Traveller is meant to be hard sci fi, not science fantasy.
Marc's been pretty explicit about it being based upon 50's and 60's space opera, not "hard sci fi"... it gained a rep for hard SF only because the early competition was pretty much just D&D with spaceships. A look in the back of Supplement 4 shows the kinds of series that were in the GDW staffers' minds... most of which are in the space opera genre.
Space Opera (the RPG) is at roughly the same hardness, but much crunchier rules and different FTL assumptions. Circa 1981. It's explicitly a reaction to Traveller and wanting crunchier rules.
Space Master likewise shares the SF hardness, but has full rules compatibility with RoleMaster. Mid 1980s. And actually ruleswise, more consistent than CT. I've heard it described by some as "Bad D&D in Space"... but it plays well enough.
Metamorphosis Alpha (1975) is a fantasy-in-a-spaceship.
Gamma World is essentially also Met Alpha 2
Starfaring (1975) is silly; play the ship as your character, the party as a convoy, people as tools of the ship.
Empire of the Petal Throne (1976) is D&D on a distant world, with different local physics. It's explicitly stated that at some point, people were taken from earth to Tekumel... Seems pretty much just Sci-Fi used as excuse for pure fantasy.
Starships & Spacemen is harder SF than MA, GW, Starfaring, and EPT, but well less than CT.
Mechanoid Invasion (1981) initially is sci-fi... but adds more and more fantasy per expansion. It's the first Palladium game released. While softer than CT, SO, and SM, it's harder than MA, GW, Starfaring, S&S, and EPT...