Nasa's Atlantis space shuttle ready for final voyage
The US space shuttle Atlantis is about to undertake what is expected to be its final mission before retirement.
The vehicle is on the launch pad at the Kennedy Space Center ready to lift off at 1420 local time (1820 GMT).
Big crowds are anticipated on the roads leading to the Nasa facility and on the beaches of Florida's Space Coast, all eager to catch a piece of history.
Atlantis will be delivering a Russian module to the space station, as well as batteries and a communications antenna.
The crew of six say they are very aware of the significance of the moment but are concentrating on the job they have before them.
"In a lot of ways you can't afford to get too distracted," said Ken Ham, who will command Atlantis.
"This is the kind of thing that's going to hit all of us after the mission, when we realise what part in history we played. I think the space shuttle is the single most incredible machine humanity has ever built."
US President Barack Obama has announced a new exploration policy that would take humans beyond the International Space Station (ISS), beyond even the Moon, to asteroids and to Mars.
The shuttles, which have been working in space since 1981, are being retired to museums; and Nasa is being asked to pass the role of taxiing astronauts to and from the ISS to private companies and to concentrate its efforts on developing the vehicles to reach more distant targets.
Three more shuttle sorties remain, including Atlantis's mission.
The Discovery orbiter is aiming for a final flight in September, with the Endeavour ship scheduled currently to conclude the shuttle programme in November.
Friday's lift-off will be the 32nd for Atlantis. Notable achievements in its 25-year career have included launching interplanetary probes from orbit and leading the Shuttle-Mir programme which saw the ship visit the Russian Mir space station more times than any other ship in the fleet.
The latest mission to the ISS will require Atlantis to carry up a 7m-long (23ft) docking and storage module known as Rassvet (Russian for "dawn").
The shuttle's cargo bay will also contain a large rack structure holding six new batteries for the orbiting platform, as well as a spare communications Ku-band antenna, and a tool tray for the station's Dextre robot system.
These items will be placed on the exterior of the platform during three spacewalks.
The trickiest moment of the mission is likely to come on flight day five when the Rassvet module is attached to the underside of the station. Russian modules are normally flown into their berthing positions, not lifted into place by a robotic arm.
The Atlantis crew has to be sure they apply sufficient pressure with the arm to engage the docking mechanism on Rassvet.
The British-born US astronaut Piers Sellers will be directing robotic operations.
"We're going to be pretending to dock this like a Soyuz or Progress spacecraft," he said.
"We're going to use the arm and very carefully approach the docking cone, and we're going to fool Rassvet into thinking it's docking itself. That's how it's going to activate all its latches and hooks."
Weather forecasters say there is a 70% chance of fine conditions at lift-off time, with low cloud being the one concern.
After it returns from the 12-day mission to the ISS, Atlantis will not go straight to a museum. It will instead be prepared as a standby shuttle ready to go rescue the astronauts on November's Endeavour flight should they get into trouble.
Nasa has not excluded the possibility that it could yet fly out this standby shuttle to take additional spares and supplies to the space station.
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