The biggest attraction at the Navy League’s annual Sea-Air-Space symposium Tuesday was not on the day’s agenda. Just before 10 a.m., the crowd at the convention center south of Washington began to thin out. People headed outside and looked toward the Potomac River. Soon enough, the space shuttle Discovery flew by, piggy-backing on a modified Boeing 747. The shuttle was traveling from Kennedy Space Center in Florida to a Smithsonian-owned hangar just outside the city. It actually made two passes.