Pioneer is a great remake of the original Elite Space Sim.
It simulates the entire galaxy (core systems are hand-built, everything else procedurally generated), allows landing on planets, trading, combat, etc.
It features the original game’s Newtonian physics, so actually arriving safely at your destination is a challenge in itself, similar to flying in Kerbal Space Program. But the HUD gives you all the info you need for that.

Oh, and it’s fully open source and moddable.