[mxvzr]

> there's not much else out there like it

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...

[0] http://orbit.medphys.ucl.ac.uk/

[garaetjjte]

> The physics model is more advanced too: I believe halo/lissajous orbits are possible, it has lagrange points & more.

There's mod for KSP for that: http://forum.kerbalspaceprogram.com/index.php?/topic/62205-w...

[emmelaich]

Another one in the same genre is Space Engineers - http://store.steampowered.com/app/244850/

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.

Haven't played it myself, but my son loves it.

[unspecified]

I just wish Orbiter was available on non-Windows. KSP is available on all of Windows, Mac, and Linux.