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]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "astronautics":
   aeronautics, aerospace medicine, aerospace research,
   aerospace science, aerospace technology, air service, airline,
   aviation, ballooning, bioastronautics, blind flying, cloud-seeding,
   commercial aviation, contact flying, cruising, escape velocity,
   flight, flying, general aviation, gliding, grand tour,
   interplanetary travel, pilotage, rocketry, sailing, sailplaning,
   science fiction, soaring, space flight, space medicine,
   space travel, space walk, step flight, trip to Mars, winging