Unexplained ‘metallic orbs’ once buzzed past a former NASA astronaut’s plane as he was flying at around 9,000 feet, he said.

American Leroy Chiao, who was the 196th NASA astronaut to fly in space and the 311th human in space, served on multiple missions during an illustrious career after qualifying in 1991.

Mr Chiao, who is also a keen light aircraft pilot, said he was flying his plane back to Houston in Texas when something odd suddenly whizzed past.

Revealing the encoutner to broadcaster NewsNation, he said saw the “two metallic spherical orbs” which were around three-feet in diameter and 20-feet away in August this year.

He added: “It’s just kinda dumb luck that they didn’t hit me. It could’ve been a bad result, if they had actually hit me. It happened so quick, there wasn’t even a chance to get scared.

“It wasn’t on radar, air traffic control certainly didn’t alert me.

“[And] it wasn’t on my display that shows other airplanes that are participating with the [Federal Aviation Administration] FAA-required transponders. I don’t know what it was.”

Mr Chiao, who has received the NASA Distinguished Service Medal and two NASA Exceptional Service Awards, explained to the US broadcaster that he only saw the objects for “a second”.

Discussing what he thought the objects might be, Mr Chiao pondered: “My first guess is that it’s some kind of military program, a drone of some kind, but you know it’s hard to say, right?’

“Frankly, I think whoever was operating the drone wasn’t aware that I was there.”

According to NASA, several phenomena can be mistaken for UFOs, from planets to military kit, and even toys.

Dave Prosper, of the Astronomical Society of the Pacific, explains on the NASA website: “Venus is one of the most confused objects in the sky, especially when it is low to the horizon.

“Sirius and Jupiter are also often reported as UFOs, as well as Mercury. When bright planets are in alignment near the horizon they can appear to be a formation of “strange lights”!

“Other ‘UFOs’ can be rocket launches, comets, military jets, weather balloons, sounding rockets, satellites, meteors and fireballs, experimental craft (if you are lucky) and odd clouds (such as lenticular clouds) and be confused, as well as artifacts from photographs that are actually lens flare or aberration.

“Remote control aircraft can also be confused for a UFO; in fact some models explicitly look like ‘flying saucers’ just to make things tricky.”