Simplify the following text for a Grade 6 native English-speaking person.
Original: The Space Shuttle Atlantis ( Orbiter Vehicle Designation : OV-104 ) is now one of the two remaining operational Space Shuttle orbiter in the Space Shuttle fleet belonging to the National Aeronautics and Space Administration ( NASA ) , the spaceflight and space exploration agency of the United States ( the other operational Space Shuttle being Endeavour ) .
Simplified: Space Shuttle Atlantis ( OV-104 ) is the fourth space shuttle run by NASA .