Nothing seemed amiss on the morning of September 3 – Labor Day in the United States – when Steve Fossett, world-renowned adventurer, took off from a private airstrip in western Nevada for an early morning flight.

It was thought he planned to scout the area for possible locations for his latest pioneering endeavour, an attempt to break the world land-speed record, but it now seems more likely Fossett just wanted to get airborne.

When he failed to return as scheduled by noon, friends at the Flying-M Ranch, a private resort where he had been staying with his wife Peggy, mounted a search. The alarm was raised that afternoon and a full-scale search-and-rescue operation began the following morning. For weeks, dozens of aircraft scanned an area of 22,000 square miles, scouring the wild, mountainous terrain of Nevada state for signs of wreckage.

Richard Branson, Fossett’s rival adventurer turned friend, called on the public to trawl satellite images of the area on Google Earth. Yet despite the most intensive search-and-rescue operation in US aviation history – boosted by the financial backing of Branson and Barron Hilton, the hotel magnate and owner of the Flying-M Ranch – not a scrap of the aircraft has been found, nor even a patch of scorched earth. The whereabouts of Fossett and his plane remain a mystery; on November 26, Peggy asked for her husband of almost 40 years to be declared dead.

While private pilots in the area continue to scan the ground for signs of the missing millionaire, the official Civil Air Patrol (CAP) search was called off in October.

Why was there no reliable radar trace of Fossett’s last flight? Why was no distress signal recorded when the plane was designed to emit one automatically in the event of an accident? Aircraft rarely vanish without trace unless, as with Amelia Earhart, one of the explorers who inspired Fossett, they are flying over an ocean.

In these days of pin-sharp satellite images, ground-penetrating radar and thermal-imaging equipment, the chances of a plane going missing without a trace range from slim to impossible.

Fossett set off some time between 8am and shortly after 9am from Hilton’s airstrip. He was in a Bellanca Citabria Super Decathlon, a tandem two-seat, single-engined aircraft made of wood, cloth and aluminium – lightweight and agile.

He was wearing everyday clothes and carrying a bottle of water. He left without his usual survival kit (known as a “go bag”), containing a satellite phone, food and clothes. According to the latest reports from the States, he also left behind his Breitling Emergency watch, a gift from Branson that features a small knob that can be twisted off to expose a tiny antenna that sends out a distress signal.

A report in Men’s Journal, a US magazine, has suggested his mind may have been on other matters in the wake of a rare argument with Peggy earlier that morning. Apparently he was visibly upset when he arrived at the airstrip. There has been some speculation that Fossett may have been going through a difficult period. He was getting older, of course, and was no longer as fit as in his youth. And having already broken more than 100 world records in sailing, ballooning and aviation, he would have known his options for future challenges were narrowing.

On the lookout for less physically gruelling adventures, he had decided to make an attempt on the world land-speed record, set in 1997, of 763mph. It was an ambitious plan and had already suffered several delays and setbacks.

The jet-powered car bought from Craig Breedlove, who had made a failed attempt at the record in 1996, had needed more work than originally foreseen, and Fossett had been advised against taking on such a risky challenge.

That he left his “go bag” and special watch behind and headed off with little more than a bottle of water may suggest that he was not in his usual, careful state of mind. But friends and colleagues of the 63-year-old former commodities broker have no time for the theory that Fossett took his own life. And they dismiss suggestions that someone as well known and recognisable could vanish in the manner of John Darwin, who reappeared recently after vanishing, presumed dead, on a canoeing trip five years ago.

Louise Ann Noeth was helping Fossett with his land-speed record attempt and is now the official spokesman for the bid, which continues without Fossett, funded by his wife but without a driver for now. She says there is no way Fossett would have ended his life. “He was in great spirits,” she says. “Are you kidding? In a few weeks he was going to drive the car. He was tickled to pieces.

“This was a serious bid and a great, great adventure for Steve. When I first met him a year ago he was test-driving another jet-powered car on the Bonneville salt flats. I remember thinking, who is this accountant-looking guy? What’s he doing here? And then he got in and he got the car straight up to 300mph. It was incredible.”

In an article in The Washington Post, shortly before his disappearance, Fossett appears to back up Noeth’s claims, saying, “The longer I wait, the more excited and eager I am to really experience this, and drive.”

So what about that missing distress signal? The Bellanca plane had an older version of the emergency locator transmitter (ELT) system, which can be unreliable and could easily have failed to go off. And for a short leisure flight Fossett may simply not have felt he needed a survival kit.

But so little can be confirmed about Fossett’s movements on the day of his disappearance that people have no choice but to cast around for explanations. He did not file a flight plan (this wasn’t unusual for this type of short leisure flight), so all that is certain is that his plane could carry up to 40 gallons of fuel, enough for four hours of flying. As the plane could travel comfortably at 100mph, Fossett could have travelled up to 400 miles from the airstrip.

Discounting a reckless suicide mission, that means he could have flown 200 miles from the ranch so as to allow for a return journey – probably less, to be on the safe side. But even the area within 100 miles of the ranch covers some pretty challenging territory, including rocky valleys and canyons, patchy forests and mountain ranges. The area is believed to have swallowed up to 200 pilots in the past 50 years, many lost without trace.

In those tricky conditions, when one is flying close to the ground – as this plane was designed to do – it is easy to clip a wing on a tree or rock and quickly end up in trouble. Not only that, but the areas alongside some of the mountain ridges are notorious for pockets of turbulence and gusty winds, creating dangerous downdraughts. If Fossett was in a mood to take risks, if he was – as reported – upset, could he have taken unnecessary chances? Noeth doesn’t think so.

“He is an extremely calm individual,” she says. “This was not a reckless person. We are not talking about a daredevil. This is one of the finest, most highly skilled pilots on the planet.”

Another pilot who was in the area on the day of Fossett’s disappearance claims to have heard a loud ELT signal – which should be activated automatically by G-force as a plane plummets to the ground – through his radio transmission. It lasted only a few seconds, consistent with a plane falling into a lake and being submerged. But no satellite picked up the signal, and Walker Lake has been thoroughly searched.

“I’m sure they know every square inch of that lake,” says Major Cynthia Ryan, spokeswoman for the Nevada branch of the CAP, which led the search. “We did complete sonar sweeps of the lake; we brought in special divers; we examined it from the air, looking for oil slicks; we walked the perimeter of it several times. We now think it’s more likely the ELT system never activated.”

As for the lack of radar data, it is not unusual for traces to be lost in such terrain, where mountain ranges can interrupt the signal, and radar cannot pick up anything flying below 1,200ft, as Fossett was probably doing.

Ryan thinks there are three possible answers to the riddle of Fossett’s disappearance. Either he had some sort of “medical emergency” – a stroke, say, or a heart attack – at the controls, or, like so many before him, he was brought down by mechanical mishap or pilot error. “Everything else is just speculation, based on nothing,” Ryan insists. “The radar traces we have lead us to believe the wreckage is somewhere on the eastern side of the mountains along Walker Lake. He flew along there quite often, according to the people at the Flying-M Ranch.

“In a place like this, all it takes is a momentary lapse and something can go wrong. It doesn’t matter how experienced you are.” If Fossett’s plane had broken into pieces before it hit the ground, or gone up in flames, it would explain why no fuselage has been found. Amid the rough Nevada wilderness, it could easily be hidden. There was a spate of fires in the area after a run of particularly dry summers, which would have made it harder to spot scorch marks.

If Fossett had been flying for more than two hours, as the last reported sightings suggest, there may have been just enough fuel to burn up the plane’s lightweight materials but not to leave a large area of blackened soil and vegetation.

“When you take people up in a plane and they look about, then they suddenly understand,” says Ryan. “This is some of the most rugged territory in the United States. There’s just a lot of empty land out there, you know? And an aeroplane like that, if it’s not an intact aircraft, you are simply not going to find it very easily. And we’ve been through this before. Even when you know exactly where a plane is, if the light’s not correct, if it’s hidden in the shadows of a ravine, or by tree branches, you just can’t see it. You are looking for bits of an aeroplane, which might be the size of a brick. You could fly over the area a hundred times and not spot it.

“People looking on Google Earth didn’t stand a chance. By the end of it I was ready to strangle the inventor of Google, and don’t even mention the psychics . . . There really isn’t any great mystery here. I am convinced someone on the ground will find the crash site eventually – they usually turn up within about two years.”

Time may prove her right. But until then, speculation will continue. And there will be plenty who claim to have spotted someone who looked just like Steve Fossett in some dusty backwater, never quite believing that a man who had repeatedly pushed back the boundaries of human endeavour could be wiped out without a trace, perhaps by a simple gust of wind.