From the description it seems like exactly what OpenStack provides (even regarding architecture, access to own network, etc.). The only place where it's explicitly mentioned is the blog (http://nebula.nasa.gov/blog/2011/may/nebula-team-releases-ne...) They might be running something more recent than Bexar though, since that one's rather old and doesn't provide many features required for large-scale operation.
Can you point at some confirmation of it? I could barely find the name "openstack" anywhere on their site and the project itself is pretty basic really (unless they follow trunk).
The advantage is probably just that it's owned by NASA rather than by a private entity. This could allow it to be used for sensitive or classified applications, or for space related applications (which could be classed as munitions, which [I'm not a lawyer, but i'm guessing...] means putting them on AWS could count as "exporting" them).
I'm trying to figure out exactly what this is, to be honest. To me it looks like their version of Amazon Web Services, for sharing compute resources exclusively within their community. But I can't tell whether it's just providing secure hardware to throw VMs at or if they've built the support infrastructure like a fault tolerant file system as well.
Edit: From http://nebula.nasa.gov/services/ it appears they have a distributed file system, but otherwise it's simply a set of networked machines.
Q Who can use Nebula? A Nebula is available to NASA’s internal project groups. Nebula is not available for use by private industry or by the public. Nebula is also currently acting as a test-bed for Federal Cloud Computing technology for The Office of Management
