The space-flight environment: the International Space Station and beyond

Spacecraft in low orbit of the earth travel at Mach 25, or about 8 km per second. These spacecraft orbit the earth once every 90 minutes. The crew controls the temperature in the crew compartment of the space shuttle and modules of the International Space Station, with an average temperature between 21 and 23°C. Much greater extremes occur outside the spacecraft. When on the sun-lit side of the earth, the temperature on the spacecraft or space station can reach over 100°C. Forty-five minutes later, during a night pass through earth's dark shadow, temperatures can plunge to -100°C.2 A high vacuum exists outside the space shuttle and the International Space Station.4 The pressure inside both spacecraft is regulated to 101.34 kPa (14.7 psi) and is equilibrated after docking and before hatch opening. NASA's suit for spacewalks (extravehicular mobility unit) is pressurized to 29.5 kPa with 100% oxygen. The Russian Orlan spacesuit is pressurized to 40 kPa. The transition from ambient pressure to vacuum during airlock depressurization causes considerable decompression stress to spacewalking astronauts. Thus, before spacewalks, astronauts perform an oxygen pre-breathe protocol that reduces the level of nitrogen in the tissues. Astronauts are trained to recognize the symptoms of decompression sickness. Treatment protocols involve repressurization, 100% oxygen therapy and, if necessary, over-pressurization of the spacesuit by use of a bends treatment adaptor. At an orbital altitude of 350 km, the International Space Station is above the earth's magnetosphere. This results in exposure of astronauts to higher fluxes of ionizing radiation. The primary radiation sources are galactic cosmic rays (energetic particles from outside our solar system), particles trapped in the earth's magnetic field (the Van Allen Belts) and solar energetic particle events (solar flares).5,6 High-energy protons and heavy ions emanate from the Sun and elsewhere in the cosmos. Even higher energy secondary particles (protons, neutrons and heavy ions) are produced when the incoming radiation strikes the spacecraft hull.