Astronauts Suni Williams and Butch Wilmore's one-week mission has now stretched to more than four.
For now the veteran test pilots are stuck 250 miles above the Earth, but say they are confident their troubled spacecraft will eventually carry them home safely.
"Human spaceflight is not easy," Wilmore said Wednesday.
Williams and Wilmore rocketed into space last month on a test flight of the Boeing Starliner. But as they approached the International Space Station for docking, the spacecraft spang helium leaks and several thrusters stopped working.
Engineers are now diagnosing Starliner's glitches and testing identical thrusters at the White Sands Missile Range in New Mexico to better understand what went wrong.
"There have been multiple issues with every spacecraft that's ever been designed. And that's the nature of what we do," Wilmore told reporters during an on-orbit press conference Wednesday.
NASA says the most optimistic estimate for a return flight home is the end of July, at the earliest.
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In the meantime, Williams and Wilmore are keeping busy as two extra members of the ISS crew, and say they are happy to get an extended stay in zero gravity.
"We've been doing science for them, maintenance, some major maintenance that has been waiting for a little while," Williams said.
NASA insists the astronauts are not stranded, and says they would be clear to fly Starliner home, but only if it's an emergency. Until then, NASA believes it is prudent to gather as much data about the spacecraft before giving it a clean bill of health.
"I feel confident that if we had to if there was a problem with the International Space Station," Williams said, "we can get in our spacecraft and we can undock, talk to our team and figure out the best way to come home."
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