Meet the most spaced-out crew in the galaxy.
Why it is hard sci-fi
Whedons short-lived 2002 series committed to a single-system setting. Every planet and moon is in the same star system, terraformed, and reachable on subluminal trajectories. There is no faster-than-light travel. There are no aliens. The crew operate a working freighter, the Serenity, with maintenance, fuel, and contracts as recurring problems. The show is a western pacing applied to a hard near-future-frontier transport setting.
Science inside it
Single-system terraforming, atmospheric processing, in-system subluminal transit, and the operational shape of a small freighter under a colonial economy. The shows one major handwave is the gravity onboard the Serenity, which the show does not attempt to justify.