Space shuttle Atlantis is ready to make one last descent to Earth.
The orbiter and its crew of four are scheduled to hit the runway at the Kennedy Space Center in Florida just before sunrise on Thursday.
At "wheel-stop", the ship will complete its 13-day mission to the space station; but more significantly, it will close the book on Nasa's 30-year shuttle programme.
Atlantis, like Discovery and Endeavour before it, is being retired.
The US space agency is turning to the private sector for future astronaut transport services, hoping that a number of commercial ventures will emerge in the coming years to ferry crews to and from the low-Earth orbit.
Atlantis has two opportunities to land on Thursday, with the first timed at 0556 EDT (1056 GMT). Weather forecasters expect conditions to be near perfect for the attempt.
Officials have resisted calls to target the second opportunity at 0733 and the better light it would afford shuttle fans to see the ship sweep in over Florida's Space Coast.
"We want to stick with what we do normally with our processes and procedures," said LeRoy Cain, the chair of the space shuttle's mission management team.
"Whether it's 40 minutes before sunrise or an hour after, it will be a good landing at Kennedy Space Center. We're going to stick with our plan."
Atlantis undocked from the International Space Station (ISS) on Tuesday. It had delivered some four tonnes of supplies to the platform, including a huge load of food.
Its landing is sure to be wound up with plenty of emotion - not least because it will trigger a big lay-off of contractor staff that have supported shuttle operations. More than 3,000 will be let go within days of touch-down.
The programme itself does not officially end for a month, but even then it is likely to take a couple of years to close all activities, such as the archiving of decades of shuttle engineering data.
For Atlantis, its retirement will be spent as a static display at the Kennedy visitors complex.
Assuming the ship takes the first landing opportunity on Thursday, its career statistics will read 33 flights, 307 days in space, logging 4,848 orbits, for a total distance travelled of 202,673,974km (125,935,769 miles).
Mind the gapThe end of the shuttle programme will leave a gap in America's astronaut-launch capability that is unlikely to be filled for at least three or four years.
In the short-term, the US will use Russian Soyuz rockets and capsules to fly its astronauts to the ISS.
A number of US national commercial carriers are then expected to enter service around the middle of the decade.
Nasa will buy seats in these astronaut "taxis". The vehicles are expected to be much simpler - and a lot cheaper - to operate than the shuttle.
"We're not asking industry to do anything cutting-edge here; what we're asking US industry to do is replicate the capabilities that were developed in the 1960s to do the relatively simple task of carrying a few people to orbit," said Dr John Logsdon, professor emeritus of the Space Policy Institute at The George Washington University. "That doesn't require new, or emerging or breakthrough technologies," he told BBC News.
This more commercial approach to low-Earth orbit activities is intended to free up resources for Nasa to build a new capsule and rocket that can take humans beyond the space station to destinations such as the Moon, asteroids and Mars.
The conical ship, known as Orion, has already been defined and is in an advanced stage of development. The rocket, on the other hand, is still an unknown quantity.
US Congress has told the agency what its minimum capabilities should be. However, Nasa is currently struggling to put those specifications into a concept it says can be built to the timeline and budget specified by the politicians. It promises to detail the rocket's baseline design before the summer is out.
The retirement of the shuttle will bring major changes to Florida's Space Coast
Source: http://www.bbc.co.uk/go/rss/int/news/-/news/science-environment-14220423
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