Starliner still hasn't returned to Earth. The repeated delays may be the next big headache for Boeing.

• Two NASA astronauts are still in space after Boeing's Starliner spacecraft faced delays.
• The spacecraft's plight adds to the pressure on Boeing.
• The company faces questions over safety after a door plug on one of its planes blew out in midair.

When Boeing announced in 2015 that its $4.2 billion spacecraft would be named Starliner, NASA officials hailed the manufacturer's innovation as a "great victory" that heralded the start of a new age of space exploration.