Shuttle: Nasa presses on with last flight
Nasa is going to give shuttle Atlantis every opportunity to launch on Friday despite a poor weather forecast.
It rained heavily over the Space Coast on Thursday and meteorologists expect conditions at the 1126 (1526 GMT; 1626 BST) lift-off time to be a "no-go".
Mission managers nonetheless gave the order to fill the orbiter's giant external tank with propellants.
They hope fortune will smile on the last ever shuttle mission, and that a weather window will suddenly open.
Forecasters believe there is a 70% chance now that stormy weather will prevent the shuttle getting away on Friday.
Whenever it launches, four astronauts will make the historic ride to orbit in Atlantis: commander Chris Ferguson, pilot Doug Hurley, and mission specialists Sandy Magnus and Rex Walheim.
Just after 0730 (1230 BST), they left their crew quarters and departed for the launch pad in their "astronaut van". The astronauts were briefed on the weather situation before they left.
They are now strapped into their seats in the shuttle's cockpit, in preparation for launch.
Although conditions have been "green" all morning, meteorologists are concerned that shower clouds, and even storms, could bubble up just at the moment of lift-off.
Huge crowds have been making their way to Cape Canaveral since before dawn. Local police think as many as three-quarters of a million people may try to get close to the Kennedy Space Center to get a good view of the ascent.
The final flight
Nasa launch managers are mindful of the impact these crowds could have on their workers' ability to do their jobs.
If they call off Friday's attempt close to the planned launch time, it is likely another attempt will not be made until Sunday.
"If we get into a scenario where we scrub very late in the count, in order to provide crew rest we may elect to go 48 hours to give our teams time to get home and back to work," said Jeff Spaulding, Nasa test director.
"With the amount of [spectators] we are expecting - upwards of half-a-million to three-quarters-of-a-million folks in the general area - getting home is going to be very challenging."
Nasa has Friday, Saturday and Sunday available to make an ascent. There is a 40% chance of favourable weather on Saturday and a 50% chance on Sunday.
If the shuttle cannot get off on any of those three days it may then have to stand down until the 16th to make way for the launch of a Delta rocket on the nearby Cape Canaveral Air Force Station.
The Delta has the launch range booked to carry a new GPS satellite into orbit.
But discussions are taking place between Nasa and the US Air Force, and it is possible that if Atlantis needs a Monday opportunity, the military will oblige.
The shuttle's mission is to deliver more than 3.5 tonnes (8,000lb) of supplies to the International Space Station (ISS).
A third of the load is food and it will ensure the platform has a year's worth of consumables aboard to sustain its residents.
Friday's ascent would be the 135th and last in the 30-year orbiter programme.
Nasa has decided to retire its shuttles because the vehicles are too costly to maintain.
The agency believes a more affordable approach to getting astronauts to the ISS can be achieved by contracting out their transport to private companies.
The first of these commercial carriers is expected to enter service sometime in the middle of the decade.
This policy, Nasa hopes, will free resources to invest in a new capsule and rocket capable of sending humans beyond the space station to destinations such as the Moon, asteroids and Mars.