The Collaborative International Dictionary of English v.0.48:

space probe \space" probe\, n.
   an unmanned rocket carrying instruments to explore objects or
   conditions in outer space; -- used mostly for vehicles
   travelling beyond the earth's gravitational field, rather
   than in orbital or suborbital flight.
   [PJC]

WordNet (r) 3.0 (2006):

space probe
    n 1: a rocket-propelled guided missile that can escape the
         earth's atmosphere; makes observations of the solar system
         that cannot be made by terrestrial observation

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "space probe":
   ATDA, ATS, Alouette, Anna, Apollo, Ariel, Atlas-Score,
   Biosatellite, Comsat, Cosmos, Courier, Diapason, Discoverer, ERS,
   Early Bird, Echo, Elektron, Explorer, GATV, Gemini, Injun,
   Intelsat, Lofti, Luna, Lunar Orbiter, Lunik, Mariner, Mars probes,
   Mercury, Midas, Nimbus, OAO, OGO, OSO, Pageos, Pegasus, Pioneer,
   Proton, Ranger, Relay, Samos, San Marco, Secor, Sputnik, Surveyor,
   Syncom, Telstar, Transit, Vanguard, Viking, Voskhod, WRESAT, Zond,
   artificial satellite, artificial satellites,
   communications satellite, geo probe, geodetic satellite,
   geophysical satellite, navigational satellite, probe,
   research satellite, robot satellite, satellite, space satellite,
   spacecraft, sputnik, unmanned satellite, weather satellite