Chemical fractionation effects during high velocity impact

Dust capture cells on board the LDEF (Long Duration Exposure Facility) satellite were designed to enable the study of the elemental and isotopic composition as well as the size distribution and flux of interplanetary dust particles (IDPs), destructively collected at 500 km altitude over a period of several years. This is a report on simulation experiments of high velocity impacts on the capture cells which have been performed in order to investigate the relationship of the particle's chemical composition before the impact to the composition of the residue on the cell. To that end, both the projectile material and the impact residues have been analyzed with secondary ion mass spectrometry (SIMS). It is found that elemental fractionation occurs during the impact and that the magnitude of fractionation is related to the volatility of the elements.     

Preliminary total dose measurements on LDEF.

After spending nearly six years in Earth orbit twenty stacks consisting of radiation detectors and biological objects are now back on Earth. These stacks (Experiment A0015 Free Flyer Biostack) are part of the fifty seven science and technology experiments of the Long Duration Exposure Facility (LDEF) of NASA. The major objectives of the Free Flyer Biostack experiments are to investigate the biological effectiveness of single heavy ions of the cosmic radiation in various biological systems and to provide information about the spectral composition of the radiation field and the total dose received in the LDEF orbit. The Biostacks are mounted in two different locations of the LDEF. Up to three layers of Lithium fluoride thermoluminescence dosimeters (TLD) of different isotopic composition were located at different depths of some Biostacks. The preliminary analysis of the TLD yields maximum absorbed dose rates of 2.24 mGy day-1 behind 0.7 g cm-2 shielding and 1.17 mGy day-1 behind 12 g cm-2 shielding. A thermal neutron fluence of 1.7 n cm-2 s-1 is determined from the differences in absorbed dose for different isotopic mixtures of Lithium. The results of this experiment on LDEF are especially valuable and of high importance since LDEF stayed for about six years in the prospected orbit of the Space Station Freedom. There is no knowledge about the effectiveness of the space radiation in long-term spaceflights and the dosimetric data in this orbit are scarce.     

Low energy ions in the heavy ions in space (HIIS) experiment on LDEF.

We present data from the Lexan top stacks in the Heavy Ions In Space (HIIS) experiment which was flown for six years (April 1984-Jan 1990) onboard the LDEF spacecraft in 28.5 degrees orbit at about 476 km altitude. HIIS was built of passive (i.e. no timing resolution) plastic track detectors which collected particles continuously over the entire mission. In this paper we present data on low energy heavy ions (10 < or = Z, 20MeV/nuc < E < 200 MeV/nuc). These ions are far below the geomagnetic cutoff for fully ionized ions in the LDEF orbit even after taking into account the severe cutoff suppression caused by occasional large geomagnetic storms during the LDEF mission. Our preliminary results indicate an unusual elemental composition of trapped particles in the inner magnetosphere during the LDEF mission, including both trapped anomalous cosmic ray species (Ne, Ar) and other elements (such as Mg and Fe) which are not found in the anomalous component of cosmic rays. The origin of the non-anomalous species is not understood, but they may be associated with the solar energetic particle events and geomagnetic disturbances of 1989.     

Overview of the space environmental effects observed on the retrieved Long Duration Exposure Facility (LDEF).

The Long Duration Exposure Facility (LDEF), which encompassed 57 experiments with more than 10,000 test specimens, spent 69 months in low Earth orbit (LEO) before it was retrieved by the Space Shuttle in January 1990. Hundreds of LDEF investigators, after studying for over two years these retrieved test specimens and the onboard recorded data and systems hardware, have generated a unique first-hand view of the long term synergistic effects that the LEO environment can have on spacecraft. These studies have also contributed significantly toward more accurate models of the LEO radiation, meteoroid, manmade debris and atomic oxygen environments. This paper provides an overview of some of the many LDEF observations and the implications these can have on future spacecraft such as Space Station Freedom.     

Meteoroids and space debris hypervelocity impact penetrations in LDEF MAP foils compared with hydrocode simulations

The continued analyses of penetrating impacts on MAP foils of Aluminium and Brass have produced data for several LDEF faces, i.e., Space, West, and East. These data have immediate bearing on the interpretation and design of devices to detect the penetration of a thin metallic film by a dust grain which have been tested both in the laboratory and in space. A crucial component of the analysis has been the theoretical calculation utilizing CTH, a Sandia National Laboratory Hydrodynamic computer code /1/ to assess the parameters of the hypervelocity penetration event. In particular theoretical hydrodynamic calculations have been conducted to simulate the hypervelocity impact event where various cosmic dust grain candidates, e.g., density = 0.998, 2.700, 7.870 (gm/cm³), and velocities, i.e., 7 - 16 km/s, have been utilized to reproduce the events. Theoretical analyses of hypervelocity impact events will be reported which span an extensive matrix of values for velocity, density and size. Through a comparison between LDEF MAP foil measurements and CTH hydrocode calculations these analyses will provide an interpretation of the most critical parameters measured for space returned materials, i.e., for thin films, the diameter of the penetration hole, D_h, and for semi-infinite targets, the depth-to-diameter ratio of craters, . An immediate consequence of a comparison of CTH calculations with space exposed materials will be an enhancement of the coherent model developed by UKC-USS researchers to describe penetration dynamics associated with LDEF MAP foils.     

Physical properties of cometary dust: Relation of DIDSY data to grain properties

The Dust Impact Detection System (DIDSY) for the Giotto Halley Mission consists of two types of sensors for the detection of cometary dust particles: two impact plasma sensors and five piezo-electric momentum sensors. One sensor of each type is covered by a penetration film. A 1 μm thick aluminum film covers an impact plasma sensor. One momentum sensor is mounted onto the rear shield behind the 1 mm front shield made from aluminum. The parameters measured are the total charge released upon impact and the amplitude of the acoustic signal generated by the impact. Both quantities depend on the mass and speed of the impacting particles. At the impact speed of 68 km/sec the mass of cometary dust particles can be determined in the mass range from 10⁻¹⁷ g to 10⁻³ g. From the difference in the countrates measured by the sensors with and without penetration film the average bulk density of dust particles of masses 10⁻¹⁴ g and 10⁻⁶ g can be determined. With appropriate calibration an accuracy of a factor of 2 for both the mass and density determination can be obtained.     

Effects of long duration space flight on rice seed (or embryo) radiation sensitivity and element microlocalizations.

In long duration space experiments Rice caryopses and embryos, which are able to remain alive 10 years (or more) and tolerate extreme physical conditions (temperature, few water content) during irradiation and post-irradiation storage, were used (8, 40, 201 and 457 days on board of Salyut 7, 2107 days on LDEF). In certain experiments (Salyut 7), samples were irradiated either before or after the flight. Effects of the flight and radiosensitivity were observed in Rice seedlings cultivated in in vitro conditions. Statistical results indicate an increase in radiosensitivity when irradiations occur before the flight. Microanalyses were made in different parts of one caryopsis and of one embryo, and the results compared with those of control samples. With caryopses and embryos of the same Rice varieties, but from LDEF, we made the same kinds of experiments to compare results.     

The influence of solid rocket motor retro-burns on the space debris environment

The ESA space debris population model MASTER (Meteoroid and Space Debris Terrestrial Environment Reference) considers firings of solid rocket motors (SRM) as a debris source with the associated generation of slag and dust particles. The resulting slag and dust population is a major contribution to the sub-millimetre size debris environment in Earth orbit. The current model version, MASTER-2005, is based on the simulation of 1076 orbital SRM firings which contributed to the long-term debris environment. A comparison of the modelled flux with impact data from returned surfaces shows that the shape and quantity of the modelled SRM dust distribution matches that of recent Hubble Space Telescope (HST) solar array measurements very well. However, the absolute flux level for dust is under-predicted for some of the analysed Long Duration Exposure Facility (LDEF) surfaces. This points into the direction of some past SRM firings not included in the current event database. The most suitable candidates for these firings are the large number of SRM retro-burns of return capsules. Objects released by those firings have highly eccentric orbits with perigees in the lower regions of the atmosphere. Thus, they produce no long-term effect on the debris environment. However, a large number of those firings during the on-orbit time frame of LDEF might lead to an increase of the dust population for some of the LDEF surfaces. In this paper, the influence of SRM retro-burns on the short- and long-term debris environment is analysed. The existing firing database is updated with gathered information of some 800 Russian retro-firings. Each firing is simulated with the MASTER population generation module. The resulting population is compared against the existing background population of SRM slag and dust particles in terms of spatial density and flux predictions.     

On the origin of trapped heavy ions at L = 1.4-1.6.

Aboard the NASA satellite Long Duration Exposure Facility (LDEF) heavy ions of nuclear charge Z = 8-26 were detected with energies between 15 and 50 MeV/nuc which are far below the cutoff energy required of fully stripped ions to reach the LDEF orbit. The arrival directions and the falling energy spectra of these particles are consistent with a trapped component incident in the South Atlantic Anomaly at L = 1.4-1.6. The trapped oxygen, neon and argon ions probably originate from the anomalous cosmic rays, whereas the origin of the other particles like magnesium, silicon and iron is not yet solved but may be associated with the October 89 solar energetic particle events.     

Comparison of model predictions with LDEF satellite radiation measurements.

Some early results are summarized from a program under way to utilize LDEF satellite data for evaluating and improving current models of the space radiation environment in low Earth orbit. Reported here are predictions and comparisons with some of the LDEF dose and induced radioactivity data, which are used to check the accuracy of current models describing the magnitude and directionality of the trapped proton environment. Preliminary findings are that the environment models underestimate both dose and activation from trapped protons by a factor of about two, and the observed anisotropy is higher than predicted.     

高层大气原子氧的半年周变

地球高层大气成分的长期变化受太阳黑子周、太阳活动程度和地磁活动程度等诸多因子所控制．本文利用国外有关高层大气成分的数据,分别讨论受上述控制因子影响的高层大气成分长期变化,讨论范围仅限原子氧半年周期变化．选用LDEF在轨飞行器1984年4月—1990年1月高度470km附近的长期资料进行统计分析,结果表明,高度470km附近原子氧在年平均太阳黑子数＜20、太阳活动程度相对低而平稳期间,半年周期的变化尤为明显,相对变幅约为40％—60％、井随平均太阳黑子数增加而增大．而年平均太阳黑子数峰值的1989年期间（＞120）,半年变化的相对变幅可达87％左右．     

Atmospheric drag modelling for orbital micro-debris at LEO altitudes

Data from satellite impact experiments and the scanning of recovered spacecraft offers an extended timebase to examine, using a consistent methodology, the microparticle fluxes. New penetration data from the TiCCE experiment on Eureca /1, 2/ adds to this database and shows that - despite an expected growth in the micro-debris flux - the observed flux is not greater than either LDEF or SMM. The question arises: “is this consistent with the micro particle flux being dominated by space debris or by meteoroids”.

To assist this assessment, numerical modelling using the Gear method /3/ of explicit time integration of the atmospheric drag lifetime of micron dimensioned orbital debris in both circular (LEO) and eccentric (GTO) orbits has been performed for the relevant space exposures. Results are applied to the data to examine whether the recent variations in flux can be attributed to varying levels of, orbital micro-debris caused by atmospheric drag and its changes during the solar cycle.     

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 Canterbury's Timeband Capture Cell Experiment (TiCCE) (flown on ESA's Eureca spacecraft) are presented. The foil perforations are used to derive the ballistic limit values, or the maximum thickness of A1 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 Earth's 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.     

Mutational effects of space flight on Zea mays seeds.

The growth and development of more than 500 Zea mays seeds flown on LDEF were studied. Somatic mutations, including white-yellow stripes on leaves, dwarfing, change of leaf sheath color or seedling color were observed in plants developed from these seeds. When the frequency of white-yellow formation was used as the endpoint and compared with data from ground based studies, the dose to which maize seeds might be exposed during the flight was estimated to be equivalent to 635 cGy of gamma rays. Seeds from one particular holder gave a high mutation frequency and a wide mutation spectrum. White-yellow stripes on leaves were also found in some of the inbred progenies from plants displayed somatic mutation. Electron microscopy studies showed that the damage of chloroplast development in the white-yellow stripe on leaves was similar between seeds flown on LDEF and that irradiated by accelerated heavy ions on ground.     

First results of particulate impacts and foil perforations on LDEF

The results of an initial examination of the LDEF MicroAbrasion Package (MAP) and limited results from other onboard hardware are presented. The intriguing tasks of interpreting these data in terms of the dynamics of a particulate distribution of natural and artificial origin are discussed. It emphasises the unique aspects of the mission and especially the attitude stabilisation which may be exploited to extract a greater range of information compared with that previously derived from space collections and exposure of similar passive sensors.     

Development of in-situ Space Debris Detector

Due to high relative velocities, collisions of spacecraft in orbit with Space Debris (SD) or Micrometeoroids (MM) can lead to payload degradation, anomalies as well as failures in spacecraft operation, or even loss of mission. Flux models and impact risk assessment tools, such as MASTER (Meteoroid and Space Debris Terrestrial Environment Reference) or ORDEM (Orbital Debris Engineering Model), and ESABASE2 or BUMPER II are used to analyse mission risk associated with these hazards. Validation of flux models is based on measured data. Currently, as most of the SD and MM objects are too small (millimeter down to micron sized) for ground-based observations (e.g. radar, optical), the only available data for model validation is based upon retrieved hardware investigations e.g. Long Duration Exposure Facility (LDEF), Hubble Space Telescope (HST), European Retrievable Carrier (EURECA). Since existing data sets are insufficient, further in-situ experimental investigation of the SD and MM populations are required. This paper provides an overview and assessment of existing and planned SD and MM impact detectors. The detection area of the described detectors is too small to adequately provide the missing data sets. Therefore an innovative detection concept is proposed that utilises existing spacecraft components for detection purposes. In general, solar panels of a spacecraft provide a large area that can be utilised for in-situ impact detection. By using this method on several spacecraft in different orbits the detection area can be increased significantly and allow the detection of SD and MM objects with diameters as low as 100 μm. The design of the detector is based on damage equations from HST and EURECA solar panels. An extensive investigation of those panels was performed by ESA and is summarized within this paper. Furthermore, an estimate of the expected sensitivity of the patented detector concept as well as examples for its implementation into large and small spacecraft are presented.     

Fullerenes in space.

The discovery and synthesis of fullerenes led to the hypothesis that they may be present and stable in interstellar space. Fullerenes have been reported in an impact crater on the LDEF spacecraft. Investigations of fullerenes in carbonaceous meteorites have yielded only small upper limits. Fullerene compounds and their ions could be interesting carrier molecules for some of the "diffuse interstellar bands" (DIBs), a long standing mystery in astronomy. We have detected two new diffuse bands that are consistent with laboratory measurements of the C60+, as first evidence for the largest molecule ever detected in space. Criteria for this identification are discussed. The inferred abundance (up to 0.9 % of cosmic carbon locked in C60+) suggests that fullerenes may play an important role in interstellar chemistry. We present new observations on DIB substructures consistent with fullerene compounds, and the search for neutral C60 in the diffuse medium.     

Face-dependent impact probabilities, velocities and angles upon LDEF by space debris and natural meteoroids

For specified geocentric orbits the impact probabilities, velocities and angles upon the different faces of the Long Duration Exposure Facility (LDEF) are calculated, and it is found that quite different distributions of micro-cratering are to be expected. In particular the flux to the East (leading) face should exceed that to the West (trailing) face by a very large ratio. The North and South faces receive exposures slightly in excess of the East face for lower-velocity impacts from low-inclination orbits, but much lower exposures than the East face for high-velocity impacts from high-inclination orbits. The Space face (pointing directly away from the Earth) and the Earth face (pointing directly towards the Earth) will have been subject to very few impacts from geocentric orbits. Therefore, whilst three sides (the East, North and South) will have been hit many times by artificial space debris, the other three (the West, Space and Earth) will have been impacted almost solely by natural meteoroids from heliocentric orbits, and may be used to determine the flux of such particles in the vicinity of the Earth. The ratios of impacts upon the East, West and Space faces will be useful indicators of the velocity/orbit distribution of meteoroids.     

原子氧和紫外线对聚酰亚胺综合作用数值模拟

在建立数学物理模型的基础上,对低地球轨道环境和地面试验环境下有无保护涂层的聚酰亚胺所受原子氧冲蚀及紫外线的综合作用进行了数值模拟,获得了具有工程应用价值的计算结果,并讨论了数学物理模型中各参数对基蚀曲线形状的影响.从数值模拟结果与美国太空试验结果的比较可以看出,得出的数值模拟的结果是正确的,对航天器设计具有重要的指导意义.     

Heavy ion measurement on LDEF.

A stack of CR-39 track detectors was exposed on the NASA satellite LDEF and recovered after almost 6 years in space. The quick look analysis yielded heavy ion tracks on a background of low energy secondaries from proton interactions. The detected heavy ions show a steep energy spectrum which indicates a radiation belt origin.