The two NASA astronauts docked with the orbiting lab June 6 for what was supposed to be an eight-day mission, but their return home may be delayed for months in what has become a star-crossed test flight for Boeing's new Starliner capsule.

Not only was the launch of the spacecraft with astronauts aboard for the first time repeatedly delayed because of multiple problems, but NASA and Boeing are taking a cautious approach in returning the pair to Earth largely because of five thrusters that malfunctioned during docking.

Four of the tiny engines that direct the craft in space are now working properly, but engineers don't have a clear understanding of what caused the shutdown, so they have decided to conduct ground tests at NASA's White Sands Test Facility in New Mexico, NASA officials said. The tests will put a Starliner thruster through its paces in a replicated space environment.

The delay also will allow engineers to further study a helium leak in the capsule's propulsion system that was first detected before launch and worsened as Starliner made its way up to the space station roughly 250 miles above Earth.