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alien extraterrestrial UFO spaceship abduction science fiction fantasy mystery unknown universe cosmos galactic intergalactic extraterrestrial life alien life intelligent life civilization interstellar space travel warp drive wormhole black hole nebula galaxy star planet moon asteroid comet meteor space exploration SETI Drake equation Fermi paradox Great Filter Rare Earth hypothesis alien invasion alien abduction alien contact first contact third kind close encounters UFO sighting Roswell Area 51 Project Blue Book