The newly discovered asteroid 2002 AA29 moves in a very Earth-like orbit that relative to Earth has a unique horseshoe shape and allows transitions to a quasi-satellite state. This is the first body known to be in a simple heliocentric horseshoe orbit, moving along its parent planet's orbit. It is similarly also the first true co-orbital object of Earth, since other asteroids in 1:1 resonance with Earth have orbits very dissimilar from that of our planet. When a quasi-satellite, it remains within 0.2 AU of the Earth for several decades. 2002 AA29 is the first asteroid known to exhibit this behavior. 2002 AA29 introduces an important new class of objects offering potential targets for space missions and clues to asteroid orbit transfer evolution.

The study of chondrules provides information about processes occuring in the early solar system. In order to ascertain to what extent these processes played a role in determining the properties of the enstatite chondrites, the physical and chemical properties of chondrules from three EL3 chondrites and three EH3 chondrites have been examined by optical, cathodoluminescence (CL), and electron microprobe techniques. Properties examined include size, texture, CL, and composition of both individual phases and bulk chondrules. The textures, distribution of textures, and composition of silicates of the EL3 chondrules resemble those of EH3 chondrules. However, the chondrules from the two classes differ in that (1) the size distribution of the EL chondrules is skewed to larger values than EH chondrules, (2), the enstatite in EL chondrules displays varying shades of red CL due to the presence of fine-grained sulfides and meetal in the silicates, and (3) the mesostatis of EH chondrules is enriched in Na relative to that of EL chondrules. The similarities between the chondrules of the two classes suggest similar precursor materials, while the differences suggest that there was not a single reservoir of meteoritic chondrules, but that their origin was fairly local. The differences in the size distribution of chondrules in EH and EL chondrites may be explained by aerodynamic and gravitational sorting during accumulation of the meteoritic material, while differences in CL and mesostasis properties may reflect differences in formation conditions and cooling rate following chondrule formation. We argue that our observations are consistent with the formation of enstatite chondrites in a thick dynamic regolith on their parent body.

Cosmic-ray exposure (CRE) ages and Mars ejection times were calculated from the radionuclide 81Kr and stable Kr isotopes for seven martian meteorites. The following 81Kr-Kr CRE ages were obtained: Los Angeles = 3.35 +/- 0.70 Ma; Queen Alexandra Range 94201 = 2.22 +/- 0.35 MA; Shergotty = 3.05 +/- 0.50 Ma; Zagami = 2.98 +/- 0.30 Ma; Nakhla = 10.8 +/- 0.8 Ma; Chassigny = 10.6 +/- 2.0 Ma; and Allan Hills 84001 = 15.4 +/- 5.0 Ma. Comparison of these age with previously obtained CRE ages from the stable noble gas nuclei 3He, 21Ne, and 38Ar shows excellent agreement. This indcates that the method for the production rate calculation for the stable nuclei is reliable. in all martian meteorites we observe effects induced by secondary cosmic-ray produced epithermal neutrons. Epithermal neutron fluxes, phi(n) (30-300 cV), are calculated based on the reaction 79Br(n, gamma x Beta)80Kr. We show that the neutron capture effects were induced in free space during Mars-Earth transfer of the meteorids and that they are not due to a pre-exposure on Mars before ejection of the meteoritic material. Neutron fluxes and slowing down densities experienced by the meteoroids are calculated and pre-atmospheric sizes are estimated. We obtain minimum radii in the range of 22-25 cm and minimum masses of 150-220 kg. These results are in good agreement with the mean sizes reported for model calculations using current semiempirical data.

We measured the sizes and textural types of 719 intact chondrules and 1322 chondrule fragments in thin sections of Semarkona (LL3.0), Bishunpur (LL3.1), Krymka (LL3.1), Piancaldoli (LL3.4) and Lewis Cliff 88175 (LL3.8). The mean apparent diameter of chondrules in these LL3 chondrites is 0.80 phi units or 570 micrometers, much smaller than the previous rough estimate of ~900 micrometers. Chondrule fragments in the five LL3 chondrites have a mean apparent cross-section of 1.60 phi units or 330 micrometers. The smallest fragments are isolated olivine and pyroxene grains; these are probably phenocrysts liberated from disrupted porphyritic chondrules. All five LL3 chondrites have fragment/chondrule number ratios exceeding unity, suggesting that substantial numbers of the chondrules in these rocks were shattered. Most fragmentation probably occurred on the parent asteroid. Porphyritic chondrules (porphyritic olivine + porphyritic pyroxene + porphyritic olivine-pyroxene) are more readily broken than droplet chondrules (barred olivine + radial pyroxene + cryptocrystalline). The porphyritic fragment/chondrule number ratio (2.0) appreciably exceeds that of droplet-textured objects (0.9). Intact droplet chondrules have a larger mean size than intact porphyritic chondrules, implying that large porphyritic chondrules are fragmented preferentially. This is consistent with the relatively low percentage of porphyritic chondrules within the set of the largest chondrules (57%) compared to that within the set of the smallest chondrules (81%). Differences in mean size among chondrule textural types may be due mainly to parent-body chondrule-fragmentation events and not to chondrule-formation processes in the solar nebula.

Natural calcium monoaluminate, CaA12O4, has been found in a grossite-rich calcium-aluminum-rich inclusion (CAI) from the CH chondrite Northwest Africa 470. The calcium monoaluminate occurs as colorless ~10 micrometer subhedral grains intergrown with grossite, perovskite, and melilite. Nebular condensation is the most likely origin for the precursor matrials of this CAI, but calculations suggest that dust/gas ratios substantially enhanced over solar are required to stabilize CaA12O4.

Asteroid and comet impacts on Earth are commonly viewed as agents of ecosystem destruction, be it on local or global scales. However, for some microbial communities, impacts may represent an opportunity for habitat formation as some substrates are rendered more suitable for colonization when processed by impacts. We describe how heavily shocked gneissic crystalline basement rocks exposed at the Haughton impact structure, Devon Island, Nunavut, Arctic Canada, are hosts to endolithic photosynthetic microorganisms in significantly greater abundance than lesser-shocked or unshocked gneisses. Two factors contribute to this enhancement: (a) increased porosity due to impact fracturing and differential mineral vaporization, and (b) increased translucence due to the selective vaporization of opaque mineral phases. Using biological ultraviolet radiation dosimetry, and by measuring the concentrations of photoprotective compounds, we demonstrate that a covering of 0.8 mm of shocked gneiss can provide substantial protection from ultraviolet radiation, reducing the inactivation of Bacillus subtilis spores by 2 orders of magnitude. The colonisation of the shocked habitat represents a potential mechanism for pioneer microorganisms to invade an impact structure in the earliest stages of post-impact primary succession. The communities are analogous to the endolithic communities associated with sedimentary rocks in Antarctica, but because they occur in shocked crystalline rocks, they illustrate a mechanism for the creation of microbial habitats on planetary surfaces that do not have exposed sedimentary units. This might have been the case on early Earth. The data have implications for the microhabitats in which biological signatures might be sought on Mars.