Little value / reg fee. With such a small amount of players in this field, I don't see any of them buying this or a well financed new comer in America calling themselves "American Space Agency" ...unconvinced, and an uninteresting space brand overall. I struggle. IMO
Indeed ''AmericanSpaceAgency'' sounds a bit grotesque. At least USSpaceAgency... even with double ' ''s'' it's better...But then it's not awesome for business either. Companies that can be called this will definitely have the money to get any one-word or short two words they like...
I myself have only one with ''space'' - SpaceContacts.com.
But ''space'' generally is a very cool (and highly competitive!) word, if you can find something worthy in two words - spend some time, brainstorm.