Shuttle Endeavour set for final flight
Nasa is preparing to launch its Endeavour shuttle one last time.
The youngest of America's reusable orbiters is set to deliver a $2bn (£1.2bn) particle physics experiment to the International Space Station (ISS).
US President Barack Obama and his family are expected at the Kennedy Space Center to witness the lift-off.
Also present will be Endeavour commander Mark Kelly's wife, Gabrielle Giffords, the Arizona politician shot in the head by a gunman in January.
Lift-off for Endeavour is targeted for 1547 local time (1947 GMT; 2047 BST). Once it completes its 14-day mission, Endeavour will be retired to a museum in Los Angeles.
That will leave just the Atlantis shuttle still in active service. It is expected to make a final outing in the next few months.
Endeavour was built to replace the Challenger ship which was lost, along with her crew of seven, in a catastrophic accident in 1986.
This final flight is the vessel's 25th overall, having already clocked a cumulative distance in space of 166 million km - an expanse just greater than that which divides the Earth and the Sun.
"We're going to take Endeavour out for about five or six million more miles," said US Navy Captain Kelly.
"It's already got about 110 million miles on it, and after 25 flights we'll hopefully land here [at Kennedy] and then Endeavour's done with its service to the country."
Notable highlights in the ship's career have included the first construction mission to the ISS by a US orbiter, and the first servicing mission to the Hubble Space Telescope. The latter carried up the equipment to correct the flaw in the observatory's vision, enabling it to take breathtaking images of the cosmos.
Nasa is hoping the payload inside Endeavour for its final mission will deliver equally astonishing science.
The Alpha Magnetic Spectrometer (AMS) has taken 17 years to prepare for launch. It has been constructed by an international collaboration of researchers across 16 nations.
The machine will be fixed to the top of the ISS where it will undertake a comprehensive survey of cosmic rays - ultra-high-energy particles that bombard the Earth from all reaches of the Universe.
It will search for exotic phenomena - anti-matter, dark matter and strange matter. Their detection could yield fascinating new insights into how the cosmos was born and why it takes the form it does now.
"All our understanding of the cosmos has come from measuring light rays," said Professor Sam Ting, the AMS principal investigator.
"Charged particles have never been studied systematically for long durations. Balloons and small satellites just hint at what's going on.
"This is the first time we've put in an accurate detector to study the other half. What you will see we don't know because you're entering a new field with a precise detector."
The survey should also provide the best assessment yet of some of the radiation dangers that exist in space. This information will prove invaluable as space agencies attempt in the coming decades to send astronaut missions to asteroids, the Moon, Mars and beyond.
Endeavour's final mission marks an important moment also for the European Space Agency (Esa). Italian Roberto Vitorri is the sole non-US citizen in the crew and will be the last non-American to board a space shuttle (Atlantis will have an all-American crew of four).
Esa, which does not have its own crew transportation system, has been heavily reliant on the orbiters to get its astronauts into orbit.
Vitorri, who will operate the shuttle robotic arm on this mission, will become one of only 24 Europeans to have taken the ride.
Esa astronauts will have far fewer opportunities to experience the journey to orbit in the years ahead as everyone waits for the new generation of US commercial launch systems to come online.
Nasa hopes that these "crew taxis" will help reduce the cost of access to low-Earth orbit.
"The shuttle is an extraordinarily expensive vehicle and it is not that safe, and those are the two reasons why we are moving on to something else," commented Michael Foale, one of the most experienced astronauts in the history of US spaceflight.
"We don't know what that something else is yet, but we're moving on. Because eventually for that amount of money and for that much risk, people believe you can do better - and I do, too; I think we can do better," he told BBC News.
One recent estimate, reported in the journal Nature, calculated that the entire shuttle programme has cost the American taxpayer close to $200bn over the past 40 years.