Large software installers are often split into smaller "parts" (part01, part02, etc.) to make them easier to download or share. To extract the contents, you generally need all parts (usually through the final part) saved in the same folder.

While older versions were free, the most stable and updated versions (including 0.990 variants) are primarily distributed via Steam . Important Security Note

The string v0.990.45.1940 identifies a specific beta or early-access build of SpaceEngine. About SpaceEngine

Because this is part of a split RAR archive (indicated by .part03 ), you cannot open or use it as a standalone file. Here is the essential context regarding this file type and the software: What is this file?