  Atom laser experiments with Bose-Einstein condensates (BECs) performed in
ground-based laboratories feature a coherent and directed beam of atoms which
is accelerated by gravity. In microgravity the situation is fundamentally
different because the dynamics is entirely determined by the repulsive
interaction between the atoms and not by the gravitational force. As a result,
the output of a space atom laser is a spherical wave slowly expanding away from
the initial BEC.We present a thorough theoretical study of this new source of
matter waves based on rf outcoupling which exhibits an isotropic distribution
both in position and momentum even for an initially anisotropic trap. The
unique geometry of such a freely expanding, shell-shaped BEC offers new
possibilities for matter waves in microgravity and is complementary to other
matter-wave sources prepared by delta-kick collimation or adiabatic expansion.
Our work paves the way for the upcoming experimental realization of a space
atom laser making use of NASA's Cold Atom Laboratory on the International Space
Station.
