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At least 1,045 records · Page 58

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NASA NTRS1982

The structure and evolution of ancient impact basins on Mars

It is pointed out that characteristic styles of degradation and modification of obvious Martian basins make it possible to recognize more subtle expressions. This approach is seen as providing not only additional basins to the existing inventory but also fundamental clues for initial impact basin structure and stratigraphy. It also reveals the long-lasting influence of basin formation on the crust of Mars in spite of extensive erosion and resurfacing. Consideration is given to five clear examples of modified impact basins, and regions around each that have undergone similar processes (fracturing, collapse, channeling) are delineated. These processes among the different basins are then compared, and similar zones of modification are correlated with concentric basin rings. Consideration is then given to the implications of these observations for current models of basin formation and to the role of impact basins in controlling regional tectonics. The results indicate that large multiring impact scars leave a major but sometimes subtle imprint on the geologic structure of stable crustal regions on Mars.

Schultz, P. H.↗

NASA NTRS1982

Experimental simulation of impact cratering on icy satellites

Cratering processes on icy satellites were simulated in a series of 102 laboratory impact experiments involving a wide range of target materials. For impacts into homogeneous clay slurries with impact energies ranging from five million to ten billion ergs, target yield strengths ranged from 100 to 38 Pa, and apparent viscosities ranged from 8 to 200 Pa s. Bowl-shaped craters, flat-floored craters, central peak craters with high or little relief, and craters with no relief were observed. Crater diameters increased steadily as energies were raised. A similar sequence was seen for experiment in which impact energy was held constant but target viscosity and strength progressively decreases. The experiments suggest that the physical properties of the target media relative to the gravitationally induced stresses determined the final crater morphology. Crater palimpsests could form by prompt collapse of large central peak craters formed in low target strength materials. Ages estimated from crater size-frequency distributions that include these large craters may give values that are too high.

Greeley, R.↗

NASA NTRS1984

Embedded gage impact study

Impact damage in graphite/epoxy laminates was characterized and transient strain history during impact was correlated. The material investigated was AS-4/3501-6 graphite/epoxy. Eight-ply and sixteen-ply quasi-isotropic laminates of 45/0/-45/90 sub s and 45/0/-45/90 sub 2s layups were fabricated with strain gages embedded between plies during the strain gages and leads from the highly conductive graphite fibers. The specimens were circular plates 12.7 cm (5 in.) in diameter and clamped along their circumference. The specimens were impacted with a 185 gm impactor, dropped from heights of 1.20 m and 1.65 m. An accelerometer was attached to the back surface of the specimen opposite the impact point and was used to trigger the recording instrumentation. The transient strain data were recorded with an eight channel waveform digitizer capable of sampling data at 0.5 microsec intervals. The data were stored, processed, and plotted by means of a microcomputer. Transient strain data were correlated with results from ultrasonic inspection of the specimens.

Schramm, S. W.↗

NASA NTRS1984

Impact basins and the volcanics of Mars

The distribution fracture patterns of old Martian impact basins were studied. The basaltic plains on the Moon generally occur inside or around impact craters and basins and it is believed that fractures generated by large impacts provided long lasting pathways for magmas. The volcanic plains and volcanic edifices on Mars which show a complex distribution are the interiors of the most obvious old basins are largely buried by wind blown deposits. The Martian ridged plains that most resemble the lunar maria typically are offset from such impact basins. The origin of the mare like plains offset and the placement of the large volcanic mountains are discussed.

Schultz, P. H.↗

NASA NTRS1982

Estimates of crater size for large-body impact Gravity-scaling results

A scaling analysis based upon dimensional invariance is used in conjunction with centrifuge experiments to estimate cratering efficiency as a function of impact velocity for kilometer-sized impactors. Complementing earlier, conventional small-scale impact experiments, centrifuge experiments were performed to substantiate these scaling laws in a scaled-size regime of interest. This technique provided empirical scaling laws used to estimate crater size for large bodies impacting the earth. From these scaling laws a 10-km-diameter body impacting at 25 km/s would be expected to produce a 66-km-diameter crater and to inject approximately 12 times its mass into the atmosphere.

Schmidt, R. M.↗

NASA NTRS1982

Impact ejecta dynamics in an atmosphere - Experimental results and extrapolations

It is noted that the impacts of 0.635-cm aluminum projectiles at 6 km/sec into fine pumice dust, at 1 atm, generate a ball of ionized gas behind an expanding curtain of upward moving ejecta. The gas ball forms a toroid which dissolves as it is driven along the interior of the ejecta curtain, by contrast to near-surface explosions in which a fireball envelops early-time crater growth. High frame rate Schlieren photographs show that the atmosphere at the base of the ejecta curtain is initially turbulent, but later forms a vortex. These experiments suggest that although small size ejecta may be decelerated by air drag, they are not simply lofted and suspended but become incorporated in an ejecta cloud that is controlled by air flow which is produced by the response of the atmosphere to the impact. The extrapolation of these results to large body impacts on the earth suggests such contrasts with laboratory experiments as a large quantity of impact-generated vapor, the supersonic advance of the ejecta curtain, the lessened effect of air drag due to the tenuous upper atmosphere, and the role of secondary cratering.

Schultz, P. H.↗

NASA NTRS1984

History of the Apollo 15 yellow impact glass and sample 15426 and 15427

Five fragments of the Apollo yellow impact glass from breccias 15426 and 15427 have been dated by the Ar-39/Ar-40 system using a laser microprobe. These glasses were generated by impact at 3.35 + or -0.05 AE and do not confirm the prediction of 1 AE or less by Delano et al. (1982). Since these glasses were probably produced by an impact into a target of moderate-TiO2 basalts, and the only known flows of such material are younger than the impact event, the target for these glasses must be covered up by later basaltic flows. The average Ar-38-Ar-37 exposure age for these glasses is 274 + or -74 m.y.

Spangler, R. R.↗

NASA NTRS1984

Compression failure characteristics of + or - 45-deg-dominated laminates with a circular hole or impact damage

An investigation of the compression failure characteristics of + or - 45-deg-dominated laminates with a circular hole or impact damage was conducted. Graphite-epoxy laminates consisting of all + or - 45-deg plies and of + or - 45-deg and 90-deg plies were evaluated. Inplane shearing between the fibers and matrix (i.e., matrix shearing) was found to be the primary compression failure mechanism for an all + or - 45-deg laminate with a hole. The failure mechanism for the + or 45-deg-dominated laminates with a hole appeared to be a combination of delamination and matrix shearing. The compression failure stresses and strains for both the all + or - 45-deg laminate and the + or - 45-deg-dominated laminates were unaffected by impact damage at an energy level of 5.5 J when compared to results for undamaged control specimens. The failure stresses and strains for all laminates with impact damage at an energy level of 17.0 J or of 34.3 J were significantly lower than the corresponding data for the undamaged control specimens. The failure stresses and strains for these impact-damaged specimens were independent of the percentage of 90-deg plies in the laminate.

Shuart, M. J.↗

NASA NTRS1985

Debris impact on Earth-orbiting spacecraft

The accumulation of Earth-orbiting space debris leads to important new design considerations. Some 5,000 orbiting objects, many of them explosion fragments, are currently being tracked and future collision of these objects with each other is predicted. These collisions will occur at high velocities. Each collision will be explosive, ejecting thousands, of new orbiting objects, in turn increasing the frequency of future collisions. The debris population may thus become self-regenerative, and the future flux of orbiting debris will exceed that of meteoroids. As a result, a large space structure in Earth-orbit for several years has a significant probability of impact by debris objects. As a design problem, debris impact is significantly different from meteoroid impact. Protection against such large objects may require structural measures. The consideration of debris impact in the design of large, Earth-orbiting spacecraft is recommended.

Smith, D. G.↗

NASA NTRS1985

Centrifuge Impact Cratering Experiments

The kinematics of crater growth, impact induced target flow fields and the generation of impact melt were determined. The feasibility of using scaling relationships for impact melt and crater dimensions to determine impactor size and velocity was studied. It is concluded that a coupling parameter determines both the quantity of melt and the crater dimensions for impact velocities greater than 10km/s. As a result impactor radius, a, or velocity, U cannot be determined individually, but only as a product in the form of a coupling parameter, delta U micron. The melt volume and crater volume scaling relations were applied to Brent crater. The transport of melt and the validity of the melt volume scaling relations are examined.

Schmidt, R. M.↗

NASA NTRS1985

Why Not a Constant Early Lunar Impact Rate?

Two distinct episodes of impacting are recorded on the Moon's surface. An early episode marked by an intense barrage that included basin-forming projectiles ended about 3.8 aeons ago when the Orientale basin was created. The second episode, after 3.2 or 3.3 aeons ago, was marked by a much lower impact rate. These very different rates are separated by a short transition period during the Late Imbrian Epoch. It is found that a constant preImbrian impact rate is consistent with all the relevant observations and with the following lunar historical scenario: (1) crustal solidification between about 4.3 and 4.25 aeons ago; (2) formation of Procellarum, South Pole-Aitken, about 22 now-obliterated basins, and about 2,850 now-obliterated 30 to 300 km craters between 4.25 and 4.1 aeons ago; and (3) formation of 39 still-preserved basins, 1,200 still-perserved craters, and 2,200 now-obliterated craters between 4.1 and 3.85 aeons ago. At the constant rate, the amount of mass that impacted the Moon since crustal solidification would not greatly exceed the amount that has left a permanent visible record.

Wilhelms, D. E.↗

NASA NTRS1985

Analysis of impact response of buckled composite laminates

The dynamic response of buckled composite plates impacted by a hard object is studied. A nine-node isoparametric finite element, developed based on the Mindlin plate theory and von Karman large deflection assumptions, is used for both postbuckling and impact analyses. In order to evaluate the contact force, an experimentally established contact law which accounts for the permanent indentation is employed. The static postbuckling problem of laminated plates is first considered. The impact responses include contact force histories, dynamic deflections, and dynamic strains in the plate for various buckling conditions. In addition, free vibration of buckled laminated plates is also solved to determine the range of natural frequencies which are needed to choose a proper time increment for time integration in the impact analysis.

Chen, J. K.↗

NASA NTRS1984

The mineralogical, chemical, and chronological characteristics of the crystalline Apollo 16 impact melt rocks

A comparative review of mineralogical, chemical, and chronological data on crystalline Apollo 16 impact melt rocks is presented. The use of such data to identify distinct impact melt complex is discussed, and 22 distinct impact melt bodies are identified. The recently detected group of feldspathic microporphyritic (FM) melt rocks was tested for chemical and isotopic homogeneity; instrumental neutron activation analysis and new Rb-Sr isotopic whole rock data indicate that FMs were probably not derived from a single impact melt sheet, but might be representative of the Descartes basement. Stratigraphical and chronological concepts for the geological development of the landing site are discussed, and a model is presented for the formation of the Cayley Plains and the Descartes formation.

Reimold, W. U.↗

NASA NTRS1985

Impact and explosion crater ejecta, fragment size, and velocity

The present investigation had the objective to develop models for the distribution of fragments which are ejected at a given velocity for both impact and explosion cratering. It is pointed out that the results have application to the physics of planetary accretion and the origin of meteorites. The impact ejection of fine dust into the earth's atmosphere has been proposed as a mechanism for extinctions which occurred at the end of the Cretaceous. A technique is developed for determining the distribution of fragments which are ejected at a given velocity. The experimental data base for the distribution fragments in the ejecta blankets of impact, explosion, and nuclear craters, are discussed. Attention is also given to impact flow field calculations, fragmentation theory, and the applications of the derived relations.

Okeefe, J. D.↗

NASA NTRS1985

Impact experiments. I - Ejecta velocity distributions and related results from regolith targets

Results are reported of laboratory measurements of the velocity distributions of powdery ejecta of 14 impacts into powders simulating regolith in near vacuum. The impact velocities ranged from 5-2321 m/sec, a velocity range believed prevalent in the early solar system. The powder was mechanically similar to lunar regolith. The projectiles comprised harder materials than the regolith, i.e., pyrex spheres, basalt spheres and cylinders, and igneous rocks. High-speed photography was employed to record the events. The differences between the present results and those from hypervelocity impacts are discussed in detail. Notably, an impact velocity threshold was detected between 10-30 m/sec, below which the amount of ejecta is less than the projectile mass.

Hartmann, W. K.↗

NASA NTRS1986

Controlled impact demonstration airframe bending bridges

The calibration of the KRASH and DYCAST models for transport aircraft is discussed. The FAA uses computer analysis techniques to predict the response of controlled impact demonstration (CID) during impact. The moment bridges can provide a direct correlation between the predictive loads or moments that the models will predict and what was experienced during the actual impact. Another goal is to examine structural failure mechanisms and correlate with analytical predictions. The bending bridges did achieve their goals and objectives. The data traces do provide some insight with respect to airframe loads and structural response. They demonstrate quite clearly what's happening to the airframe. A direct quantification of metal airframe loads was measured by the moment bridges. The measured moments can be correlated with the KRASH and DYCAST computer models. The bending bridge data support airframe failure mechanisms analysis and provide residual airframe strength estimation. It did not appear as if any of the bending bridges on the airframe exceeded limit loads. (The observed airframe fracture was due to the fuselage encounter with the tomahawk which tore out the keel beam.) The airframe bridges can be used to estimate the impact conditions and those estimates are correlating with some of the other data measurements. Structural response, frequency and structural damping are readily measured by the moment bridges.

Soltis, S. J.↗

NASA NTRS1986

Impact dynamics instrumentation

One of the tasks specified in the NASA Langley controlled impact demonstration (CID) work package was to furnish dynamic instrumentation sensors. The types of instrumentation sensors required were accelerometers for aircraft structural loads measurements, seat belt load cells to measure anthropomorphic dummy responses to the aircraft impact, and strain gage bending bridges to measure the aircraft fuselage and wing bending during impact. The objective in the selection of dynamic instrumentation for the CID was to provide 352 of the highest quality transducers and remain within budget allocation. The transducers that were selected for the CID evaluation process were each subjected to rigorous laboratory acceptance tests and to aircraft fuselage section drop tests at the LaRC Impact Dynamics Research Facility. Data compiled from this series of tests showed the selected transducers to be best suited for the CID mission requirement. The transducers installation technique on the airframe proved successful. The transducer quality assurance was guaranteed through rigorous acceptance testing. Data acquired was 97.0%.

Mccormck, R. F.↗

NASA NTRS1986

The size distributions of fragments ejected at a given velocity from impact craters

The mass distribution of fragments that are ejected at a given velocity for impact craters is modeled to allow extrapolation of laboratory, field, and numerical results to large scale planetary events. The model is semi-empirical in nature and is derived from: (1) numerical calculations of cratering and the resultant mass versus ejection velocity, (2) observed ejecta blanket particle size distributions, (3) an empirical relationship between maximum ejecta fragment size and crater diameter, (4) measurements and theory of maximum ejecta size versus ejecta velocity, and (5) an assumption on the functional form for the distribution of fragments ejected at a given velocity. This model implies that or planetary impacts into competent rock, the distribution of fragments ejected at a given velocity is broad, e.g., 68% of the mass of the ejecta at a given velocity contains fragments having a mass less than 0.1 times a mass of the largest fragment moving at that velocity. The broad distribution suggests that in impact processes, additional comminution of ejecta occurs after the upward initial shock has passed in the process of the ejecta velocity vector rotating from an initially downward orientation. This additional comminution produces the broader size distribution in impact ejecta as compared to that obtained in simple brittle failure experiments.

Okeefe, J. D.↗