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spaceship rocket galaxy stars nebula space travel science fiction interstellar exploration universe cosmos celestial astronomy astrophysics cosmology spacecraft satellite planet moon asteroid comet meteor black hole supernova star cluster spiral galaxy elliptical galaxy irregular galaxy dwarf galaxy quasar blazar pulsar magnetar white dwarf red dwarf brown dwarf rogue planet habitable planet exoplanet alien extraterrestrial UFO SETI interstellar travel time travel faster-than-light travel