The threat posed to Earth by a newly discovered asteroid has been downgraded. But others could be headed our way
What was the recent scare all about?
It was caused by a football pitch-sized asteroid designated 2024 YR4. Detected last December by a telescope in Chile, the space rock was flagged for its trajectory – which put it on track to possibly hit Earth on 22 December 2032 – and for its size: it is 130 to 300ft in size, big enough to "collapse residential structures across a city", as Nasa put it. The space agency initially put the chance of an impact at 1%. The odds soon jumped to 3.1%, or one in 32, the highest-threat asteroid ever detected. But after closer study of its orbit, the threat of a hit was downgraded to a negligible 0.004%. While humanity's plans for Christmas 2032 were saved, the scare highlighted a threat that scientists say needs more focus: the millions of giant rocks that are hurtling through space, some of which may be on a collision course with our planet. "Take it as a warning shot across our bow," said astrophysicist Neil deGrasse Tyson. "These things are out there."
How many space rocks pose a threat?
Nasa's Centre for Near Earth Object Studies (CNEOS) tracks more than 37,900 asteroids whose trajectories approach Earth's orbit. About a third are 30ft wide or less; many are the size of a car or smaller and pose no risk, because they will burn up in our atmosphere. At the other extreme are "planet killers": asteroids a kilometre or more across that could potentially wipe out civilisation. About 900 have been identified. In between are a range of potential threats. A 160ft-wide asteroid could cause "local devastation"; those are thought to strike once every 1,000 years. A 500ft-wide rock could inflict mass casualties across a metropolitan area; those arrive every 20,000 years. Of course, "these numbers are very approximate", said planetary geologist Gordon Osinski, "and they don't really help us figure out when the next one might happen".
When did the last one hit?
On 15 February 2013, a roughly 60ft-wide asteroid entered the atmosphere and exploded 18.6 miles above the Russian city of Chelyabinsk. It set off a blinding flash and a shockwave that damaged 7,320 buildings over 200 square miles, and injured more than 1,600 people, many of them hit by shattered glass. The light from the blast was like "the end of the world", said Valentina Nikolayeva, a teacher. In 1908, a 130ft-wide asteroid exploded six miles above a remote stretch of Siberia, releasing 185 times more energy than the atomic bomb dropped on Hiroshima. Some 800 square miles of forest were levelled in the so-called Tunguska event. Still, that rock was a pebble compared with the one, six to nine miles wide, that wiped out the dinosaurs 66 million years ago. There's no current fear of anything of that magnitude hitting Earth, but other Tunguska-sized threats are out there.
Can we stop them?
Possibly. But first we have to see them coming. In 2005, the US Congress directed Nasa to find and track, by 2020, 90% of near-Earth objects – asteroids or comets that come within 30 million miles of our planet's orbit – that are 460 feet or larger. Right now "we're at something like 45%", said CNEOS director Paul Chodas. Nasa has built a network of telescopes, including the one in Chile that detected 2024 YR4, to identify threats. An infrared space telescope, NEO Surveyor, that will further boost detection, is scheduled for launch in 2027. When a new object is found, data is shared with a global web of space agencies and observatories that go to work determining its shape, size and orbital path. If one is judged to be headed for Earth, the next task would be to try to alter its path – something Nasa recently proved feasible.
How did Nasa do that?
In 2022, it launched the golf cart-sized Double Asteroid Redirection Test (Dart) spacecraft, which slammed into Dimorphos, a 525ft "moonlet asteroid", at 14,000 miles per hour. The collision, which took place 10 months' journey away in the Didymos system, successfully slowed the asteroid down and altered the projectile's orbit. Researchers are also studying the use of a "gravitational tractor", a spacecraft that would orbit alongside an asteroid, exerting gravitational pull that would gradually alter the rock's course. All these efforts require years of advance planning and may not be effective against a giant asteroid. If a space rock is too large for deflection – or due to hit with relatively short notice – humanity would need to use a nuclear bomb to deflect or vaporise it.
Would a nuclear bomb work?
A 2021 study showed that a one-megaton bomb launched at least two months before impact could annihilate a 330ft asteroid. But dealing with a dinosaur-killer-sized one would be immensely difficult, probably requiring the Earth's entire nuclear arsenal, according to Andrew Rivkin, of Johns Hopkins Applied Physics Laboratory. And that might just turn one large asteroid into 1,000 smaller ones – like a radioactive shotgun shell. Besides, setting off a nuclear bomb in space "could be very awkward geopolitically", says Robin George Andrews, author of "How to Kill an Asteroid".
Are different nations working together on this?
The Chelyabinsk explosion led to the creation of Nasa's Planetary Defence, in part to link efforts with groups such as the UN-sponsored International Asteroid Warning Network. And last year, for the first time, international representatives attended Nasa's biennial Planetary Defence Interagency Tabletop Exercise, which modelled how to deal with the (hypothetical) discovery of a massive asteroid with a 72% chance of impacting Earth in 2038. Scientists say this cooperation is a step in the right direction, but global asteroid defence is still in its infancy. What's the best approach? Who would be in charge? "Asteroid impacts are one of the few natural disasters that we actually have the means to both foresee and prevent," said Nasa aerospace engineer Brent Barbee. We must be "as prepared as possible".
The Chicxulub impact
Some 66 million years ago, an asteroid at least six miles wide slammed into Mexico's Yucatán Peninsula. The impact dug a crater 125 miles wide and 12 miles deep, known as the Chicxulub crater (now partly underwater and filled in with sediment). It triggered earthquakes and tsunamis, and firestorms that may have spanned the globe. Gas, soot and dust blanketed the planet, blotting out the Sun and sending global temperatures plummeting. That extinction event wiped out 76% of life on Earth – including the dinosaurs. But some experts believe the impact would have been far less calamitous if the rock had landed elsewhere. A 2017 study concluded that the asteroid struck a spot unusually rich in hydrocarbons, which worsened the blackout effect. The dinosaurs might have survived if not for that happenstance, the researchers believe, and the rise of the mammals – including humans – might never have occurred. So it "is maybe a lucky coincidence that everything came into place like it is today", said geochemist Mario Fischer-Gödde.
