Scene Context: During their first hours in weightlessness, the crew experiences severe nausea and facial swelling ("Puffy Face, Bird Legs" syndrome).
Essence of the Phenomenon: On Earth, gravity constantly pulls blood and other bodily fluids toward the lower extremities. The cardiovascular system has evolved to work against this pull, exerting additional effort to pump blood up to the brain. In microgravity, hydrostatic pressure vanishes, but the heart continues to pump in its "Earth-normal" mode.
Scientific Basis: This triggers a rapid cephalad fluid shift (the upward redistribution of bodily fluids). Approximately 2 liters of fluid migrate from the legs into the thorax and head. This induces mucosal congestion, elevated intraocular pressure, and characteristic facial puffiness. Simultaneously, the vestibular system, deprived of a gravity vector, sends chaotic signals to the brain that contradict visual inputs. The brain interprets this profound sensory conflict as neurotoxic poisoning, deploying its only available defense mechanism: severe nausea.
Current Limitations: No amount of Earth-based training grants 100% immunity to SAS. Pharmacological countermeasures can suppress the nausea but carry the side effects of drowsiness and diminished reaction times—a critical liability during complex orbital maneuvers.