Microgravity environment conditions--from Spacelab to the International Space Station.

Since the early eighties Spacelab has been the primary platform for experimentation under the state of virtually weightlessness (microgravity) in spaceflight. The International Space Station (ISS), planned to be operational by end of the century, will extend the scientific opportunities of Spacelab in many respects, e.g. by offering the opportunity for long lasting and continuous microgravity investigations. Stringent design and operational requirements have been defined for the ISS to keep residual accelerations as low as possible. These guidelines are based on investigations (Langbein, Alexander, and others) of upper allowable residual accelerations for most physical phenomena involved in space experiments. To meet these requirements, severe perturbations from operating the Station (reboosts to correct the orbit, docking maneuvers, etc.) are grouped outside dedicated microgravity-experiment phases. For the microgravity-phases, stringent criteria on upper limits of residual accelerations are specified. Recent predictions for the present ISS concept indicate that additional measures have to be taken. The results of the microgravity-studies on Spacelab missions (D-1, D-2, etc.) contain a wealth of valuable information for developing optimal design principles. Passive measures must be applied first ('perturbation reduction at the source' and timelining) before considering complex active isolation concepts which should be reserved for high sensitive investigations only.