The article says 'Gnu Debian Linux 6 (Stable)'. However Debian 6 is "oldstable" since May 6th (the article was written after the release).
The new stable release is Debian 7 or 'Wheezy'.
I wonder why NASA didn't pick Wheezy though. Then again, at the time of the decision the prospect of wheezy being released as stable was probably still a wet dream.