É mostrado no final que os seres humanos estão vivendo em uma nave espacial orbitando perto de Saturno. Mas quando esse navio foi construído? Não houve menção de nave espacial em grande escala sendo mencionada.
Como o @BobbyAlexander comentou, o navio estava sendo construído nas instalações da NASA que Cooper visita. E a estação espacial que orbita Saturno poderia ter sido um navio completamente diferente - são décadas (ou mesmo um século) depois! A Humanidade na Terra fez muitas outras coisas enquanto Cooper se foi.
Em The Science of Interstellar pelo físico Kip Thorne, o capítulo 31 fala sobre eles usando sua nova teoria da gravidade quântica, descoberta por Murph com a ajuda dos "dados quânticos" transmitidos por Cooper (veja meu responder aqui para obter detalhes sobre as idéias de física envolvidas), para diminuir temporariamente a gravidade da Terra e lançar as enormes colônias fora da Terra:
Early in Interstellar, when Cooper first visits the NASA facility, he is shown a giant, cylindrical enclosure being constructed to carry thousands of humans into space and house them for many generations: a space colony. And he's told there are others being constructed elsewhere.
"How does it get off Earth?" Cooper asks the Professor. "Those first gravitational anomalies changed everything," the Professor replies. "Suddenly we knew that harnessing gravity was real. So I started working on the theory—and we started building this station."
...
How did it get lifted into space? The key, of course, was the quantum data (in my scientist's interpretation, the quantum gravity laws) that TARS extracted from Gargantua's singularity (Chapters 26 and 28) and Cooper transmitted to Murph (Chapter 30).
...
Murph must have figured out how to reduce Newton's gravitational constant G inside the Earth ... In my interpretation, with Newton's G reduced inside the Earth to, say, a thousandth of its normal value for, say, an hour, rocket engines could lift the enormous colonies into space.
As a byproduct, in my interpretation the Earth's core—no longer compressed by the enormous weight of the planet above—must have sprung outward, pushing the Earth's surface upward. Gigantic earthquakes and tsunamis must have followed, wreaking havoc on Earth as the colonies soared into space, a terrible price for the Earth to pay on top of its blight-driven catastrophe. When Newton's G was restored to its normal strength, the Earth must have shrunk back to its normal size, wreaking more earthquake and tsunami havoc.
But humanity was saved. And Cooper and ninety-four-year-old Murph were reunited. Then Cooper set out in search of Amelia Brand in the far reaches of the universe.
Tags interstellar