RESUMO
An asteroid's history is determined in large part by its strength against collisions with other objects1,2 (impact strength). Laboratory experiments on centimetre-scale meteorites3 have been extrapolated and buttressed with numerical simulations to derive the impact strength at the asteroid scale4,5. In situ evidence of impacts on boulders on airless planetary bodies has come from Apollo lunar samples6 and images of the asteroid (25143) Itokawa7. It has not yet been possible, however, to assess directly the impact strength, and thus the absolute surface age, of the boulders that constitute the building blocks of a rubble-pile asteroid. Here we report an analysis of the size and depth of craters observed on boulders on the asteroid (101955) Bennu. We show that the impact strength of metre-sized boulders is 0.44 to 1.7 megapascals, which is low compared to that of solid terrestrial materials. We infer that Bennu's metre-sized boulders record its history of impact by millimetre- to centimetre-scale objects in near-Earth space. We conclude that this population of near-Earth impactors has a size frequency distribution similar to that of metre-scale bolides and originates from the asteroidal population. Our results indicate that Bennu has been dynamically decoupled from the main asteroid belt for 1.75 ± 0.75 million years.
RESUMO
NASA'S Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) spacecraft recently arrived at the near-Earth asteroid (101955) Bennu, a primitive body that represents the objects that may have brought prebiotic molecules and volatiles such as water to Earth1. Bennu is a low-albedo B-type asteroid2 that has been linked to organic-rich hydrated carbonaceous chondrites3. Such meteorites are altered by ejection from their parent body and contaminated by atmospheric entry and terrestrial microbes. Therefore, the primary mission objective is to return a sample of Bennu to Earth that is pristine-that is, not affected by these processes4. The OSIRIS-REx spacecraft carries a sophisticated suite of instruments to characterize Bennu's global properties, support the selection of a sampling site and document that site at a sub-centimetre scale5-11. Here we consider early OSIRIS-REx observations of Bennu to understand how the asteroid's properties compare to pre-encounter expectations and to assess the prospects for sample return. The bulk composition of Bennu appears to be hydrated and volatile-rich, as expected. However, in contrast to pre-encounter modelling of Bennu's thermal inertia12 and radar polarization ratios13-which indicated a generally smooth surface covered by centimetre-scale particles-resolved imaging reveals an unexpected surficial diversity. The albedo, texture, particle size and roughness are beyond the spacecraft design specifications. On the basis of our pre-encounter knowledge, we developed a sampling strategy to target 50-metre-diameter patches of loose regolith with grain sizes smaller than two centimetres4. We observe only a small number of apparently hazard-free regions, of the order of 5 to 20 metres in extent, the sampling of which poses a substantial challenge to mission success.
Assuntos
Meio Ambiente Extraterreno/química , Planetas Menores , Voo Espacial , Exobiologia , Origem da Vida , Voo Espacial/instrumentação , Propriedades de SuperfícieRESUMO
Vascular injuries during or after total hip replacements have often been reported. In 2257 artificial hip joint replacements, we observed 8 cases of vascular injuries requiring reconstruction (3 cases), suturing (2 cases), end-to-end anastomosis (1 case), or ligature (2 cases) performed by a vascular surgeon. A separate skin incision for vascular repair was necessary in all patients. Most of the injuries occurred as a result of technical errors through lack of caution when using bone hooks or rough handling, resulting in dislocation of the hip joint. The latter occurs mainly in revision procedures. Six patients recovered fully, but two died, one of hemorrhagic shock and the other of cardiac decompensation.
Assuntos
Prótese de Quadril , Complicações Intraoperatórias/cirurgia , Músculo Liso Vascular/lesões , Idoso , Idoso de 80 Anos ou mais , Anastomose Cirúrgica , Feminino , Artéria Femoral/lesões , Artéria Femoral/cirurgia , Veia Femoral/lesões , Veia Femoral/cirurgia , Hemorragia/cirurgia , Humanos , Músculo Liso Vascular/cirurgiaRESUMO
The bony refill of the distraction-space in leg-lengthening operations is analysed by X-ray control and histological technic as well in animal experiments as clinical patients. Based on our observations a theory of the formation of bone is postulated: We figured out, that a) presumable besides the periosteal potentials of bone formation there are also other active mechanisms of bone growth to be found. b) frequently histological recognised formations of bones cannot yet be proved by X-ray, that means, that the X-ray changes follow the histological ones. c) optimal formation of callus needs most possible stability during the time of consolidation. d) most careful distraction prevents deformities.
Assuntos
Alongamento Ósseo/métodos , Modelos Animais de Doenças , Adolescente , Adulto , Animais , Regeneração Óssea , Calo Ósseo , Feminino , Membro Posterior/diagnóstico por imagem , Humanos , Desigualdade de Membros Inferiores/cirurgia , Masculino , Radiografia , OvinosRESUMO
The author reports on 175 cases of leg lengthening operations for leg equalisation by means of Wagners and Andersons procedure. None of the occuring complications prevented the operation being successful. So the methods offer an useful alternative to shortening osteotomies and epiphysiodesis.