Orbiter [0] is commonly mentioned. You trade a bit of the "fun" angle of KSP for more realism. The physics model is more advanced too: I believe halo/lissajous orbits are possible, it has lagrange points & more.
> with a world that has a ton of depth to explore
Very deep: on Duna's surface (KSP's Mars equivalent), there is a rock emitting an SSTV signal...
There's mod for KSP for that: http://forum.kerbalspaceprogram.com/index.php?/topic/62205-w...
Still pre-release but very playable, 4/5 stars.
Space Engineers is a sandbox game about engineering,
construction, exploration and survival in space and
on planets. Players build space ships, space stations,
planetary outposts of various sizes and uses, pilot ships
and travel through space to explore planets and gather
resources to survive.