The radiation environment inside a shielded volume is highly complex, consi
sting of both charged and neutral particles. Since the inception of human s
pace flights, the charged particle component has received virtually all of
the attention. There is however, a significant production of secondary neut
rons, particularly from the aluminum structure in low earth orbiting spacec
rafts. The interactions of galactic cosmic rays (GCR), and solar energetic
particles with the earth's atmosphere produce a non-isotropic distribution
of albedo neutrons. Inside any reasonable habitable module, the average rad
iation quality factor of neutrons is about 4-5 times larger than the corres
ponding average quality factor of charged particles. The measurement of neu
trons and their energy spectra is a difficult problem due the intense sourc
es of charged particles. This paper reviews the results of Shuttle flight e
xperiments (made during both solar maximum and solar minimum) to measure th
e contribution of neutrons to the dose equivalent, as well as theoretical c
alculations to estimate the appropriate range of neutron energies that cont
ribute most to the dose equivalent. Published by Elsevier Science Ltd.