Usually, a major attraction goes through a period called soft opening for several days or weeks prior to its official opening. This is not advertised, and the ride may shut down at any time during the day. It may not necessarily reopen during the same day either. I don't know the official reason why this is done, but I assume it is to prepare the cast members to operate the attraction at full capacity so that things go smooth on the official opening date and beyond.
Mission: SPACE is somewhat of an oddity because it has an officially announced preview period that is going to last from August 15 to October 8, with the official opening being October 9. It seems increasingly likely that there will be a "traditional" soft opening period prior to August 15, which based on past experiences may begin as early as this month. THAT PART IS NOT OFFICIAL, HOWEVER!
So, anyone who will be visiting WDW after August 15 will definitely get to experience Mission: SPACE until something strange happens. People visiting before that *may* get to ride it. When/if soft openings start, it'll be all over these boards and I'll add it to my Mission: SPACE FAQ, so you probably won't miss it.