The retrieval of Eureca has given the Agency a rare opportunity to study the fluxes and resulting effects of meteoroid and space-debris impacts on exposed spacecraft surfaces in Low Earth Orbit (LEO). With its fixed Sun-pointing attitude, large areas of identical surface materials, and 1992-1993 exposure period, Eureca has provided impact data that are complementary to those from LDEF. The detailed optical survey of all outer Eureca surfaces, which was the first main task of the impact analysis, has now been completed.
Every spacecraft in Earth orbit is exposed to a flux of space debris and meteoroid particles. Currently more than 7000 large man-made objects orbiting in near-Earth space can be tracked from the ground with radar or by optical means. A much larger number of smaller man-made debris items and micrometeoroids that are orbiting the Earth cannot be detected from the ground. These particles are a hazard for both long-term missions and large spacecraft.
While the risk of collision with a large piece of debris or a large meteoroid is very small, particles less than one millimetre in size cause craters visible to the naked eye. Typical impact velocities are 10 km/s for space debris and 20 km/s for meteoroids. Larger particles can penetrate the outer shielding of a spacecraft and can damage its internal equipment. As a result of this threat, designers have to consider the risk of particle impacts in the planning of every space mission. In addition, particle fluxes in space are also of considerable scientific interest.
Knowledge of the solid-particle population of millimetre- and micron-sized particles is gained either from dedicated space experiments or through the analysis of material that has been returned from space. After just a short period of exposure to the space environment, surfaces are covered with impacts from small pieces of debris and meteoroids. Investigating the nature and the morphology of the impact features on ESA's European Retrievable Carrier 'Eureca' is therefore an exercise crucial to the understanding of the meteoroid and the evolving debris environments. This activity extends the analyses that NASA performed in the late eighties on the Solar Max, Palapa and Westar satellites and the Long-Duration Exposure Facility (LDEF), after they too had been recovered from space.
Compared to LDEF, Eureca has a large Sun-pointing surface, resulting in different types of meteoroid and debris impact signatures. Impact data from the Eureca post-flight analysis will therefore contribute to the validation and improvement of the current meteoroid and debris models for low Earth orbit.
Figure 1 shows Eureca in the cargo bay of Space Shuttle 'Atlantis' prior to launch. The external surface of the Carrier, which provides accommodation and resources for 1000 kg of payload, is almost entirely covered by thermal Multi-Layer Insulation (MLI) blankets. The only exceptions are the radiators and some boxes mounted on the bottom of the spacecraft, which are painted, and the solar-array wings. The total area of the exposed external surface is about 140 m ² , including 99 m ² of solar arrays (front and back surfaces), which span 20 m tip-to-tip when deployed.
The Shuttle Atlantis lifted off on 31 July 1992 and subsequently released Eureca into a nearly circular orbit, with a 508 km initial altitude and 28.5 deg inclination. The Carrier was retrieved again in June 1993, after 326 days of space exposure. Throughout the mission, it had been in pointing towards the Sun and its 'Y-axis' in the orbital plan (Fig. 2).
In early 1993, the Agency initiated a Post-Flight Investigation Programme to make a detailed analysis of the effects of long-duration exposure to space on the Eureca hardware. It included a micrometeoroid and debris analysis of all of the Carrier's exposed surfaces, with the goal of:
The main steps in this micrometeoroid and debris analysis were:
Having retrieved Eureca, on 1 July 1993 the Space Shuttle 'Endeavour' landed at Kennedy Space Center (KSC), where a full inspection of the Carrier's upper surface was made in the Orbiter Processing Facility. Eight days later, Eureca was placed in its transport container, which provided an opportunity to make the first full visual and photographic survey of the complete spacecraft.
The general impression was that there had been a significant change in the colours of the exposed surfaces, where outgassing products had been deposited on the thermal blankets. Areas of paint delamination and many micro-meteoroid impacts were clearly visible.
On 14 July, Eureca was transported to the Astrotech Facility in Titusville (Florida) in order to depressurise the Carrier's propulsion and attitude-control systems, to remove the remain-ing propellant, and to remove the payloads. During that payload de-integration, the 35 m² of thermal blankets that had covered the majority of Eureca's external surfaces were removed and put at the disposal of the micrometeoroid and debris investigation team for scanning. The other main exposed surfaces were the solar-array panels, the two scuff plates used for protection and housing Eureca in the Shuttle's payload bay, the two ESA logo plates, and the grapple fixture.
Over a six-week period during the summer of 1993, an investigation team formed by Prof. T. McDonnell of Unispace, led by T. Stevenson and made up by B. Carey (SAS), S. Deshpande (Unispace), W. Tanner (Unispace) and C. Maag (T&M Engineering), inspected and scanned the thermal blankets, the two scuff plates, the two ESA logo plates, and the grapple fixture. The Astrotech: they were scanned at Fokker, in The Netherlands, between December 1993 and March 1994, by the same team that had worked on the thermal blankets, who were joined by J.C. Mandeville (ONERA).
The Figure 1. The European Retrievable Carrier 'Eureca' in the payload bay of the Space Shuttle prior to launch (STS-46)
Figure 2. The Eureca space-craft's configuration, and the numbers of impact events recorded on its various components
The best methodology for crater location and identification proved to be visual inspection under good illumination by an experienced observer. Often this procedure was used to identify sites for magnified inspection at high resolution in a later stage.
Scanning at Astrotech
The thermal blankets were visually examined and those containing impacts were set aside for scanning with the aid of a Photometric CCD camera system linked to an Apple Macintosh II running a commercial software package, to record the digital images. Standard lenses could be attached to the CCD camera and thus a low-resolution scan (10 cm x 10 cm field of view) made of the entire sample to identify features to be revisited for recording at higher resolution (1.5 cm x 1.5 cm FOV).
The thermal blankets have an outer layer made of Beta-cloth, which is a very fibrous synthetic material. Scanning this cloth proved to be difficult because only features larger than 100 microns penetrating the outer layer could be recognised as hypervelocity impact sites. During the scanning, the investigation team took 3445 low-resolution and 100 high-resolution images of sites of special interest.
Scanning at Fokker
The solar arrays were the largest exposed surfaces on Eureca, having a total area of about 99 m² . The glass-covered cells mounted on the front side of the solar arrays are very sensitive 'impact detectors' in that impinging particles shatter the glass and cause a damaged area much larger than the central crater size. Impact features caused by particles as small as 10 microns are visible to the naked eye on the solar-array cover glasses. On the rear side of the array, only impacts from much larger particles are visible because the panels are covered by Kapton foil and other more ductile material.
The solar-array survey was documented with a Nikon 35 mm camera for the global scanning (24 cm x 16 cm or 24 cm x 17.5 cm field of view), and with an HIROX microscopy system with 20-100x magnification zoom optics for the detailed appraisal. The images were again acquired and stored on a PC equipped with picture-archiving software. The left-hand strip of each solar panel was scanned recording all features that could be detected with the naked eye under optimal illumination, which corresponded to a pit diameter of about 50 microns. Due to the high number of very similar impact features on the solar-cell glasses, the remaining swaths were scanned recording high-resolution pictures of impact features with a minimum size of about 650 microns, which is half of the solar-cell electrode spacing.
Approximately 3000 low-resolution images from both the front and rear faces of the solar arrays have been transferred to Photo-CDs. All of the high-resolution images of selected impact sites have been transferred to CD-ROM.
The distribution of the numerous impact events recorded is shown in Figure 2.
Solar Array Front Rear (Kapton) Rear (wiring) Circular Elliptical Circular Elliptical Wires Interconnect 478 225 120 15 1 8 Other Surfaces TICCE (Exp.) Beta-Cloth ESA Logo Scuff Plate Radiators Mesh Foil (MLI) (aluminium) (aluminium) (aluminium) 24 94 71 3 11 =5
The main results of the Eureca main-body survey are as follows:
More than 1000 impacts are visible to the naked eye on the front side of each wing. The impact features range in size from about 100 microns, to the largest crater which is 6.4 mm across (Figs. 5-7).
A typical impact feature can be described via the following parameters:
Comparisons indicate that, in terms of feature sizes larger than about 0.5 mm, Eureca encountered a higher impact flux than the LDEF. Some caution is required in interpreting these data, however, because of the low total number of impacts. To make a true comparison with the LDEF data, calibration tests are needed to translate the Eureca thermal-blanket and solar-cell data to aluminium targets. These tests are currently in process.
Figure 3. Typical thermal-blanket puncture hole
Figure 4. A 2 mm impact hole in one of the aluminium ESA logo plates
Impact damage to Eureca hardware caused no system or subsystem failures. This is partly because the multi-layer (MLI) structure retains particles up to a certain size very efficiently. The critical size for complete penetration of the 20 layers of insulation was only exceeded in two places, luckily causing no further damage at either site. Any loss in thermal-control function due to the particle impacts was negligible.
As far as the solar arrays are concerned, due to the massive redundancy and cross-strapping, even the most extensive form of damage - for example if a cell was completely cracked perpendicular to the current flow - would have caused only a small power loss. There is no evidence to suggest that extreme damage of this sort occurred anywhere on the Eureca arrays. Rather, it was confined to localised shattering of cells and cell cover glasses, thereby removing a tiny fraction of the sunlight-collecting area (curiously, it is possible to increase the output of solar cells slightly by introducing centres of scattering which bring in Earth albedo radiation not normally seen by Sun-pointing arrays). A significant number of impacts penetrated cells, and in some cases the structure. Despite the large number of impact sites recorded, their overall effect at system level was trivial. There is, however, evidence from elsewhere to suggest that impacts may cause electromagnetic and shock effects such as those seen on ESA's Giotto spacecraft during its Comet Halley encounter and on the Olympus geostationary telecommuni-cations satellite. No link has yet been established between the failures seen on Eureca and the disruptive effect of any impact. Nevertheless, work in this field continues, and definitive results may depend upon further measurements of in-orbit phenomena.
The main findings of the detailed visual survey of Eureca's external surfaces can be sum-marised as follows:
Figure 5-7. Typical solar-cell impact features
All images and data have been stored on Compact Disc (CD) or magnetic tape, and will be made available to both the engineering and scientific communities.
Questions like the number of Eureca impact features that can be related to natural micro-meteoroids and the number caused by man-made debris are still being investigated. It is planned to use the ESABASE/DEBRIS analysis tool developed at ESTEC to compare the observed impact data with the predictions of the current reference flux models. It will also allow the micrometeoroid and space-debris fluxes to be computed in the future for user-specified spacecraft geometries and mission parameters.
Presently, additional analyses are being carried out on the thermal-blanket impact holes to identify the chemical composition of the impact residues with a view to distinguishing between meteoroid and space-debris damage. Impact calibration tests are also being performed, on both thermal-blanket and solar-array samples, to relate the observed impact features to the parameters of the impacting particles. These tests will allow the Eureca survey results to be compared with existing flux models.
A similar post-flight analysis has just been started by ESA for the Hubble Space Telescope solar array that was retrieved during the first HST servicing mission in December 1993.
Once the Eureca and HST studies are complete, the present meteoroid and debris flux models will be re-examined and, if shown to be necessary, updated and/or refined.
Figure 8. Spacial distribution of the impacts on the front side of the ten solar-array panels