NASA has warned a "potentially hazardous" asteroid is set to skim the Earth's orbit.

The space rock called 4660 Nereus and as big as the Eiffel Tower is set to make a close pass of the earth this week.

But should humanity be worried - and how does Nasa know this asteroid could be hazardous to our planet?

Here’s all you you need to know.

Will 4660 Nereus hit earth?

On 11 December 2021, the 330m-long asteroid 4660 Nereus is set to whistle past the earth at a speed of 6.58 kilometres-per-second at a distance of 2.4 million miles.

While that might not seem particularly close - after all, you’d have to drive around the earth 100 times to go that far - Nasa has classified the asteroid as a “potentially hazardous” near-earth object.

This is because it is bigger than 150m and will approach the earth at less than half the distance from the earth to the sun (around 93 million miles), meaning any slight deviation in its orbit could put it on a collision course with our planet.

Although 4660 Nereus appears to pose no threat in 2021, it is set to come much closer to the earth in the future.

On Valentine’s Day in 2060, it will come within 745,000 miles of earth.

How does Nasa track ‘dangerous’ asteroids?

Nasa tracks asteroids which could destroy the earth through its Centre for Near-Earth Object Studies.

Since 1968, it has tracked more than 1,000 asteroids which have passed close to our home planet using radar - a technique which allows Nasa to accurately map the orbit, size and shape of space rocks.

And using telescopes, Nasa has mapped 27,323 asteroids that could come close to the earth.

Included in this number are 9,886 asteroids that are 140m or larger and 891 that are more than a kilometre in size.

Nasa says there is no “significant chance” any of these asteroids will hit earth in the next 100 years.

But it estimates there could be more than 25,000 near-earth objects in space that are more than 140m in size, meaning we have recorded less than half of the potentially fatal asteroids out there.

By tracking these asteroids and finding out more about their size, shape, mass, structure and what they’re made of, the US Space Agency says it hopes to determine methods of how we could divert one should it threaten earth.

What is an asteroid?

An asteroid is a rocky fragment left over from when the solar system formed about 4.6 billion years ago.

Most of these rocks orbit the sun between Mars and Jupiter, with scientists estimating there are millions of asteroids in this part of space - some of which are hundreds of kilometres in size.

Sometimes, these asteroids change their orbits if they are influenced by the gravitational tug of planets.

They can also collide with one another in events which can throw out smaller, but potentially deadly, shards of space rock.

It is one such stray asteroid, believed to have measured around six miles in size, that wiped out the dinosaurs 66 million years ago and led to mammals’ dominance of the planet.