Their return caps a protracted space mission that was fraught with uncertainty and technical troubles and turned a rare instance of NASA's contingency planning - and the latest failures of Boeing's Starliner - into a global spectacle.

On Tuesday, two NASA astronauts who had been in orbit since June, Suni Williams and Butch Wilmore, splashed down in calm, azure waters off the coast of the Florida Panhandle, concluding a saga that had captivated the country since last summer.