US space shuttle Endeavour prepared for its final landing early on Wednesday, becoming the second American shuttle to enter retirement as the program draws to a close after 30 years.
Endeavour and its crew of six astronauts -- five Americans and one Italian -- were to glide in for a nighttime touchdown at Florida's Kennedy Space Center at 2:35 am (0635 GMT), the US space agency NASA said on Tuesday.
The astronauts are wrapping up STS-134, a 16-day mission to the International Space Station, where they installed a $2 billion physics experiment to probe the origins of the universe, and also conducted four spacewalks.
The final flight by US shuttle Atlantis is set for July 8, and NASA planned to send the shuttle rolling out to the launch pad one last time beginning Tuesday at 8:00 pm (0000 GMT).
The event will mark a major milestone toward the end of a three-decade program of human space flight and exploration, and was expected to draw thousands of employees and media.
Meanwhile NASA interim flight director Tony Ceccacci said landing weather for Endeavour looks "very promising," and crosswinds were expected to be about 10 knots, well below the upper limit of 12 knots for a night landing.
"We are very confident that trend is going to stay the same until tomorrow," he said.
If the shuttle is unable to land at 2:35 am, a second landing opportunity would open at 4:11 am (0811 GMT).
Deorbit preparations are set to begin at 9:26 pm (0126 GMT). If NASA decides to go ahead with the landing, the shuttle commander will fire Endeavour's engines about an hour ahead of landing to allow the shuttle to fall out of orbit.
After the shuttle era, the world's astronauts will rely on Russia's space capsules for transit to the orbiting lab at a cost of $51 million per seat until a new US crew vehicle can be built by private enterprise.
NASA has estimated that a shuttle replacement could be expected sometime between 2015 and 2021.
"There is going to be a period of time when Americans aren't flying on US spacecraft, so that's a challenge," said shuttle commander Mark Kelly in an interview with US media, broadcast from space on the last day of Endeavour's mission.
"People leave, you know, engineers and operations people will move on and do other things, so it is the corporate memory that I think I am most worried about," said Kelly, 47.
"But over time, we will get the right mix of people. NASA has an incredible workforce, it is very talented and you know, from the late 1950s to today we have taken on great challenges and we have never failed."
Endeavour is the youngest of the space flying fleet, which also includes Discovery and Atlantis. Discovery retired after returning from its final mission in March.
Two of the original fleet were destroyed by explosions in flight -- Challenger in 1986 and Columbia in 2003. A total of 14 astronauts were killed in the disasters.
Endeavour was commissioned in the wake of the Challenger explosion, and first flew to space on May 7, 1992. It is now ending its 25th mission, after amassing a total of 122.8 million miles (198 million kilometers), NASA said.
The other shuttle is Enterprise, which was a prototype that never flew in space and has long been on display in a museum outside Washington.
Endeavour's crew includes five Americans and Italian Roberto Vittori of the European Space Agency.
During nearly 11 days at the space station, the crew delivered and installed the Alpha-Magnetic Spectrometer-2, which will be left at the space station to scour the universe for clues about dark matter and antimatter.
They also brought up a logistics carrier with spare parts and performed some maintenance and installation work during four spacewalks, the last to be carried out by an American shuttle crew.
A spacewalk is planned during the Atlantis mission in July but it will be done by space station crew, not astronauts who arrive aboard the US shuttle.
After the final shuttle missions, the three spacecraft in the flying fleet and the prototype Enterprise will be sent to different museums across the country.