Shuttle Endeavour fixed and ready to go
The American shuttle Endeavour will try again on Monday to launch to the International Space Station (ISS).
Engineers believe they have fixed a heater problem in the aft of the ship that kept the orbiter from making an ascent two weeks ago.
Endeavour is undertaking its final mission before retirement to a Los Angeles museum.
It will carry to the ISS a $2bn particle physics experiment known as the Alpha Magnetic Spectrometer (AMS).
Also in the shuttle's payload bay will be a tray of critical spare parts for the station.
"Endeavour is pretty close to my heart as it's the first space shuttle I flew on in 2001," said shuttle commander Mark Kelly. "So I'm glad it's the one I'm going to fly in last.
"It's the baby of the fleet; it's coming up on 19 years in service - the 25th flight. Twenty-five is a good round number to end on."
Lift-off from the Kennedy Space Center in Florida is timed for 0856 local time (1256 GMT; 1356 BST).
Local police say they expect perhaps half a million people to gather around the spaceport to try to get a good view of the orbiter climbing into the morning sky.
Among the thousands of guests inside Kennedy will be Mark Kelly's wife, Arizona congresswoman Gabrielle Giffords, who is recovering from a gunshot wound to the head.
On Endeavour's return, the only active orbiter left in the US space agency's (Nasa) shuttle fleet will be Atlantis.
It should make a final sortie sometime in July.
Endeavour's last flight will be historic on many fronts, not just because it is the orbiter's swansong:
* Its cargo will be the single most expensive science experiment to be delivered to the ISS
* The mission is expected to see a shuttle crew make a spacewalk for the last time
* Italian crewmember Roberto Vitorri will be the last non-American to fly aboard a shuttle
* And it will see mission specialist Mike Finke become the most experienced American astronaut in history. On his return, Finke's cumulative time spent in orbit will be 381 days, passing that of Peggy Whitson.
The delayed arrival at the station means the sleep patterns of the shuttle's crew members and those of the station's residents will be slightly misaligned.
The daily routine for the Endeavour crew will begin roughly four hours before that of the ISS team.
Three of the six station astronauts will actually depart in a Soyuz capsule while Endeavour is present and return to Earth. It will be the first time some of the "hosts" have left while "guests" are still in the "house".
It will mean just nine astronauts are left aboard the platform to accomplish all the tasks set out for Endeavour's visit.
But Nasa managers have extended the mission from 14 days to 16 days to make sure nothing gets lost from the schedule.
The key objective is the delivery of the Alpha Magnetic Spectrometer, a highly sensitive machine that will carry out a comprehensive survey of cosmic rays. These are the high-energy particles that our accelerated in Earth's direction from all corners of the cosmos.
Scientists hope it can help them to address similar sorts of questions to those pursued in particle accelerators in Earth laboratories - how did the Universe come into being and how it is constructed?
But AMS principal investigator Professor Sam Ting said his space experiment could explore domains beyond the reach of ground technology.
"The highest energy particles are from the cosmos," he told BBC News. "No matter how large the accelerator you build on Earth, you cannot compete with cosmic rays." These particles are absorbed by the atmosphere and the only way to study them properly is to get into orbit, he added.
The AMS machine, built by a collaboration of 16 nations, will:
* look for clues to the nature of "dark matter", the material that shapes galaxies and makes up most of the mass of the Universe
* hunt for evidence of the existence of antimatter, the mirror of the material from which stars and planets are made
* try to catch strangelets, or strange matter, which is built from a slightly different mix of sub-atomic particles to that of normal matter
* characterise the space radiation environment so that the dangers for astronauts and satellites of being in space is fully comprehended
Endeavour's late lift-off has had a knock-on delay to the final flight of Atlantis. It had been scheduled to launch on 28 June, but Nasa thinks it will now be early July before the ship can attempt its ascent.
On completion of the 30-year shuttle programme, America will use Russian Soyuz capsule to fly its astronauts to the ISS.
A number of US national commercial carriers should enter into service around the middle of the decade.
Nasa will then buy seats in these astronaut "taxis". The arrangement will represent a major change in the way the agency does business. No longer will it own the vehicles it uses to go to low-Earth orbit.
This servicing plan should free up resources for Nasa to build an astronaut Space Launch System capable of travelling beyond the station, to the Moon, asteroids and to Mars.BBC News