The Mechanical Properties of Chelyabinsk LL5 Chondrite Under Compression and Tension

The mechanical properties of Chelyabinsk LL5 chondrite (Chelyabinsk meteorite) were studied by uniaxial compression and diametral compression/indirect tension test. Twenty cylindrical samples, 10 for compression and 10 for tension, with the diameter 3.3 mm and 1.65 mm in height have been prepared for testing. It was shown that the strength of the tested samples under compression almost 45 times greater than it is at tension: 372 ± 10 MPa and 8.2 ± 0.7 MPa, respectively. Fracture behaviour under compression and tension was similar and can be characterised as brittle. The obtained compression strength of the Chelyabinsk meteorite lies close to the maximal values of strength for many other chondrites, whereas its tensile strength magnitude resides in the bottom quarter of the range of similar measurements. It may be caused by the small sizes of the investigated samples together with a large number of tiny cracks between the grains in the Chelyabinsk chondrite. Our estimations have shown that if one assumes that the initial shape of the Chelyabinsk fireball was spherical or ellipsoidal, then its fragmentation stress is close to the experimental tensile strength and much lower than the compression strength. Hence, a stress state equivalent to one appearing at the indirect tension test could occur in the Chelyabinsk fireball during its fall in the Earth atmosphere.

URAL

The article shows the monitoring results of the Ural region seismic network in 2013. It describes the seismic stations and registration abilities of the network. The analysis of seismic activity in Ural in 2013 and infor-mation about changes of the regional seismic regime since 2006 are given. The seismicity in the Ural is unique as it is presented by a wide spectrum of natural earthquakes (tectonic, earthquakes due to collapse, impact) as well as induced earthquakes (explosions, rock falls, rockbursts). Whereby the number of explosions in the region predominates among other seismic events, the number of rockbursts is much more than tectonic earth-quakes. A structural ordering can be seen for tectonic earthquakes. They tend to the basic geologic structure of the region to the Main Ural Fault. The induced events tend to mining regions. Also, there is a weak scat-tered seismicity that is typical for platform territories. Acting since 1999 the regional seismic network pro-vides the representative registration on the magnitude level ML≥2.5. In general in 2013 in Ural it was regis-tered 173 seismic events, and their basic seismic parameters were determined. The common number of in-dustrial explosions was 173. The summarized seismic explosions energy was 3.99E+9 Joules. The number of rockbursts was 29; their seismic energy was 2.82E+9 Joules. The five tectonic earthquakes made the min-imal contribution to the seismicity of the region. The unique event registered by seismic network was the ex-plosion of Chelyabinsk meteorite, and its parameters are shown in the article. Parameters of all mentioned above seismic events are presented in catalogue. The strongest events with ML≥3.0 including Chelyabinsk meteorite explosion, are considered separately, including their seismograms and parameters provided by other international seismic centers. The article shows the map with the actual locations of regional seismic stations and event epicenters in 2013. Generally the seismic regime of the region in 2013 was quite calm; the summarized seismic explosions energy was low. The trend to the seismic activity decay continues since 2010. The location of the natural and induced seismic events in space confirms the active zones previously determined.

Mechanical properties of particles from the surface of asteroid 25143 Itokawa

Aims. Asteroids have been exposed to impacts since their formation, and as a consequence their surfaces are covered by small particles, pebbles, and boulders. The Japanese JAXA/ISAS Hayabusa mission collected micron-sized particles from the regolith of asteroid 25143 Itokawa. The study in terrestrial laboratories of these particles provides a scientific opportunity as their physical properties can be compared with those characteristic of chondritic meteorites that are often considered proxies of the building materials of potentially hazardous asteroids (PHAs). Methods. Here we present the results from a study of the mechanical properties of three of these particles using a precise technique called nanoindentation. The derived results are compared with those obtained via a methodology similar to that used for the Chelyabinsk meteorite. Results. The reduced Young’s modulus values obtained for the Itokawa samples are higher than those measured for the Chelyabinsk chondrite, so these specific particles of asteroid regolith are more compacted than the minerals forming the particular LL chondrite associated with PHAs. This might be a natural consequence of particles surviving long exposure times on the surface of a (near-Earth asteroid) NEA. The Double Asteroid Redirection Test (DART) mission plans to excavate a crater in the surface of the (65803) Didymos satellite. Our results suggest that excavating a crater with a kinetic impactor in an area of significant fine-grained regolith will increase the momentum transfer. As this will facilitate the release of particles carrying target mass in the opposite direction to the movement of the projectile, there is no need to grind up the target during the mechanical excavation phase.