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 have for the first time calculated the population characteristics of the Earth's irregular natural satellites (NES) that are temporarily captured from the near-Earth-object (NEO) population. The steady-state NES size-frequency and residence-time distributions were determined under the dynamical influence of all the massive bodies in the solar system (but mainly the Sun, Earth, and Moon) for NEOs of negligible mass. To this end, we compute the NES capture probability from the NEO population as a function of the latter's heliocentric orbital elements and combine those results with the current best estimates for the NEO size-frequency and orbital distribution. At any given time there should be at least one NES of 1-meter diameter orbiting the Earth. The average temporarily-captured orbiter (TCO; an object that makes at least one revolution around the Earth in a co-rotating coordinate system) completes $(2.88\pm0.82)\rev$ around the Earth during a capture event that lasts $(286\pm18)\days$. We find a small preference for capture events starting in either January or July. Our results are consistent with the single known natural TCO, 2006 RH$_{120}$, a few-meter diameter object that was captured for about a year starting in June 2006. We estimate that about 0.1% of all meteors impacting the Earth were TCOs.Comment: 63 pages, 29 figures. Accepted for publication in Icarus (December 13, 2011

No abstract

An explosion on comet 17P/Holmes occurred on 2007 Oct 23, projecting particulate debris of a wide range of sizes into the interplanetary medium. We observed the comet using the midinfrared spectrograph (5-40 µm), on 2007 Nov 10 and 2008 Feb 27, and the imaging photometer (24 and 70 µm), on 2008 Mar 13, on board the Spitzer Space Telescope. The 2007 Nov 10 spectral mapping revealed spatially diffuse emission with detailed mineralogical features, primarily from small crystalline olivine grains. The 2008 Feb 27 spectra, and the central core of the 2007 Nov 10 spectral map, reveal nearly featureless spectra, due to much larger grains that were ejected from the nucleus more slowly. Optical images were obtained on multiple dates spanning 2007 Oct 27 to 2008 Mar 10 at the Holloway Comet Observatory and 1.5-m telescope at Palomar Observatory. The images and spectra can be segmented into three components: (1) a hemispherical shell fully 28 on the sky in 2008 Mar, due to the fastest (262 m s −1 ), smallest (2 µm) debris, with a mass 1.7×10 12 g; (2) a 'blob' or 'pseudonucleus' offset from the true nucleus and subtending some 10 on the sky, due to intermediate speed (93 m s −1 ) and size (8 µm) particles, with a total mass 2.7 × 10 12 g; and (3) a 'core' centered on the nucleus due to slower (9 m s −1 ), larger (200 µm) ejecta, with a total mass 3.9 × 10 12 g. This decomposition of the mid-infrared observations can also explain the temporal evolution of the mm-wave flux. The orientation of the leading edge of the ejecta shell and the ejecta 'blob,' relative to the nucleus, do not change as the orientation of the Sun changes; instead, the configuration was imprinted by the orientation of the initial explosion. The distribution and speed of ejecta implies an explosion in a conical pattern directed approximately in the solar direction on the date of explosion. The kinetic energy of the ejecta > 10 21 erg is greater than the gravitational binding energy of the nucleus. We model the explosion as being due to crystallization and release of volatiles from interior amorphous ice within a subsurface cavity; once the pressure in the cavity exceeded the surface strength, the material above the cavity was propelled from the comet. The size of the cavity and the tensile strength of the upper layer of the nucleus are constrained by the observed properties of the ejecta; tensile strengths on > 10 m scale must be greater than 10 kPa (or else the ejecta energy exceeds the binding energy of the nucleus) and they are plausibly 200 kPa. The appearance of the 2007 outburst is similar to that witnessed in 1892, but the 1892 explosion was less energetic by a factor of about 20.

Impact investigations will be an important aspect of the InSight mission. One of the scientific goals of the mission is a measurement of the current impact rate at Mars.

We surveyed 23 comets using the Infrared Array Camera on the Spitzer Space Telescope in wide filters centered at 3.6 and 4.5 µm. Emission in the 3.6 µm filter arises from sunlight scattered by dust grains; the 3.6 µm images generally have a coma near the nucleus and a tail in the antisolar direction due to dust grains swept back by solar radiation pressure. The 4.5 µm filter contains scattered sunlight by, and thermal emission from, the same dust grains, as well as strong emission lines from CO 2 and CO gas. The 4.5 µm images are often much brighter than could be explained by dust grains, and they show sometimes distinct morphologies, in which cases we infer they are dominated by gas. Based on the ratio of 4.5 to 3.6 µm brightness, we classify the survey comets as CO 2 +CO 'rich' and 'poor'. This classification is correlated with previous classifications by A'Hearn based on carbon-chain molecule abundance, in the sense that comets classified as 'depleted' in carbon-chain molecules are also 'poor' in CO 2 +CO. The gas emission in the IRAC 4.5 µm images is characterized by a smooth morphology, typically a fan in the sunward hemisphere with a radial profile that varies approximately as the inverse of projected distance from the nucleus, as would apply for constant production and free expansion. There are very significant radial and azimuthal enhancements in many of the comets, and these are often distinct between the gas and dust, indicating that ejection of solid material may be driven either by H 2 O or CO 2 . Notable features in the images include the following. There is a prominent loop of gas emission from 103P/Hartley 2, offset toward the sunward direction; the loop could be due to an outburst of 1 CO 2 before the Spitzer image. Prominent, double jets are present in the image of 88P/Howell, with one directed nearly toward the Sun and the other closer to the terminator (but still on the daytime hemisphere). A prominent single jet is evident for C/2002 T7 (LINEAR), 22P/Kopff and 81P/Wild 2. Spirals are apparent in 29P/Schwassmann-Wachmann 1 and C/2006 W3 (Christensen); we measure a rotation rate of 21 hr for the latter comet. Arcs (possibly parts of a spiral) are apparent in the images of 10P/Tempel 2, and 2P/Encke.

Abstract.Observations of meteor showers allow us to constrain several cometary parameter and to retrieve useful parameters on cometary dust grains, for instance the dust size distribution index s. In this first paper, we describe a new model to compute the time and level of a meteor shower whose parent body is a known periodic comet. The aim of our work was to use all the available knowledge on cometary dust to avoid most of the "a priori" hypotheses of previous meteoroid stream models. The ejection velocity is based on a hydrodynamic model. Because of the large amount of particles released by the comet, it is impossible to compute the orbits of all of them. Instead, we link each computed particle with the real number of meteoroids ejected in the same conditions, through a "dirty snowball" cometary model calibrated with the [A f ρ] parameter. We used a massive numerical integration for all the particles without hypotheses about size distribution. The time of maximum is evaluated from the position of the nodes of impacting meteoroids. The model allows us to compute ephemerides of meteors showers and the spatial density of meteors streams, from which a ZHR can be estimated. At the end a fit of our predictions with observations allows us to compute the dust size distribution index. We used 2002 and 2003 leonid meteor showers to illustrate our method. The application of our model to the Leonid meteor shower from 1833 to 2100 is given in Paper II.

Abstract-This article explores what the recovery of 2008 TC 3 in the form of the Almahata Sitta meteorites may tell us about the source region of ureilites in the main asteroid belt. An investigation is made into what is known about asteroids with roughly the same spectroscopic signature as 2008 TC 3 . A population of low-inclination near-Earth asteroids is identified with spectra similar to 2008 TC 3 . Five asteroid families in the Main Belt, as well as a population of ungrouped asteroids scattered in the inner and central belts, are identified as possible source regions for this near-Earth population and 2008 TC 3 . Three of the families are ruled out on dynamical and spectroscopic grounds. New near-infrared spectra of 142 Polana and 1726 Hoffmeister, lead objects in the two other families, also show a poor match to Almahata Sitta. Thus, there are no Main Belt spectral analogs to Almahata Sitta currently known. Space weathering effects on ureilitic materials have not been investigated, so that it is unclear how the spectrum of the Main Belt progenitor may look different from the spectra of 2008 TC 3 and the Almahata Sitta meteorites. Dynamical arguments are discussed, as well as ureilite petrogenesis and parent body evolution models, but these considerations do not conclusively point to a source region either, other than that 2008 TC 3 probably originated in the inner asteroid belt.