The Collaborative International Dictionary of English v.0.48:

astronautics \as`tro*naut"ics\ n.
   The theory and practice of navigation through the upper
   atmosphere or outer space; the science of travel beyond the
   Earth's atmosphere.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

astronautics
    n 1: the theory and practice of navigation through air or space
         [syn: aeronautics, astronautics]