Nick R.G. Shrine

Laboratory investigations of hypervelocity impact cratering in ice

Abstract Hypervelocity impact experiments on water ice targets have been performed using a two stage light gas gun. The resulting craters were measured to obtain the crater depth and diameter. From the data set for 23 impact craters, damage equations have been obtained which give the crater depth (diameter) in terms of the dependence on impact velocity, projectile diameter and projectile density. The resulting damage equations are compared to those for another brittle material, glass. Scaling of the excavated crater volume with energy is shown to obey a simple power law over 10 orders of magnitude in energy (10 −7 to 10 3 J).

Optical survey of micrometeoroid and space debris impact features on EURECA

Abstract The results of an optical survey of impact features from micrometeoroids and space debris on the EUropean REtrievable CArrier (EURECA) are presented. EURECA was returned from space in June 1993 after almost 11 months in Low Earth Orbit. Impact features on all outer surfaces (mainly Multi Layer Insulation blankets and front and rear sides of solar arrays) were systematically recorded and measured. The size of recorded impact features ranges from about 30 μm to 6.5 mm. High resolution microphotographs were taken from 932 of the larger impact sites. The visibility of impacts differs largely for the different surfaces. Impacts are most easily detected on the cover glass of the solar cells. On these surfaces the measured cumulative flux of impact craters with a central pit diameter of 50 μm or larger is ≈1.5×10 −6 /m 2 s −1 . On these glass surfaces, 225 out of 703 impacta which were measured in detail showed signs of directionality. The measurements, derived impact fluxes and a first evaluation of more detailed impact feature parameters and correlations are presented.

Impacts on HST and EuReCa solar arrays compared with LDEF using a new glass-to-aluminium conversion

Abstract In order to compare impact fluxes measured on solar arrays in Low Earth Orbit (LEO) to those on aluminium surfaces, as measured by the Long Duration Exposure Facility (LDEF) for example, a conversion between the conchoidal diameter impact feature ( D co ) (as observed on solar array surfaces) and the ballistic limit in aluminium ( F max) has been developed. We find good agreement between the converted HST and EuReCa solar array fluxes and LDEF fluxes at large sizes ( F max > 30 μ m) where meteoroids dominate. We also find the converted HST and EuReCa fluxes are in broad agreement with the microdebris enhancement ( F max μ m) apparent on LDEF, indicating that the microdebris flux has not changed significantly between the LDEF epoch (1984–1990), and the EuReCa (1992–1993) and HST (1990–1993) epochs.

Hypervelocity impact on brittle materials of semi-infinite thickness: Fracture morphology related to projectile diameter

Hypervelocity impact on brittle materials produces features not observed on ductile targets. Low fracture toughness and high yield strength produce a range of fracture morphologies including cracking, spallation and shatter. For sub-mm diameter projectiles, impact features are characterised by petaloid spallation separated by radial cracks. The conchoidal or spallation diameter is a parameter in current cratering equations. An alternative method for interpreting hypervelocity impacts on glass targets of semi-infinite thickness is tested against impact data produced using the Light Gas Gun (LGG) facility at the University of Kent at Canterbury (UKC), U.K. Spherical projectiles of glass and other materials with diameters 30–300 μm were fired at ∼5 km s−1 at a glass target of semi-infinite thickness. The data is used to test a power law relationship between projectile diameter and crack length. The results of this work are compared with published cratering/spallation equations for brittle materials.

Micro-particle impact flux on the timeband capture cell experiment of the Eureca spacecraft

Measurements of hypervelocity impact fluxes (in both thick and thin targets) detected by the University of Kent at Canterburys Timeband Capture Cell Experiment (TiCCE) (flown on ESAs Eureca spacecraft) are presented. The foil perforations are used to derive the ballistic limit values, or the maximum thickness of Al perforated, for the impacting particles. This data is then combined with the thick target data to derive a unified ballistic limit flux. A significant enhancement in the observed large particle flux compared with LDEF is found, possibly due to the pointing history of Eureca compared to the Earths orbital direction. Comparisons are also made to predictions from ESABASE modelling. Preliminary results of a study of perforation morphology are also presented, providing insight into particle shape, density and directionality.

Laboratory investigations of the survivability of bacteria in hypervelocity impacts

It is now well established that material naturally moves around the Solar System, even from planetary surface to planetary surface. Accordingly, the idea that life is distributed throughout space and did not necessarily originate on the Earth but migrated here from elsewhere (Panspermia) is increasingly deemed worthy of consideration. If life arrived at the Earth from space, its relative speed will typically be of order many km s-1, and the resulting collision with the Earth and its atmosphere will be in the hypervelocity regime. A mechanism for the bacteria to survive such an impact is required. Therefore a programme of hypervelocity impacts in the laboratory at (4.5 +/- 0.6) km s-1 was carried out using bacteria (Rhodococcus) laden projectiles. After impacts on a variety of target materials (rock, glass and metal) attempts were made to culture Rhodococcus from the surface of the resulting craters and also from the target material ejected during crater formation. Control shots with clean projectiles yielded no evidence for Rhodococcus growth from any crater surface or ejecta. When projectiles doped with Rhodococcus were used no impact crater surface yielded colonies of Rhodococcus. However, for four shots of bacteria into rock (two on chalk and two on granite) the ejecta was afterwards found to give colonies of Rhodococcus. This was not true for shots onto glass. In addition, shots into aerogel (density 96 kg m-3) were also carried out (two with clean projectiles and two with projectiles with Rhodococcus). This crudely simulated aero-capture in a planetary atmosphere. No evidence for Rhodococcus growth was found from the projectiles captured in the aerogel from any of the four shots.

Laboratory investigations of the temperature dependence of hypervelocity impact cratering in ice

Abstract Laboratory investigations by hypervelocity impact cratering in water ices are usually carried out at temperatures of approximate 250–265 K. However, icy surfaces in the Solar System are typically at lower temperatures. Accordingly a study of the temperature dependence of cratering in water ice has been carried out using a two-stage light gas gun firing mm-sized projectile at 5 – 6 km s −1 . The temperature of the water ice targets has been varied in the range 152–253 K. The variation in depth, diameter and volume of the resulting craters is presented as a function of temperature.

Microscopic and chemical analyses of major impact sites on timeband capture cell experiment of the eureca spacecraft

Abstract Extensive studies of over 100 impact sites on aluminium foils and mesh supports of the Timeband Capture Cell Experiment (TiCCE) on the European Retrievable Carrier (EuReCa) spacecraft were conducted with scanning electron microscope and energy dispersive X-ray spectrum analyser. Chemical elements of residues in and around the perforations and craters were examined to identify the origin of impactors. 73% of the impacts were classified: the minimum of 15% was due to natural particle impacts and the rest indicated high silicon presence. Possible origins of these silicon profiles were discussed. For micrometeoroid craters, the depths to diameter ratios were compared with those of meteoroid and orbital debris impacts on the Solar Maximum Mission satellite.