I read on another Disney fan board that Space Mountain will be opening late and closing early on certain dates due to maintence. Below is the info from the site:
"Starting July 10, Space Mountain will open late on Mondays and Wednesdays, and close early on Sundays and Tuesdays due to scheduled maintenance:
Late openings (10am) - July 11, 13, 18, 20, 25 & 27
Early closing (10pm) - July 10, 12, 17, 19, 24 & 26 "
If your visiting during these dates, make note of this.