Space rock 7482 (1994 PC1), which is the same size as three Empire State Buildings, came within a mere whisker of our home planet earlier in January.
It followed 4660 Nereus - an asteroid larger than Paris’ Eiffel Tower and The Shard in London - that’s set to come dangerously close to us once again on Valentine’s Day 2060.
As the recent Leonardo DiCaprio Netflix film Don’t Look Up showed, there’s nothing scarier than a monster space rock hurtling towards the earth.
But work on how to stop them hitting us is still very much in its infancy, with humanity’s first attempt to alter the orbit of an asteroid having only just blasted off.
So how close did 4660 Nereus come to earth, how did Nasa know about it - and what is the space agency doing to prevent an Don’t Look Up scenario?
Here’s what you need to know.
What did 4660 Nereus do?
On 11 December 2021, 330m-long asteroid 4660 Nereus rocketed past earth at a speed of 6.58 kilometres-per-second (14,719mph) at a distance of 2.4 million miles from earth.
While that might not seem particularly close - after all, you’d have to drive around the earth 100 times to travel that far - Nasa has classified the asteroid as a “potentially hazardous” near-earth object.
This is due to it being larger than 150m and its orbit meaning it passes our planet at less than half the distance from the earth to the sun (roughly 93 million miles).
It means any slight deviation in its orbit could put it on a collision course with earth in the future.
In fact, on Valentine’s Day in 2060, it will come within 745,000 miles of earth - just over twice the distance between the earth and the moon.
So it might be worth buying those flowers or getting that proposal done well in advance of that date, just in case.
How does Nasa track ‘dangerous’ asteroids?
Nasa tracks dangerous asteroids through its Centre for Near-Earth Object Studies.
Since 1968, it has tracked more than 1,000 asteroids that have passed close to our home planet using radar - a technique which allows the space agency to accurately map the orbit, size and shape of space rocks.
Through telescopes, Nasa has also managed to spot 27,323 asteroids that could endanger the earth.
Just under 10,000 of these are 140m or larger and 891 are more than a kilometre in size.
To put into perspective the potential impact these rocks could have if they hit the earth, the asteroid which caused the Chelyabinsk explosion in Russia in 2013 was just 20m in size.
When it exploded in the atmosphere, this space rock gave off up to 33 times as much energy as that which was released by the atomic bomb the US dropped on the Japanese city of Hiroshima in World War Two.
The shockwaves from this blast blew out windows in more than 3,600 apartment blocks and injured 1,200 people.
Some suffered skin and retinal burns due to the asteroid shining up to 30 times brighter than the sun as it burned up in the earth’s atmosphere.
Nasa says there is no “significant chance” any near-earth asteroids it has seen will hit earth within the next 100 years.
But it estimates it has spotted just half of the dangerous space rocks out there, as there are potentially 25,000 large near-earth objects in space.
By tracking them and finding out more about their size, shape, mass, structure and what they’re made of, the US space agency hopes to come up with ways of diverting one should it hurtle towards earth.
What is the Nasa Dart asteroid mission?
On 24 November 2021, Nasa launched its Double Asteroid Redirection Test (Dart) mission.
This one-way experimental voyage will see a spacecraft smash into an asteroid in a bid to alter the space rock’s course.
Should it prove successful, it could provide a way of protecting the earth from asteroids.
Dart will be heading to the 780-metre rock Didymos and a 160-metre asteroid which orbits it, called Dimorphos.
These asteroids have been chosen because they are easily traceable through earth-based telescopes, so any changes to their courses can be tracked accurately.
Neither of them are thought to be a danger to the earth.
When the spacecraft reaches the pair of space rocks at the end of September 2022, it will hit Dimorphos at four miles per second - a speed scientists think should be enough to change the rock’s path.
In four years’ time, a European Space Agency (ESA) spacecraft named Hera will arrive at the asteroids to inspect Dart’s crater and survey their respective masses.
What is an asteroid?
Asteroids are rocky fragments left over from the birth of the solar system 4.6 billion years ago.
They are different from comets in that they are mostly made up of metals, whereas comets contain ice, gases and fragments of rock.
Space rocks are believed to be the remnants of collisions between planets and moons.
Many of these rocks orbit the sun between Mars and Jupiter in a part of space known as the Asteroid Belt.
Scientists think there could be millions of asteroids in this part of space - some of which are hundreds of kilometres in size.
An even larger collection of massive asteroids can be found at the farthest reaches of the solar system, known as the Kuiper Belt.
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 still potentially deadly, shards of space rock.
It is one such stray asteroid, believed to have measured around six miles (10km) in size, that wiped out the dinosaurs 66 million years ago and led to mammals’ dominance of the planet.
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