ATDA ATS Alouette Anna Apollo Ariel Atlas-Score Biosatellite Castor and Pollux Comsat Cosmos Courier Diapason Discoverer ERS Early Bird Echo Elektron Explorer GATV 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 Siamese twins Sputnik Surveyor Syncom Telstar Transit Tweedledum and Tweedledee Twin Stars Vanguard Viking Voskhod WRESAT Zond artificial satellites exact mates fraternal twins identical twins pair of twins spacecraft twins