The Gravity of Microgravity
On April 12, 1961, 27-year-old Soviet cosmonaut Yuri Gagarin entered his tiny Vostok spacecraft atop the massive R-7 booster and rocketed into Earth orbit in his legendary three-orbit, 108-minute flight. Meanwhile the Western medical establishment was in the midst of a debate about whether human beings could tolerate extended periods of near weightlessness. The dramatic physiological effects of microgravity are multifaceted, but over the short term they are tolerated remarkably well by the human body. Prolonged weightlessness does not seem to impose a clearly identifiable limitation to survival, but it has been the source of concern in at least six areas of human biology: space sickness, cardiovascular function, lung function, immune function, loss of bone and muscle mass, and interactions with radiation.
A great deal of effort has been devoted to studying the effects and mechanisms of microgravity, but nearly half a century into the Space Age, debate continues about the mechanisms for some of them and about how important they will be in future missions of long duration (Fitts et al., 2000). To date the longest space mission has been the 437-day epic of Valeri Polyakov, the Russian physician– cosmonaut aboard the MIR station in 1994 and 1995. Indeed, as of 2002 only sixteen people had spent a total of more than one year in space (Fig. 18.1). However, such missions have provided evidence that certain effects of microgravity have more important implications than do others for long-duration spaceflight.