phase) leads to very large batteries which seriously charge the elements' mass budget. The fact that only very limited experience is available with respect to the long-term LEO behaviour of such large batteries, underlines the criticality of this area for COLUMBUS. Another specific characteristic of the low earth orbit is the plasma environment which has a four orders of magnitude higher charged particle density than the geostationary orbit. This conductive plasma can act as a low-ohmic bypass to the solar array and may cause significant leakage currents and discharges under certain solar array operation conditions. Recent data from Shuttle flights indicate that the amount of atomic oxygen in low earth orbits has an eroding effect on several materials and may also change their thermo-optical data. Specifically organic materials and some metals seem to be affected most and will have to be reviewed for future long missions. Solar array blanket materials and solar cell interconnectors are of this category of materials and hence, need special attention. On the other hand some environmental characteristics for LEO are less severe than for geostationary orbits (GEO). The ionizing particle environment, for example, which is concentrated in the Van-Allen belts, is very moderate compared to GEO missions. Hence, array oversizing in order to cope with end of life power requirements is not so severe (appr. 10% degradation for a ten-year LEO mission). However, LEO radiation is high enough to affect power F.E.T.'s (as e.g. used in solid state switches) such, that it is uncertain whether they are in on- or in off-state. Hence, sufficient margins and measures are to be incorporated in the design in order to eliminate radiation effects. 4. The Solar Array In view of its physical size and its complexity, the solar array is by far the most important element of the power system. The power demands in the multi-kW range can be satisfied best by a semi-rigid or a rigid array as it is used for example on EURECA. This array-type is composed of deployable rigid panels which carry the solar cell strings. Better properties with regard to growth capability offers the flexible foldable array type as used e.g. on the OLYMPUS spacecraft. This solar array is characterized by a thin flexible blanket on to which the solar cells are bonded. In stowed configuration this electrical blanket is folded and stowed in a stowage box at the space craft walls. Once in orbit, special deployment mechanisms can deploy this blanket fold by fold to its full size and keep it under sufficient tension. The present COLUMBUS baseline is a rigid array. However, a foldable array shall be studied as option. The solar array reference configuration for the Resource Module is shown in Fig. 2. The entire solar array (SA) consists of two identical, deployable/retractable wings, two bearing and power transfer assemblies (BAPTAs) including SA drive, the SA harness and electrical/mechanical interface devices. The SA output power shall be 16 kW (end of life). Deployment/retraction of the SA is performed by the extendable and retractable Mast (ERM) developed by Dornier System GmbH. The ERM is constructed such that partial deployment of the SA is possible. The most important components of the solar array are the solar cells which convert the incident sun light directly to electrical power. From aspects such as efficiency, cost
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