By Jonathan Amos
The Kennedy Space Center in Florida became enveloped in smoke on Wednesday as a fire raged on brushland close to the spaceport's media complex.
At one point, the smoke was so thick it obscured completely the view to shuttle Endeavour sitting on its launch pad.
As helicopters dropped water on the blaze, Nasa said there would be no impact on lift-off preparations.
The US shuttle Endeavour is due to begin its final mission this Friday to the International Space Station (ISS).
It will be delivering a $2bn (£1.2bn) particle physics experiment called the Alpha Magnetic Spectrometer.
Before the drama of the fire, mission managers said they were very pleased with the progress of pre-launch activities.
"We had a management team meeting this morning, a standard meeting on launch minus two days - just to review our status and see where we're at," explained Mike Moses, chair of the mission management team.
"Everything's in great shape; [it was a] very short meeting. The teams all polled 'ready to go' and proceed with the countdown."
Shuttle Endeavour will be using a relatively old external tank for its final launch.
ET-122, as it is known, was originally built in 2002, but was damaged during Hurricane Katrina at its New Orleans manufacturing facility.
It has been refurbished for this mission, not just to repair the storm defects but also to incorporate measures designed to limit the amount of insulation foam that can fall off a tank during launch - a persistent hazard down the years, and one that fatally damaged the Columbia shuttle on launch in 2003.
Mr Moses said Nasa had no reservations about using ET-122.
"If you take a look at it out on the pad, it looks a little more suntanned than other tanks because the ultraviolet [light] makes that orange foam insulation turn a little darker, and it's got some repair spots. But it's in more than good shape to fly."
In what he said was a tribute to re-usability in the shuttle programme, Mr Moses also revealed that a few components in the Solid-fuel Rocket Boosters for Endeavour were also used in the very first shuttle flight in 1981.
The huge interest in the end days of the shuttle programme means large crowds will again gather around the Kennedy Space Center on launch day.
For the Discovery orbiter's final lift-off in February, up to half a million people lined local roads and beaches. The police are expecting perhaps up to 750,000 people to do the same on Friday.
There will be about 40,000 guests inside KSC itself. These are likely to include President Barack Obama and his family.
Also present will be Arizona Congresswoman Gabrielle Giffords, the wife of Endeavour's commander, Mark Kelly. Ms Giffords is currently recovering from a gunshot wound to the head, sustained during a shooting spree by a gunman in her constituency on 8 January.
Mission managers insist these visitors will not be a distraction to the launch teams as they prepare the second-from-last shuttle mission.
Only the Atlantis orbiter will be left in active service once Endeavour returns. Atlantis is expected to make it final journey into orbit sometime in June or July.
All of Nasa's ships are being retired to make way for astronaut "taxis" to be provided by the private sector later this decade.
"The end of the programme is coming one at a time," said shuttle launch director Mike Leinbach. "Discovery is now being taken apart and being made safe for final display, and that will start on Endeavour when she gets home.
"It's kinda sad to see the end, of course, but we're dealing with it. I just hope the next programme comes more quickly than a lot of people are expecting."