image sourceVirgin Galacticimage captionVirgin Galactic said Unity encountered unexpected winds at high altitude

The US Federal Aviation Administration says it's investigating how Sir Richard Branson's recent space flight drifted off course during its climb skyward.

The British billionaire fulfilled his life's dream on 11 July by riding his Virgin Galactic rocket plane, Unity, above 85km (52 miles) in altitude.

But the New Yorker magazine has revealed how the vehicle flew for a period outside its pre-agreed airspace.

Unity landed safely and Virgin Galactic says it is cooperating with the FAA.

Sir Richard's company issued a statement in which it strongly disputed what it described as "misleading characterisations and conclusions" in the New Yorker article.

And Virgin Galactic said it would be pushing ahead with its flight programme.

Thursday saw it announce details of its next mission – a research flight for the Italian Air Force.

This would likely be conducted at the end of September or in early October, a Virgin Galactic spokesperson told BBC News.

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media captionWatch Sir Richard Branson's flight to the edge of space (and back)

The New Yorker article was penned by Nicholas Schmidle, who's written a book about Sir Richard's near two-decade effort to develop a rocket plane capable of taking fare-paying passengers above Earth's atmosphere.

Mr Schmidle's magazine story said the pilots on the 11 July flight – with Sir Richard and three other company employees aboard – received cockpit warnings about the trajectory of their rocket-powered ascent.

Left uncorrected, this shallower than planned climb to altitude could have left Unity unable to reach its landing location on the glide back down to Earth. Mr Schmidle claims company protocols for the cockpit warnings would normally have led to the pilots aborting the ascent.

But Virgin Galactic rejected the New Yorker's assessment.

The company said the pilots had encountered unexpected winds at high altitude and took the necessary actions to complete a safe climb to space and return to Earth.

"Our pilots responded appropriately to these changing flight conditions exactly as they have been trained and in strict accordance with our established procedures," Virgin Galactic said.

"Although the flight's ultimate trajectory deviated from our initial plan, it was a controlled and intentional flight path that allowed Unity to successfully reach space and land safely at our spaceport in New Mexico. At no time were passengers and crew put in any danger as a result of this change in trajectory."

Virgin Galactic said it is discussing the issues with the FAA.

The New Yorker magazine told the BBC it was standing by its account of what happened.

Meanwhile, the company has released details of Unity's next mission.

This will take aloft three Italian nationals – Walter Villadei and Angelo Landolfi from the Italian Air Force; and Pantaleone Carlucci, an Italian national research council engineer.

The three men will conduct 13 experiments during the flight, and in particular during those few minutes of weightlessness they'll experience at the top of Unity's climb.

The Italians will be supervised in the back of the rocket plane by Virgin Galactic's chief astronaut instructor, Beth Moses. It will be her third mission to the edge of space.

Following the flight, Virgin Galactic expects to enter an extended period of maintenance and upgrades for both Unity and its carrier/launch plane, known as Eve.

It will be the middle of next year before these vehicles resume space missions. The company said one further test outing would be conducted before full commercial service began, probably in the second half of 2022.

Some 600 individuals put down deposits a number of years ago to buy seat tickets costing $200,000-250,000. Tickets sales resumed last month with prices from $450,000 (£325,000) per seat.

image sourceVirgin Galacticimage captionUnity and its carrier/launch plane, Eve, will undergo extended maintenance