Here's an explanation of what happened and why the site was still up for some people but not for others.
The NSF web server can be reached via its IP address, which is currently 208.43.2.163. Because that's impossible to remember, and may change, it also has a domain name, nasaspaceflight.com. This means that when you go to the site, your browser needs to convert "nasaspaceflight.com" into 208.43.2.163. It does this by contacting a server at your internet service provider called the DNS (Domain Name System) server.
This DNS server keeps a list of all IP addresses for recently requested domain names, so if you or some other customer has recently visited the site, it'll just get the IP address from its local list and return it to you.
If it doesn't have the IP address already, it will contact the server that controls the .com domain, ask it who is responsible for nasaspaceflight.com, then contact that second server and ask it for the IP address. It'll then store that in its local list, and return it to you.
Every once in a while, the DNS server will throw out the current IP address, and request it again if someone wants to know. This way, if the IP address changes, it isn't handing out the old one forever and misleading everyone.
Now, what happened is that nasaspaceflight.com was registered 10 years ago, for 10 years. That registration expired yesterday, and the DNS was updated to reflect that today. Thus, any DNS server having to re-request the IP address didn't get any answer, and those people got an error message. Others whose DNS server still had the IP address in its list could get to the site however.
This is actually a fairly common occurrence, as DNS registrations often run for a long time (many years), so it's easy to lose track of them. Even Microsoft and Google have had domains unintentionally expire.
There is a workaround if something like this happens: you can tell your computer directly which IP address is associated with a particular domain name by
editing your hosts file. So for those real addicts, adding the following to their hosts file if something like this happens again should get you back on the site:
208.43.2.163 nasaspaceflight.com
208.43.2.163 forum.nasaspaceflight.com
208.43.2.163 www.nasaspaceflight.com
208.43.2.163 content.nasaspaceflight.comOf course, this only works if the IP address hasn't changed in the mean time, so try this only if things are down, don't put it in permanently.
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