Seconds after firing its engine, Virgin Galactic’s spaceship was breaking through the sound barrier. As the craft made its ascent, the pilots were pinned against their seats, gasping for breath. G-forces crushed their eyes into their sockets, trapping the supply of blood and fading their vision from colour to black and white.

It was at this point, in the three disorientating seconds it took for the craft to climb from Mach 0.94 to Mach 1.02, that co-pilot Mike Alsbury made what many close to the event believe was the fatal mistake that led to the disintegration of SpaceShipTwo. The pilot, Peter Siebold, survived the 10-mile fall back to earth.

“It will be regarded as one of the most amazing test flight survival stories of all time,” says Virgin Galactic chief executive George Whitesides, paying tribute to the pilot who escaped the accident with just a shoulder injury. “It is truly incredible and we are all incredibly thankful that he was able to parachute safely to the ground.”

Speaking to the Guardian after the accident that threatens to extinguish Sir Richard Branson’s dream of creating a commercial space service in the Mojave desert, Whitesides was able to give only minimal details about how Siebold escaped. But by piecing together his information about previous test flights, accounts from witnesses, investigators and anonymous sources, one week on a clearer picture is now emerging of the sequence of events which led to the disaster.

Debris from Virgin Galactic’s SpaceShipTwo sits in a desert field. Photograph: Sandy Huffaker/Getty Images

SpaceShipTwo was on its most ambitious test flight yet when the tragedy struck last Friday. Siebold and Alsbury were planning to push the craft higher than ever. To reach space, SS2 must fly under its own power for about 60 seconds. In its first three powered tests, its engine had burned for no more than 20 seconds.

“Our plan was to burn for a longer duration than we had in the past,” said Whitesides, a former Nasa chief of staff who joined Galactic in 2011. Another crucial element of the flight plan was that the hooks which held SS2’s folding wings in place were not to be released until the plane had reached Mach 1.4.

Both pilots would have been aware that unlocking the wings too near the Mach 1 sound barrier would have been disastrous. As objects reach this speed, the aerodynamic forces around them become unpredictable. So unpredictable that when test pilots first broke the sound barrier last century, they lost control of their planes.

SS2 was shaped something like a swallow, its two wings stretching back towards its tail. Wing booms were held in place by two hooks. Once the hooks were removed, a second handle at the controls would set in motion two pistons which pushed the wings upwards, transforming SS2’s profile from a sleek line to an L shape. The manoeuvre was designed to happen just before the craft began its descent.

Investigators from the national transportation safety board inspect some of the wreckage from the crash of Virgin Galactic’s SpaceShipTwo. Photograph: David McNew/Reuters

By this time, the engine would have stopped burning and the momentum propelling SS2 higher would have run out. Feathering was designed to right the ship, so that it drifted back down belly first. On approaching landing, the wings would straighten again, allowing the ship to land like a glider, without the help of an engine.

On its final flight, SS2’s engines were fired at 21 seconds past 10.07am. Nine seconds after ignition, a cockpit camera shows Alsbury pushed the lever to unhook the wings. At this point, according to a source who has seen the footage, Alsbury appears to have realised his mistake. As panic set in, he apparently tried to shut off the engine. But to no avail. At 34 seconds past, the wings began to deploy. From that point, all data is lost, which means disintegration was almost immediate.

While the pistons would have held the wings flat at Mach 1.4, or even Mach 1.2 as on previous test flights, SS2 was at that moment breaking the sound barrier and aerodynamic turbulence forced the pistons and pushed the wings upwards. The effect was like slamming on the brakes at the moment of peak acceleration.

“There’s nothing that tells me that this was anything other than pilot error, sadly,” said Will Whitehorn, the former president of Virgin Galactic, speaking from the UK. “I think he just unlocked at a time he shouldn’t have. The [National Transportation Safety Board] are trying to understand why he did that, was there something in the system that told him he should do it, or did he just make a mistake, and I think it’s going to be the latter.”

A piece of wreckage lies within the large debris field created by the destruction of Virgin Galactic’s SpaceShipTwo. Photograph: David McNew/Reuters

SS2 did not have ejector seats. It had a door immediately to the rear of the pilot station, on the left side, and its emergency hatch was the middle passenger window on the right side. But in the three or four seconds between when the pilots would have realised the mistake and the destruction of their ship, there may not even have been time to unbuckle from their seats.

The accident happened at around 50,000 feet. At this height, 10 miles above sea level, temperatures are at -60C. It is likely that Siebold and Alsbury fell with their oxygen masks, attached to portable tanks, still in place. But at that altitude, the sudden decompression within the cabin can cause lungs to rupture as the air inside them expands in an instant.

Once outside the plane, folding limbs into the body so that they are not ripped off by a fall that can reach speeds of 200mph is the next challenge. Blacking out is also difficult to avoid.

“Even if you breathe 100% oxygen it’s still a problem,” a senior aerospace physiologist told the Guardian. “At that altitude you’ve got nine, 10 seconds of useful consciousness left. It’s like being strangled.”

Siebold was either thrown out of his seat as the plane fell apart, or managed to unbuckle himself. Either way, his chute opened at an estimated 17,000 feet.

Inspectors survey a tail section from the crashed Virgin Galactic SpaceShipTwo where it lies in the arid Californian desert. Photograph: Josh Edelson/AFP/Getty Images

He was spotted by a chaser plane, sent up as part of the flight plan to monitor SS2’s progress. It was piloted by Virgin Galactic employee CJ Sturckow, who had joined the company to become a full time space tourism pilot once commercial flights began. A highly experienced astronaut who had piloted two Nasa Shuttle flights and commanded two others, Sturckow is said to have seen Siebold make the thumbs up as he drifted past, indicating that he was alive and well. “It’s the sign that says I am OK,” said Whitehorn. “They would immediately know he’s alive and conscious. If the chase plane was circling me I’d want to give them a sign.”

Alsbury was not so lucky. Local journalist Doug Messier said the co-pilot had landed strapped in his seat. “A small part of the parachute had deployed but he didn’t have a chance to get out or deploy”.

Branson’s ambition of creating a spaceline that could one day take over where Concorde left off hangs in the balance. Some commentators have described his ship, now facing more delays after a decade in development, as little more than a Heath Robinson machine. Whitesides dismisses any suggestion that the replacement Virgin Galactic craft, under construction in Mojave and expected to begin test flights within six months, has anything in common with the British cartoonist’s fantastical inventions.

Whitesides said: “This was a vehicle that was designed by arguably the top aerodynamic new vehicle designer in the world, owned by Northrop Grumman, one of the finest aerospace companies in the world. I believe that it will accomplish its design goals. While I don’t know who Heath or whatever Robinson is, my guess is that it’s pretty far away from Northrop Grumman.”