✓ If you enjoyed Mars 2120, expect deep RPG systems, open world exploration and melee combat
≠ Where it differs: brings deep simulation, sandbox freedom and turn-based strategy, while skipping action RPG combat
A game from the X series of games.
The whole series consists of four main games - X: Beyond the Frontier, X2: The Threat, X3: Reunion and X Rebirth - and the sequels to them.
Sequels, which include X3: Terran Conflict, can be considered as stand-alone games, their difference from the "main" games is that the sequels develop the ideas that first appear in the "main" games.
It predates Mars 2120 by 16 years, with what that implies technically.