The space rock – called 3200 Phaethon – is being monitored by the US space agency after it was described as “potentially hazardous”.

It whizzed by so close to Earth over Christmas, space boffins were able to snap high-resolution images of its inside.

And what they captured is a mystery.

The radar images reveal the asteroid is utterly empty on the inside – a space measuring hundreds of metres.

NASA said the tennis ball-shaped rock “has a large concavity, or depression, at least several hundred meters in extent near its equator, and a conspicuous dark, circular feature near one of the poles.”

The most powerful astronomical radar system on our planet recently provided the highest-resolution images to date of a near-Earth asteroid, 3200 Phaethon, during its Dec. close approach to Earth, ~1.1 million miles away: https://t.co/h1DhS6t9xY pic.twitter.com/XGziN6vtBn

— NASA (@NASA) December 23, 2017

3200 Phaethon has been on experts’ radars since the the 1980s and is the second largest asteroid close to Earth that has been described as “potentially hazardous”.

This is because of its sheer size and closeness to the planet.

The newest pictures, which were taken when it was just 6.4 million miles away from Earth, of the asteroid reveal it is approximately 3.6 miles wide.

Patrick Taylor a scientist at Arecibo Observatory, which is monitoring the space rock, said the dark void at Phaethon’s centre might be “a crater or some other topographic depression that did not reflect the radar beam back to Earth.”

NASA recently revealed a huge astroid shaped like a human skull  would fly past Earth.

The vast object – which measures a whopping 500m wide – will pass our planet at 78,000mph in November next year.

Asteroid 2015 TB145 has been monitored since it had a near miss two years ago.

It zipped past as Brits went trick-or-treating on October 31, 2015, relatively close to Earth at 486,000km away – 1.3 times the distance to the moon.

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