Here is a site for FREE Documentaries. http://documentaryheaven.com/category/space/
To the right side of the screen scrolling down you can see all of the categories noted on this site.
Not of Caution: Documentary Heaven does have some videos in the "lifestyle" category that might not be appropriate for K-12 use so you might not want to have students search the site on their own.
courtesy post of Freetech http://www.freetech4teachers.com/2011/09/documentary-heaven-watch-free.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+freetech4teachers%2FcGEY+%28Free+Technology+for+Teachers%29