The Soviet satellite Sputnik is widely considered the first man-made object sent to space in October 1957, beating the launch of the American Explorer I by four months.

But an astrophysicist believes the US may have inadvertently beaten Sputnik to the title much earlier in the year.

During underground nuclear tests in New Mexico in August, experts launched a manhole cover into the sky with enough power to blast it into space - and the cover's whereabouts have never been found.

An underground nuclear test conducted by the Americans just two months before Sputnik launched a manhole cover into the sky at five times Earth's escape velocity, possibly putting it into orbit (file image). Earth's escape velocity is the minimum speed needed for an object to break free of its gravity and begin to orbit

Astrophysicist Dr Robert Brownlee, now 91 years old, revealed the bizarre tale in an interview with The Register.

Between May and October 1957, American scientists were working on Operation Plumbbob which involved a series of nuclear explosions in the Nevada desert to test their weaponry.

Worried about the amount of nuclear radiation being released into the air, bosses at Los Alamos National Laboratory in New Mexico asked Dr Brownlee to figure out a way to test nuclear weapons underground.

The team drilled a 500ft (152 metre) borehole to conduct what was to become the world's first underground nuclear test, codenamed Pascal A on 26 July.

The Soviet satellite Sputnik (pictured) is widely known as the first man-made object sent into orbit in 1957, a year before the Americans launched the Explorer I. This triggered what later became known as the space race

But the yield from the test was 50,000 times greater than expected, shooting flames hundreds of feet into the air from the mouth of the shaft.

The next month they tried again.

To test how changing the pressure in the chamber affected the explosion, they welded a four-inch (10cm) thick concrete and metal cap to the top of the shaft.

Before the explosion, Dr Brownlee had calculated the pressure from the blast would blow off the cap, so they set up a high-speed camera to capture the moment it popped.

The cap was in frame for a millisecond of the camera footage before disappearing from view, according to Dr Brownlee, never to be seen again.

But according to his calculations, taking the yield and the pressure into account, it would have been blasted into the air at more than 37 miles per second (60km per second), which is more than five times the escape velocity of our planet.

Worried about the amount of nuclear radiation being released into the air, the Americans began to test nuclear weapons underground. Pictured, a cameraman films an atomic mushroom cloud in a project named Operation Plumbbob on July 19, 1957

Earth's escape velocity is the minimum speed needed for an object to be travelling at to break free of its gravity and begin to orbit, instead of falling back to the ground.

The cap was likely destroyed before making it into space, but the possibility remains that it survived long enough to enter into Earth's orbit, according to Dr Brownlee.

'I have no idea what happened to the cap, but I always assumed that it was probably vapourised before it went into space. It is conceivable that it made it,' he said.

'Many years later, when I was in Baikonur, the subject of Russia being the first to launch something into space came up. I did not raise my hand to add to the discussion, though I thought about doing so.'