By BlueShift and AFP | 2025-02-20

PARIS -- There is a very small chance that an asteroid capable of taking out an entire city could strike Earth in less than eight years. But even if this asteroid is hurtling our way, humanity is now able to defend itself against such a threat, experts say.

For more than 24 hours, the 2024 YR4 asteroid, which was first detected in December, had the highest probability of hitting Earth -- 3.1% -- of such a big space rock in modern forecasting.

But the odds were cut in half to about 1.5%, according to new calculations that NASA announced February 19.

Separate calculations by the European Space Agency (ESA) drove the odds down to 1.38%.

The drop in odds had been widely anticipated by the global astronomical community, which now broadly expects the probability the asteroid will hit Earth on December 22, 2032, to fall down to zero.

"Don't panic," said Richard Moissl, director of the ESA's planetary defense office.

The percentage chance will likely still "wiggle slightly up and down a bit" but is now expected to soon fall below 1%, he told AFP.

There is still a 0.8% chance that the asteroid will hit the Moon, according to NASA.

'We are not defenseless'

Data from the James Webb Space Telescope -- the most powerful space observatory -- will be key in better understanding its trajectory, Bruce Betts, chief scientist for the nonprofit Planetary Society, told AFP.

"Webb is able to see things that are very, very dim," he said -- which is key because the asteroid's orbit is currently taking it out towards Jupiter, and its next close approach will not be until 2028.

The telescope will turn its powerful gaze to the asteroid next month.

The International Asteroid Warning Network (IAWN), a worldwide planetary defense collaboration, issued a warning memo on January 29 after the impact probability had crossed 1%.

If the risk rises over 10%, IAWN would issue a formal warning, leading to a "recommendation for all UN [United Nations] members who have territories in potentially threatened areas to start terrestrial preparedness," explained Moissl.

However, even in the unlikely event that the probability does keep rising up to 100%, "we are not defenseless," he emphasized.

Here are some of the ways that humanity could deflect or destroy the asteroid known as 2024 YR4.

Smash a spacecraft into it

Only one planetary defense strategy has been tried out on an actual asteroid.

In 2022, NASA's Double Asteroid Redirection Test (DART) deliberately smashed a spacecraft into the 160-meter-wide Dimorphos asteroid, successfully altering its orbit around a larger space rock.

An advantage of this plan is that we could hit the 2024 YR4 asteroid with multiple spacecraft, observing how each one changed its trajectory, Bruce Betts, chief scientist for the nonprofit Planetary Society, told AFP.

The asteroid discovered in December is estimated to be 40-90 meters wide -- roughly half the size of Dimorphos.

"You have to take care not to overdo it," Moissl warned.

If the spacecraft partially destroyed the asteroid, it could send "bits flying off" that still head towards Earth, he said.

If this kind of mishap changed the eventual impact site on Earth -- for example, "from Paris to Moscow" -- that would likely cause major problems back home, Betts added.

Tractor, ion beams, paint

A separate idea called a gravity tractor involves a large spacecraft flying up near the asteroid and -- without touching it -- using its gravitational pull to tug it away from Earth.

Another non-contact plan would put a spacecraft near the asteroid armed with thrusters that would exert a "constant stream of ions" to shove the asteroid off course, Moissl said.

Scientists have also considered spray painting one side of the asteroid white, increasing its reflectiveness so it slowly changes trajectory.

These subtler strategies would require reaching the asteroid sooner than for some more severe options.

The nuclear option

Or we could also just blow it up with a nuclear bomb.

Rather than drilling a nuclear weapon deep into an asteroid -- as depicted in the 1998 sci-fi action movie "Armageddon" -- this would likely involving detonating a bomb nearby.

Last year, US researchers testing out this theory on a marble-sized mock asteroid in the lab found that the X-rays from a nuclear blast would vaporize its surface and send it shooting off in the opposite direction.

Even setting aside the ethical, political and legal issues of sending nukes into space, this is considered more of a last-ditch plan for multi-kilometer-wide asteroids like the one that killed off the dinosaurs.

And again, there is a risk that a nuclear explosion could still send unpredictable chunks hurtling towards Earth.

Lasers

Along less dangerous but similar lines, another idea is to shoot laser beams from a spacecraft to vaporize the side of an asteroid, pushing it away.

Lab experiments suggest this plan is viable, but it is not one of the "top techniques" being looked at, Betts said.

If all else fails

If necessary, deflecting this asteroid is "doable, but it depends on the speed at which we move as a planet," Moissl said.

While scientists and space agencies will make their recommendations, ultimately the decision on how to tackle the asteroid will be made by world leaders.

If all else fails, we will have a good idea of the strike zone of the asteroid -- which is not a "planet killer" and at most could threaten a city, Moissl said.

This means that preparing for impact, potentially including evacuation if the area is populated, will be the last line of defense.

"Seven and a half years is a long time to prepare," Moissl said, re-emphasizing that there is a roughly 97% chance the asteroid will miss Earth.