Starbase is described as 'hybrid voxel/vertex-based space MMO with a fully destructible and infinitely expanding universe, with a focus on building and designing spaceships and stations, exploration, resource gathering, crafting, trading, and combat'. There are four alternatives to Starbase for Steam, Windows, Linux and Mac. The best alternative is Space Engineers. It's not free, so if you're looking for a free alternative, you could try StarMade. Other great apps like Starbase are Empyrion (Paid) and Avorion (Paid).
Space Engineers is a sandbox game about engineering, construction and maintenance of space works. Players build space ships and space stations of various sizes and utilization (civil and military), pilot ships and perform asteroid mining.
Avorion is a procedural co-op space sandbox where players can build their own space ships out of dynamically scalable blocks. Fight epic space battles, explore, mine, trade, wage wars and build your own empire to save your galaxy from being torn apart by an unknown enemy.