6,672 research outputs found

    Impact and collisional processes in the solar system

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    The mechanical and thermodynamic aspects of shock impact cratering and accretionary processes on solid planets and satellites are being investigated experimentally. The recently proposed model of Melosh, describing the physics controlling the size and velocity of only lightly shocked spalled ejecta surrounding the crushed rock region of an impact crater, was studied at the Ames gun facility. Spall velocity measurements using lead and aluminum bullets were conducted. In addition, shock temperatures of silicates and volatile-bearing minerals were measured using radiative techniques. Finally impact devolatilization of minerals and accretion of planetary atmospheres were examined. Measurements of the release isentropes of CaCO3 were carried out. The effect on the water budgets of planets of reactions which occur when metallic iron (which would be present in chondritic material) is introduced into a simple accretion model is under investigation

    Proposed Earth based cratering experiments at low G in hard vacuum

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    In order to address the question of whether the cratering scale which was developed can be extrapolated to low velocity (of planetesimals appropriate for conditions during accretion of the planets and the impact mechanics of encounters of both asteroids and the solid objects which comprise the rings of the outer major planets), a series of experiments at low gravity and at high vacuum are proposed. Specific issues which could be addressed include: the effect of very low gravity on cratering efficiency and final crater shape; and the dynamics of impact into a strengthless spherical and ellipsoidal liquid target

    Impact of an asteroid or comet in the ocean and extinction of terrestrial life

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    Finite difference calculations describing the impact mechanics associated with a 10 to 30 km diameter silicate or water object impacting a 5 km deep ocean overlying a silicate solid planet demonstrate that from 12 to 15% of the bolide energy resides in the water. It is speculated that minimal global tsunami run-up heights on the continents would be 300-400 meters, and that such waves would inundate all low altitude continental areas, and strip and silt-over virtually all vegetation. As a result the terrestrial animal food chain would be seriously perturbed. This could in turn cause extinction of large terrestrial animals

    A planetary ultra hypervelocity impact mechanics and shock wave science facility

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    Using the concept of intercepting orbits from a pair of Space Station serviced free flyers, a class of impact and shock wave experiments pertinent to planetary science can be performed. One proposed free flying vehicle is an impactor dispensor, and the second is the impact laboratory. How collision is achieved by utilizing essentially twice orbital velocity is demonstrated. The impactor dispensor contains a series of small flyer plates or other projectiles which are launched into the trajectory of the impactor laboratory at appropriate positions. The impactor laboratory is a large impact tank similar to those in terrestrial gun laboratories, except that it contains a supply of targets and instrumentation such as high speed cameras, flash X-ray apparatus, and digital recorders. Shock and isentropic pressures of up to 20 Mbar are achievable with such a system which provides 15 km/sec impact velocities for precisely oriented projectiles

    Penetration depth time history measurement method

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    A new method for measuring the depth time history of rigid body penetration into brittle materials under a deceleration of ~10^5 g. The method includes: sabot-projectile, sabot-projectile separation and penetration depth detection systems. Relatively small intrinsic time error (3%) and depth error (0.3–0.7 mm) results. Penetration depth time history in a series of 4140 steel projectile penetrations into a mortar are measured at velocities of 100 to 500 m/sec with sufficient accuracy such that differentiation with respect to time yields stopping force, via Newton's second law

    Exact ground states of spin-2 chains

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    We use the matrix product approach to construct all optimum ground states of general anisotropic spin-2 chains with nearest neighbour interactions and common symmetries. These states are exact ground states of the model and their properties depend on up to three parameters. We find three different antiferromagnetic Haldane phases, one weak antiferromagnetic and one weak ferromagnetic phase. The antiferromagnetic phases can be described as spin liquids with exponentially decaying correlation functions. The variety of phases found with the matrix product ansatz also gives insight into the behaviour of spin chains with arbitrary higher spins.Comment: 7 pages, 2 figures, to be published in europhysics letters, uses epl.cl

    Superheating systematics of crystalline solids

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    Systematics of superheating (theta= T/Tm–1) of crystalline solids as a function of heating rate (Q) are established as beta= A(Q)(theta+ 1)theta2, where the normalized energy barrier for homogeneous nucleation is beta= 16pigammasl3/(3kTmDeltaHm2), T is temperature, Tm melting temperature, A a Q-dependent parameter, gammasl interfacial energy, DeltaHm heat of fusion, and k Boltzmann's constant. For all elements and compounds investigated, beta varies between 0.2 and 8.2. At 1 and 10^12 K/s, A = 60 and 31, theta= 0.05–0.35 and 0.06–0.45, respectively. Significant superheating is achievable via ultrafast heating. We demonstrate that the degree of superheating achieved in shock-wave loading and intense laser irradiation as well as in molecular dynamics simulations (Q~10^12 K/s) agrees with the theta–beta–Q systematics

    The relation between the shock-induced free-surface velocity and the postshock specific volume of solids

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    The release of solids from a state of shock compression at a free surface is examined. For isentropic release, the postshock specific volume V[prime]0 is shown to be constrained by V[prime]0? (Ufs–Up)2/P1+V1, where (P1,V1) is the pressure-volume Hugoniot state of shock compression and Ufs and Up are the free-surface and shock particle velocities, respectively. When a sudden phase change occurs during the release process, this lower bound is increased, subject to simplifying assumptions about the phase transition
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