Space ships Space flight 6 Interplanetary voyages 4 Robots 3 Exploration 2 Human-alien encounters 2 McCoy, Leonard (Fictitious character) 2 more ... Science and civilization 2 Scott, Montgomery (Fictitious character) 2 Space colonies 2 Space warfare 2 Sulu, Hikaru (Fictitious character) 2 War 2 Astronauts 1 Enemies 1 Friendship 1 Manned space flight 1 Perseveration (Psychology) 1 Radio 1 Refugees 1 Survival 1 Transmitters and transmission 1 Treaties 1 less ...