I don't know if HAL is homicidal, suicidal, neurotic, psychotic, or just plain broken.
Why it is hard sci-fi
A direct sequel that respects the rules of the first film. Joint Soviet-American mission to Jupiter, aerobraking through the upper atmosphere to shed velocity, careful attention to fuel budgets and orbital windows. The crew argue about real engineering tradeoffs. Even the diagnosis of HAL gets a procedural answer rather than a mystical one.
Science inside it
Aerobraking, free-return trajectories, crew dynamics under months of confinement, the engineering of restarting a long-derelict spacecraft in deep space. The film also walks through the Roche limit and tidal physics around Jupiter. The HAL post-mortem treats the AI failure as a software fault to be debugged rather than evil.
Spoiler alert
Jupiter is converted into a second sun by an external intelligence, which is the soft note. The hard work happens before that. HAL is reactivated, told the truth this time, and chooses to stay behind to relay messages while Discovery and Leonov burn for Earth.