Previews of Mission Space begin today for Epcot Cast Members. The previews are scheduled to run for several days.
The walls have now been taken down from in front of the attraction. It is now garden by Cast Members. (Thanks to Sean for the photos below)
Update on the staffing situation as I currently have it...
There are 26 CM positions at Mission Space, requiring 90 CMs total to run the attraction on an everyday basis. Operations have currently pulled 50% of those 90 needed cast members. This will be enough for Cast Previews and limited guest previews. The second wave of staffing is set to take place after the July 4 holiday, when the cast allocation will be 100%.