by the gravitational pull of planets to cause meteor showers on Earth. He includes the findings of recent space missions that have visited comets and asteroids, the risk of meteoroid impacts on Earth, what showers to expect on other planets, and how meteor showers may have seeded the Earth with the ingredients that made life possible. All known meteor showers are identified, accompanied by fascinating details on the most important showers and their parent comets. The book predicts when exceptional meteor showers will occur over the next 50 years, making it a valuable resource for both amateur and professional astronomers.

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.

Selected area electron diffraction is used to monitor structural changes of vapor-deposited water ice in vacuum during warm-up from 15 to 188 K. A progression of three amorphous forms of water ice is found with well-defined transitions. The formation of a high-density amorphous form (Iah) at 15 K is confirmed, and the transition to the more familiar low-density form (Ial) occurs gradually over the range 38 to 68 K. At 131 K, the ice transforms into a third amorphous form (Iar), which precedes the crystallization of cubic ice (Ic) and coexists metastably with Ic from 148 K until at least 188 K. These structural transformations of amorphous water ice can be used to explain hitherto anomalous properties of astrophysical ices. The structural transition from Iah to Ial is responsible for the diffusion and recombination of radicals in ultraviolet-photolyzed interstellar ices at low temperatures. The occurrence and persistence of Iar explains anomalous gas retention and gas release from water-rich ices at temperatures above 150 K.