Warning: Doomsday asteroid taller than Empire State building cannot be stopped by Nasa

Nasa will be unable to defend our planet against a doomsday asteroid that could smash into Earth in 2135, a study has revealed.

Researchers found that it might be impossible to stop the space rock - which is the size of the Empire State building - from careering into Earth next century, the Daily Mail reported.

Scientists revealed that even their most advanced technology would prove "inadequate" for deflecting such a space rock, which has been dubbed Bennu.

The consequences would be "dire" experts have warned, and the asteroid has sparked fears that it could even wipe out life for good.

Scientists are investigating the possibility of using a specially designed spacecraft to nuke approaching cosmic objects, in hopes that doing so could deflect the threat.

While it could be possible to divert a near-Earth object (NEO) by slamming into it with a so-called impactor, experts say a nuclear explosion may be the best bet when time is running out.

In a new paper, scientists from NASA and the National Nuclear Security Administration have laid out a plan for the Hypervelocity Asteroid Mitigation Mission for Emergency Response (HAMMER).

The 8.8-ton HAMMER spacecraft could be used to steer itself directly into a small asteroid, or blow the space rock up using a nuclear device, according to BuzzFeed News.

The team has devised a proposal around a potential impact with the 1,600-foot-wide asteroid Bennu, which is currently the destination for NASA's Osiris-Rex sample return mission.

While Earth isn't at risk of a collision with Bennu any time soon, there's a 1 in 2,700 chance it will slam into our planet sometime next century.

Bennu is also the best-studied asteroid of all the known NEOs, the researchers note.

'The two realistic responses considered are the use of a spacecraft functioning as either a kinetic impactor or a nuclear explosive carrier to deflect the approaching NEO,' the authors wrote in the study, published to the journal Acta Astronautica.

But, there are several factors that would determine the best approach.

The asteroid's size and mass must be taken into consideration, along with the amount of time available before it hits Earth.

Plus, there are always "various uncertainties".

"Whenever practical, the kinetic impactor is the preferred approach, but various factors, such as large uncertainties or short available response time, reduce the kinetic impactor's suitability and, ultimately, eliminate its sufficiency," the authors wrote.

By driving multiple spacecraft into the asteroid's path, it could be possible to slow the object down and throw it off course to avoid a collision with Earth.

If the conditions aren't right to use the kinetic impactor, however, nuking it might be the only option.

"If the asteroid is small enough, and we detect it early enough, we can do it with the impactor," physicist David Dearborn of the Lawrence Livermore National Laboratory told BuzzFeed News.

"The impactor is not as flexible as the nuclear option when we really want to change the speed of the body in a hurry."

Despite ongoing efforts to catalogue potential hazards in Earth's vicinity, scientists have increasingly warned that there are countless large objects that remain undetected.

Given the risk of an inevitable impact sometime in the future, the experts say it's necessary to plan for the worst.

The researchers will present their proposal at a conference in Japan this coming May.

For now, however, whether or not the HAMMER plan will ever come to fruition remains up in the air.

WHAT IS NASA'S HAMMER ASTEROID DEFLECTING VEHICLE?

NASA is working on a Hypervelocity Asteroid Mitigation Mission for Emergency Response vehicle, dubbed HAMMER.

The nine metre tall, 8.8-ton spacecraft features a modular design that would enable it to serve as a kinetic impactor, essentially a battering ram.

It could also be used as a transport vehicle for a nuclear device.

Its possible mission is to deflect 101955 Bennu, a massive asteroid around 500 metres (1,640 ft), more than five football fields, in diameter.

It weighs around 79 billion kilograms (174 bn lbs), which makes it 1,664 times as heavy as the Titanic, and is circling the sun at around 63,000 mph (102,000 kph).

Based on observational data, Bennu has a 1 in 2,700-chance of striking Earth on Sept. 25, 2135.

It is estimated that the kinetic energy of this impact would be equivalent to 1,200 megatons, around 80,000 times the energy of the Hiroshima bomb.

The preferred approach to mitigating an asteroid threat would be to deflect it by ramming a kinetic impactor into it, delivering a gentle nudge large enough to slow it down, but not so large that the object breaks apart.

However, recent studies have suggested that the nuclear option may be required with larger objects like Bennu.