The U.S. space agency says it is preparing to bring the Space Shuttle home Wednesday, before the U.S. military launches a missile intended to destroy a falling spy satellite.
Defense officials have said they will not make the shootdown attempt until the shuttle has landed.
The Pentagon says the satellite is carrying toxic fuel that could pose a public health threat if it reaches the ground.
The crew Tuesday performed several pre-landing checks for Wednesday's return to Earth, including a test of the steering jets that will be used to position the orbiter for reentry.
The returning astronauts include Daniel Tani, whose 90-year-old mother was killed in a car accident while he was on a four-month stay on the International Space Station.
NASA says the shuttle is scheduled to land at the Kennedy Space Center in Florida if weather conditions permit. If the weather changes, the shuttle will aim for a landing at Edwards Air Force Base in California.
The crew has been working with the crew of the International Space Station to install the European Space Agency's $2 billion lab.