<P> The first group consists of the experiments that involve gravitational fields of less than one g, termed hypogravity . Without artificial gravity, a space station or a spacecraft in a spaceflight will be in hypogravity . Therefore, understanding of the effects of hypogravity on the human body is necessary for prolonged space travel and colonization . </P> <P> The second group consist of those involving gravitational fields of more than one g, termed hypergravity . For brief periods during take - off and landing of space craft astronauts are under the influence of hypergravity . Understanding the effects of hypergravity are also necessary if colonization of planets larger than the Earth is ever to take place . </P> <P> Recent experiments have proven that alterations in metabolism, immune cell function, cell division, and cell attachment all occur in the hypogravity of space . For example, after a matter of days in microgravity (<10 g), human immune cells were unable to differentiate into mature cells . One of the large implications of this is that if certain cells cannot differentiate in space, organisms may not be able to reproduce successfully after exposure to zero gravity . </P> <P> Scientists believe that the stress associated with space flight is responsible for the inability of some cells to differentiate . These stresses can alter metabolic activities and can disturb the chemical processes in living organisms . A specific example would be that of bone cell growth . Microgravity impedes the development of bone cells . Bone cells must attach themselves to something shortly after development and will die if they cannot . Without the downward pull of a gravitational force on these bone cells, they float around randomly and eventually die off . This suggests that the direction of gravity may give the cells clues as to where to attach themselves . </P>

Role of gravitation in supporting life on earth