Humans evolved to live on Earth, and to move around under the influence of Earth’s gravitational force. The acceleration of gravity is defined as 1 g and equals 9.81 m/sec2. It is technically known as 1 gz, because the force acts through the z axis – through the head and feet for an upright human (Fig. 1). Humans (like other animals) can easily cope with the changed patterns of the various accelerative forces involved in running, jumping and swimming. Mechanised travel causes more difficulty, because it involves large variations in accelerative forces in one or more axes. Humans can adapt to these changes to some extent, but they usually show an initial impairment in motor skills, and may suffer from motion sickness.
Astronauts in orbital spaceflight live in a microgravity (low g) environment, which is close to zero gravity (0 g) because the acceleration of the spacecraft cancels out that of Earth’s gravity. There is thus no constant accelerative force in any axis, and no gravitational ‘up’ or ‘down’. The same effect can be produced for about 20–30 s during parabolic flight, but this is preceded and followed by about 20 s of hypergravity (high g) of up to 2 g (Fig. 2). Repeated parabolas offer an opportunity to examine perceptual and motor changes during both high g and low g, and compare them with performance under 1 g during straight and level flying. However, it is difficult to adapt to rapidly changing g levels, and only prolonged spaceflight allows for the study of long-term adaptation to microgravity.
Changes in g affect many aspects of human physiology, only some of which are relevant to haptic perception. Perceptual-motor performance is usually slower in orbital or parabolic flight than on the ground, and there may be several reasons for this: a floating or poorly restrained astronaut has difficulty executing manual tasks; microgravity may directly affect the control of limb movement; and the general stresses of spaceflight may affect cognitive and other functions. Current research suggests that perceptual-motor performance is impaired rather than cognitive performance . This chapter concentrates on the direct effects of microgravity on hand control and haptic perception.