The US space agency has begun a three-day countdown to the launch of its next space shuttle mission.
Discovery is "go" for launch on Saturday, though delays are likely
At 2200 BST (1700 EDT) on Wednesday, launch team members at Kennedy Space Center in Florida set the clock running ahead of Saturday's planned lift-off.
The shuttle Discovery is set to visit the International Space Station on a 12-day mission to deliver supplies and equipment and test safety improvements.
But Nasa says there is a strong chance of weather delaying the launch.
Thunderstorms and anvil clouds brought the threat of lightning strikes, the agency said, estimating the chance of postponement at about 60%.
During a countdown status briefing at Kennedy, Nasa test director Jeff Spaulding announced: "Discovery is in excellent shape, and we're tracking no issues in our preparation at this point.
"Our teams have been working tirelessly during this last year to help make this flight and all shuttle flights as safe as possible for the crews."
In the run-up to launch, some final "tireless work" for the five-man, two-woman crew of Discovery will involve training for the descent phase, Florida Today newspaper reports.
Mission commander Steve Lindsey and pilot Mark Kelly will practise landing in a Gulfstream aircraft modified to mimic the shuttle's steep trajectory during final approach.
Fellow astronauts Mike Fossum, Stephanie Wilson and Piers Sellers will be at the pad, training to take pictures of the shuttle's re-designed external fuel tank once it is jettisoned from the orbiter nine minutes into flight.
It is only the second shuttle launch since Columbia broke up as it re-entered the Earth's atmosphere in 2003.
A suitcase-sized foam chunk broke away from the external tank on lift-off and punched a hole in Columbia's wing.
This allowed super-heated gases into the wing during descent towards Earth on 1 February 2003, tearing the craft apart.
Foam was also shed during the first post-Columbia launch in July 2005. This forced Nasa managers to ground the fleet while further modifications were made to the tank.
At a Flight Readiness Review on 17 June, Nasa managers were split on whether the problem was fixed.
The agency's chief engineer Christopher Scolese and its top safety officer Bryan O'Connor each cast a negative vote for Discovery's lift-off on 1 July. Their concerns centred on the risk of losing the orbiter, but not its astronaut crew.
The option of using the International Space Station (ISS) as a lifeboat if the shuttle was damaged provides this mission with additional security, Scolese has said.
On this mission, a new crew member, the European Space Agency astronaut Thomas Reiter, will be dropped off at the station for a six-month-long stay.
If the flight is successful, Nasa plans another 16 missions to finish construction of the half-built ISS, and - potentially - one last servicing call to the Hubble Space Telescope before the shuttle fleet is retired in 2010.
Nasa's launch team is conducting the countdown from the newly renovated Firing Room Four of Kennedy's Launch Control Center.