Starfleet
Starfleet explores space using space vessels called starships.  The USS Enterprise is the most well known starship, and there have been several of them in Starfleet from the 22nd Century through at least the 24th Century.  One of the most famous Starfleet captains is James T. Kirk (from the television series Star Trek).  Another famous Starfleet captain is Jean-Luc Picard, the captain of the USS Enterprise-D.