Description of the Opportunity

This AO addresses the access to three ''Express Pallet Adapters' able to carry instruments, integrated on one or two of the four NASA-developed Express Pallets. As indicated in Figure 3, the latter are to be attached to specific payload accommodation sites on the ISS truss structure. Depending on the location of the Express Pallet (zenith- or nadir-facing), it will be possible to fly instruments requiring solar or celestial viewing, as well as Earth- or Earth-limb-viewing experiments. Experiment exposure in the ram or wake direction of the Space Station trajectory will also be possible. The final locations of the Express Pallet Adapters available to ESA will depend on the instruments selected following this AO, and on a feasibility assessment of their compatibility with the NASA-selected external payloads.

Figure 3. Close-up of a truss site with two Express Pallets

ESA will provide a coarse pointing device able to compensate for changes in the Space Station's attitude and to point to the Sun with an accuracy of 1 degree. The performance characteristics of this pointing device, as well as technical Space Station interfaces and detailed operational aspects, are described in a Technical Appendix to this AO (Ref. 1). A concept for the pointing device, together with a pre-integrated set of model solar instruments, is shown in Figure 4.

Figure 4. Concept for a pointing device, with a set of model instruments

The Express Pallet accommodates six Adapters (Figs. 5a,b), each of which is able to carry up to 225 kg of payload and has a mounting surface of roughly 1 m x 1.2 m. The maximum allowable payload height is approximately 1.25 m. This height, which is limited by constraints imposed by in-orbit robotic handling of the Adapter, is presently subject to confirmation by NASA.

Figure 5a. Express Pallet with payloads on the six Adapters

Figure 5b. The Express Pallet Adapter

The launch to the Space Station of the first two Express Pallets with pre-integrated payload Adapters, which will also carry the European instruments selected via this present AO, is foreseen for 2001. Consequently, the instrument engineering models and flight models will have to be delivered in 1999 and 2000, respectively. On later Space Shuttle flights to and from the Station, the Express Pallet Adapters will be carried individually on an unpressurised logistics carrier, thus allowing the changeout of payloads on a modular level. Once the assembly and outfitting of the Station are complete, there will be one unpressurised logistics flight per year with the Space Shuttle, carrying both Station and payload hardware. It may be also possible to transport hardware to the Station with other space transportation systems, e.g. the European Ariane-5/Automated Transfer Vehicle (ATV).

The present assumption for this AO is that there will be no payload changeout or robotics servicing at Pallet Adapter level during the three year in-orbit period from 2001 to 2004. Specific payload requirements for changeout during this period would have to be considered on a case-by-case basis. Once deployed from the Shuttle and installed at the truss attachment sites, payloads will obtain the basic resources essential for their operation (electrical power, onboard data management and transmission, attitude knowledge, etc.) via standardised Express Pallet interfaces.

The Space Station's quasi-circular orbit will have an inclination of 51.6° (Fig. 6) and an altitude ranging between about 400 and 450 km for the operational period from 2001 to 2004. The altitude variations are caused by atmospheric drag, which depends on variations in solar activity. The resulting orbital decay will be compensated for by periodic Station reboosts. The Station's attitude will vary by a few degrees per orbit from the ideal local horizontal/local vertical (LH/LV) attitude.

Figure 6. The International Space Station's orbit and ground track

The details necessary for a first assessment of the feasibility of conducting an envisaged experiment or observation onboard the Station, including technical and operational interfaces, natural and expected induced external environment, field-of-view limitations, etc. are described in the Technical Appendix (Ref. 1). This document and an Instrument Proposal Data Sheet (Ref. 2) are designed to assist investigators in preparing their proposals in reply to this AO. Preliminary interface details for instruments to be mounted on the Express Pallet Adapters are described in this document, which also gives preliminary information about ESA- provided payload-support equipment, such as power or data distribution hardware, payload monitoring support plates, etc.