In fact, there is a key that you can get without it. If you don't believe me, you can create an archive, and after a pass (just to save people), you can see if there are two keys. After you get all 50 keys, you will open a hidden X-space checkpoint. The mission didn't say what you wanted, but there was actually one person (Cesandra) who could save it. (PS: Actually, it will be cleared after the ninth level. )