As the countdown clock ticks towards the scheduled launch on Saturday night at Cape Canaveral, Florida, space agency workers who have toiled for three years and spent $1.3 billion (£700 million) trying to save the shuttle from the scrapyard have had one more challenge thrown at them — this time by Nature.

But the lightning strikes that have delayed the fuelling process are nothing compared with the rumblings from within Nasa ranks over warnings that the vehicle may not be fit to fly — and the storm that will be unleashed if they are proved right.

Two senior Nasa officials, Chris Scolese, the chief engineer, and Bryan O’Connor, the chief safety officer, have refused to sign off on mission STS-121, contradicting the assessment of others that the vehicle is “go for launch”.

“I am a no go,” they both wrote on the shuttle’s flight-readiness certificate, crossing out a pre-printed statement that read: “I concur with proceeding with this mission.”

Mr O’Connor said that his prime concern was Nasa’s failure to redesign sections of the fuel tank known as the ice-frost ramps that have proved problematic in tests. “This got into the area of what I would say is unacceptable risk,” he said.

For the family of Piers Sellers, a British astronaut who is among the crew of five men and two women, the launch will be a tense affair. They lost friends among the seven astronauts who were killed in the 2003 Columbia disaster, which was caused by the same problem that engineers fear will recur on Discovery: debris falling from the fuel tank. If any of the pieces strike the orbiter during launch, the shuttle’s homecoming could be in jeopardy.

Engineers estimate that there is a one in 75 chance that the heat shield could become damaged and a one in 100 chance that it could cause catastrophic impact around the landing-gear doors and other critical areas.

“I’ll just stand there with my fingers crossed and my arms round my kids,” said Dr Sellers’s wife, Mandy, who plans to watch from the roof of the launch control room at the space centre with daughter Imogen, 21, son Tom, 18, and dignitaries including Dick Cheney, the US Vice-President.

She added: “After Columbia we are all far more wary . . . there’s something there that wasn’t before. I’m paying more attention to the details. But so many people worked so hard on this and I’m convinced they wouldn’t launch it if it wasn’t safe.”

Dr Sellers’s mother, Lyndsay, 78, has flown in from Elstead, Surrey, along with her other four sons and their families. “Every little problem has been talked over the dinner table for a long time,” she said. “There’s a ‘try-not-to-think-about-it’ kind of feeling. I have spent an awful lot of time talking to God — He’s getting an earful from me at the moment.”

Michael Griffin, who took charge of Nasa after the Columbia tragedy, has said that he does not believe the crew is at risk, but has conceded that if the vehicle were irretrievably damaged when it reached orbit, a rescue mission could be required.

In the Vehicle Assembly Building three miles from the launch pad, Discovery’s sister ship, Atlantis, is being tuned up, just in case. Enough food and water has been delivered to the International Space Station aboard a Russian Progress vehicle to help to sustain seven extra guests for up to 81 days, should the need arise.

The longer-term implications of failure would be severe. The shuttle fleet is not scheduled to retire until 2010, by which time it must fly 16 more missions to complete the construction of the space station. But Mr Griffin said: “If we were to lose another vehicle, I would tell you right now that I would be moving to figure out a way to shut the programme down. At that point, we’re done.”

Dr Sellers arrived in America with his wife, from Hebden Bridge, Yorkshire, in 1982, with three suitcases and $60 between them. He worked for Nasa for 14 years as a climate change scientist before joining the astronaut corps in 1996. He has discussed the risks with his family.

“Most of the time, like most of the families, they don’t think about it,” he said. “Most of the days go by and you can almost pretend you’re doing a normal job. You know the risk is there. It’s not huge, but it’s definitely present.”

The shuttle will spend up to 13 days in orbit, circling Earth once every 90 minutes at a speed of 17,500mph, and dock with the International Space Station to drop off more than 5,000 pounds (2,300kg) of supplies.

Dr Sellers, who earned a degree in earth sciences at Edinburgh University and a doctorate from Leeds University, will spacewalk outside the shuttle as it races 250 miles (400km) above Earth to test new repair methods on its thermal shield and fix a broken piece of equipment on the space station.

He traces his fascination with space travel to his childhood, when his father used an orange to represent Earth as he described how the space pioneers orbited the planet. “I followed the whole Gemini programme and the Apollo landing on the Moon, which just completely captivated me.” he said. “And from then on, there was always the hope that I would get to do this.

“Fabulous.”

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