Astéroïde

Résumé

vignette|upright=1.4|L'astéroïde de la ceinture principale (243) Ida ainsi que sa lune Dactyle. Dactyle est la première lune astéroïdale à avoir été découverte. vignette|upright=1.4|L’astéroïde géocroiseur (101955) Bénou photographié par la sonde OSIRIS-REx en décembre 2018. vignette|upright=1.4|Modélisation basée sur des images radar de l'astéroïde géocroiseur (4769) Castalie. Il s'agit d'un corps binaire à contact. Un astéroïde (du grec ancien , « qui ressemble à une étoile ») est une planète mineure composée de roches, de métaux et de glaces, et dont les dimensions varient de l'ordre du mètre (limite actuelle de détection) à plusieurs centaines de kilomètres. L'appellation « en forme d'étoile » vient de l'aspect irrégulier des astéroïdes au télescope, différent du disque parfait des planètes, lors des premières observations astronomiques. En 1801, le premier astéroïde découvert est nommé Cérès. C'est le plus gros astéroïde de la ceinture principale, groupe d'astéroïdes le plus imp

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Asteroid deflection entails multiple sources of epistemic uncertainties and stochastic uncertainties. Epistemic uncertainties can be reduced by replenishing incomplete information with a better means of observation, whereas stochastic uncertainties are inherent and irreducible owing to their randomness. Our understanding of asteroid deflection is largely limited by lack of in-situ data or full-scale experiments. Reducing these epistemic uncertainties in (1) the physical properties of asteroids or (2) the consequences of human interference will drastically benefit our mitigation efforts against their hazards. Much of previous literature concentrated on single-stage missions or simple asteroid models with fixed physical attributes, making it inadequate to handle epistemic or stochastic uncertainties. To fill this gap, this paper tries to incorporate different kinds of uncertainties within a campaign design framework. In a multi-stage campaign, a precursor is deployed first to reduce epistemic uncertainties. Because stochastic uncertainties cannot be reduced by nature, the campaign should be optimized to maximize its robustness against the worst random situations. Finally, whether or not a two-stage campaign is better than a single-stage mission can be visualized as a decision map for decision-makers. The paper analyzes a hypothetical scenario of deflecting 99942 Apophis and discusses future work.

Ieee2016

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Unique large diamonds in a ureilite from Almahata Sitta 2008 TC3 asteroid

Philippe Gillet

The Almahata Sitta MS-170 ureilite (a piece of a breccia originating from the asteroid, 2008 TC3) consists mainly of olivine, with many diamond and graphite grains existing between the olivine grains. The occurrences of the diamonds are unique; i.e., (i) some diamonds exhibit sub-euhedral habits and (ii) some diamonds have large grain-size (up to about 40 mu m). Several diamonds are segmented into many fragments by fractures. Individual fragments have similar crystallographic orientation, which implies that the adjacent diamond segments were originally a single crystal. Large diamond assemblages occur besides such individual diamond grains. In one of the largest assemblages (almost about 100 mu m in size) has also the same crystallographic orientation. They can be regarded as the pieces of a previously unique single diamond, which provides evidence for large single-crystals diamond in meteorites. Almahata Sitta MS-170 is a meteorite fragment from the 2008 TC3 asteroid that underwent less shock than other ureilitic meteorites. It is unlikely that such large diamonds were formed from graphite through a shock-induced phase transformation during planetesimal collision, despite this idea being now widely accepted as the diamond formation mechanism of ureilites. Fine-scale heterogeneous distribution of impurities (hydrogen, nitrogen, and oxygen) exists in single crystal diamonds, indicative of sluggish growth. This distribution is reminiscent of sector zoning growth. Its grain size, the shock features of MS-170, and the C- and N-isotopic composition signatures allow us to revive classical and but not widely accepted models for diamond formation in ureilites; i.e., a diamond formed from partially melted magma or a C-O-H fluid in the deep interior of the ureilite parent-body or, alternatively, through a chemical vapor deposition (CVD) process in the solar nebula. Considering present mineralogical and isotopic features, the former scenario is more favorable. (C) 2015 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2015

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Samples of the Jilin H5 chondrite were experimentally shock-loaded at the peak pressures of 12, 27, 39, 53, 78, 83, 93, and 133 GPa. The aim of this study is to compare experimentally shock-induced phenomena with those in naturally shocked chondrites and to test the feasibility of experimentally calibrating naturally induced shock phenomena in Hand L-chondrites. Planar fractures, mosaicism, brecciation in olivine and pyroxene, as well as transformation of plagioclase into diaplectic glass were observed in the Jilin samples shocked at pressures lower than 53 GPa. Shock-induced chondritic melts were first obtained at P > 78 GPa and more than 60% of the whole-rock melting was achieved at P similar to 133 GPa, and that shook-induced silicate melt consists of quenched microcrystalline olivine and pyroxene, metal, troilite and vesicular glass. No high-pressure phases were observed in any of the experimentally shocked samples, neither in the deformed nor in the molten regions. Deformation features in Jilin samples shock-loaded below 53 GPa are comparable to those found in H- and L-chondrites. The mineral assemblages in the molten regions in the shocked Jilin samples are also comparable to those encountered in the heavily shocked Yanzhuang (H6) and some Antarctic H-chondrites, but differ considerably from those found in heavily shocked Sixiangkou and many other L6 chondrites. Shock melt veins in L6 chondrites contain high-pressure polymorphs of olivine, pyroxene, plagioclase and high-pressure liquidus phases, whereas shock melt veins in heavily shocked H-chondrites contain mainly low-pressure mineral assemblages. The differences in the mineral constituents of shock melt veins in L- and H-chondrites clearly indicate differences in the shock histories of these meteorites. While crystallization in the shook melt veins in L-chondrites took place at high pressures, crystallization in shock-induced melt in most H-chondrites took place after decompression. It is evident that the thickness and abundance of shock melt veins and size of melt regions is not necessarily a quantitative measure of the degree of shock. The duration of the high-pressure regime, the time of the cooling and the P-T regime during the crystallization path, and the post-shock temperatures are stringent parameters that control the evolution of the shock-induced melt. So, scaling from shock experiments on millimeter-sized samples to natural shock features on kilometer-sized asteroids poses considerable problems in quantifying the P-T conditions during natural shock events on asteroids. (C) 2001 Elsevier Science B.V. All rights reserved.

2001

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Unique large diamonds in a ureilite from Almahata Sitta 2008 TC3 asteroid

Philippe Gillet

Pergamon-Elsevier Science Ltd2015

2001