An asteroid twice the height of the Empire State Building will narrowly miss Earth in less than a month, flying past our planet at 23,100 km/h.

Asteroid 200 QW7 will make its close call on September 14, as one space entrepreneur warns we have "no defence" against killer asteroids.

The giant rock is 650m in diameter, around double the size of Auckland's Sky Tower.

The revelation comes from Nasa's Centre for Near Earth Object Studies (CNEOS), which says that the asteroid will only be 5.3 million km from Earth when it flies by.

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The asteroid will pass closely by the earth. Photo / File
The asteroid will pass closely by the earth. Photo / File

The news comes as Space X CEO Elon Musk sounded a chilling warning about our vulnerability to asteroid strikes.

Replying to a tweet about Apophis, an asteroid due to come close in 2029, Musk pointed out we have no plans in place to repel a killer space rock.

Musk said: "Wouldn't worry about this particular one, but a big rock will hit Earth eventually & we currently have no defence."

'Doomsday' space rock

Apophis, named for the Egyptian god of chaos and darkness, is expected by scientists to make several passes of the Earth this century.

Its visit in 2029 will see the asteroid pass closer to the planet than our own weather satellites and one space buff has released terrifying video of what would happen if it collided.

The video, by You Tube user Space Sim, shows an impact in the Atlantic Ocean that results in a devastating fireball that travels as far as the US state of Kansas, over 4000km away from the impact.

Those who survived the first strike would then have to contend with debris raining back down to earth as fiery asteroids, though he notes the rock "has a very low chance of actually hitting the Earth in real life".

Nasa rates the chance of a collision as just one in 250,000.

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"Apophis is a representative of about 2000 currently known Potentially Hazardous Asteroids," said Paul Chodas of Nasa.

"By observing Apophis during its 2029 flyby, we will gain important scientific knowledge that could one day be used for planetary defence."