On 4 September, an asteroid was spotted curving towards Earth. Astronomers quickly established that it would impact the planet in 10 hours’ time. The Philippines island of Luzon was in the line of fire, and there was nothing they could do about it but watch. Sure enough, at 16.39 UTC (17.39 in the UK), just as predicted, the space rock plunged into the world and burst into flames.
If you’re wondering why you’re still around to read this, it’s because that meteor was only a metre in length. Far too puny to cause any damage, the asteroid instead harmlessly ignited in the upper atmosphere, temporarily painting the sky in a blue-green streak of light. As it turns out, small asteroids hit the planet all the time. They’re nothing to worry about – but it doesn’t take a massive leap in size for one to become a threat.
An asteroid just 20m long exploding in the sky could implode windows and knock people off their feet. A 50m space rock could ruin a town, causing widespread infrastructural damage, injuries and deaths many miles away from the site of the mid-air explosion. And an asteroid 140m in length would make its way to the ground, slice a hole in the face of the planet, and instantly destroy a sprawling metropolis.
For billions of years, Earth has been at the mercy of such cosmic threats – but oh, how times have changed. Today, there exists a field of applied science known as planetary defence, which is exactly what it sounds like: scientists and engineers working around the clock to protect the world from apocalyptic space rocks. One of the ways in which they do this is by spying on the heavens, scanning the night sky for asteroids that may be heading our way. In the next few years, two next-generation telescopes are coming online that will find almost all the space rocks that have been eluding even the most eagle-eyed astronomers. And if these missions achieve their considerable promise, all 8 billion of us will be significantly safer than we are now.
Planetary defence falls into two categories. The first is more offensive, using technology to deflect or destroy an incoming asteroid of those 140m-long city-killing or 50m-long town-trashing dimensions. In 2022, Nasa conducted the first planetary defence experiment in history. As part of the Double Asteroid Redirection Test, or Dart mission, it crashed an uncrewed spacecraft into a (harmless) asteroid to see if it could deflect it. Dart passed this test – a dress rehearsal for a genuine global emergency – with flying colours, suggesting that an asteroid big enough to vaporise a metropolis could be knocked out of Earth’s way, should we rush to meet it with force and precision.
There is, however, a huge caveat to this technique: we can’t deflect asteroids if we don’t know where they are. That’s why planetary defence is a tag team effort. While space agencies are building spacecraft and developing technology to deflect (or destroy) incoming asteroids, others have their eyes on the sky, seeking any near-Earth asteroids that may imperil us.
At the moment, Earth’s continued safety relies on optical astronomy: telescopes that look for sunlight glinting off space rocks that have yet to be discovered. Many observatories conduct all sorts of astronomical quests; finding asteroids is something that happens opportunistically during those surveys. Some telescopes, including a select few funded by Nasa, are solely dedicated to finding errant asteroids.
This method of space rock sleuthing has proved fairly effective, especially for the heftier asteroids. Nasa estimates that most of the planet killers – asteroids a kilometre long or above – zooming about near Earth’s orbit around the sun have been found. (The asteroid that swiftly ended the reign of the non-avian dinosaurs 66 million years ago was 10km long, and easily fits into the planet killer category.) It also suspects it has spotted just under half of those 140m, near-Earth, city-killer-size asteroids. (And, thankfully, none of them are on a collision course with Earth.)
But this crop of asteroid-seeking surveys is insufficient to shield the planet. There are roughly 14,000 near-Earth asteroids with city-flattening potential still out there to be found. And only a handful of near-Earth asteroids 50m long have been identified; Nasa reckons there are hundreds of thousands of town-trashing space rocks hiding nearby. Astronomers have been crying out for a better instrument to scan the stars to find these asteroids before they find us. Fortunately, they’re about to get two.
The first is Nasa’s Near-Earth Object Surveyor, or NEO Surveyor, mission. It’s essentially a sniper that is going to be hidden in outer space. Within 10 years of being launched, it will find 90% or more of those city-killer asteroids that have yet to be found by conventional means.
This planetary defence mission has been through developmental hell, having to spend years competing for attention with other space mission concepts that were purely about planetary exploration in the name of scientific curiosity. But today it is a separate, dedicated mission with its own funding stream – and Nasa recently gave the green light to begin building it. Its secret sauce comes from the fact that, instead of using reflected starlight to find asteroids, it’s going to seek out their heat signatures.
Using visible light to spy asteroids allows astronomers to catch sight of moving objects and get an estimate of their size. But there is an issue with this method: a small asteroid that has a shiny rocky coating reflects as much light as a bigger asteroid that has a dull, charcoal-like coating. That means it’s hard to tell the size of an asteroid using reflected light, which is problematic if you’re trying to work out whether you have a town trasher or a city killer coming at you.
A second issue is that there are probably many asteroids hidden in the glare of the sun. If you try to look at it with your naked eye – which I wouldn’t advise – you’d struggle to see anything. The same applies to Earth’s telescopes: if they point towards the sun, many asteroids will be invisible, like lit matches in front of a raging bonfire.
NEO Surveyor circumvents both problems. Sitting far from Earth, and covered in a sun shield, it will be one of the coldest objects ever built. And that allows its infrared eye to be extremely sensitive to any heat sources, including those of city-killer asteroids warmed by the sun. It will be so perceptive that even asteroids concealed by the sun’s glare will quickly show up on its scopes.
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It is to be launched sometime in the next five years. And when it does, it will already have a ground-based partner tallying up its own near-Earth asteroid count: the Vera C Rubin Observatory, under construction now in the mountains of Chile.
Unlike NEO Surveyor, Rubin is not a dedicated asteroid hunter, and it relies on reflected starlight, not infrared emissions. But it has the most technologically advanced mechanical eye ever made. With a colossal mirror that collects even the faintest, most distant starlight, and a 3,200-megapixel digital camera the size of a car, it will see and chronicle anything that moves in the dark sky above, from distant exploding stars to interstellar comets.
It will also create a detailed inventory of pretty much everything in the solar system, including the host of objects flying around close to our planet. The first asteroid was spotted in 1801, and it took two centuries to find a million more. In the first six months of operations, which begin in 2025, Rubin will double that number. It is, in other words, a polymathic telescope; one that, among all its other tasks, will find asteroids of all shapes and sizes faster than any other spotter on Earth.
Like any ground-based observatory, Rubin must still deal with bad weather and an increasing number of reflective artificial satellites occluding its view. But, along with NEO Surveyor, it will accomplish what traditional telescopes have often struggled to do: find potentially cataclysmic asteroids. In fact, the combined power of NEO Surveyor and the Rubin Observatory means that, by the 2040s, we should know whether Earth is in danger of being hit by a city killer-sized asteroid within the next century.
If we did discover that we were in the line of fire, it would be terrifying. But at least we could do something about it: space agencies could launch a mission to deflect it – either hitting it with a Dart-like spacecraft, or aggressively irradiating one side of it with a nuclear explosion – or blast it into tiny pieces, or at the very least (and once the impact zone is more precisely known) plan to get those in harm’s way to a place of safety. And if it’s found that none of these asteroids are heading our way for the foreseeable future, then humanity can breathe a collective sigh of relief, and have one less existential risk to worry about.
For most of our species’s history, we had no dominion over space. It was something that affected us, not the other way around. Even after setting up space stations in orbit around the planet, after visiting the moon with astronauts, and after sending spacecraft into interstellar space, we have remained passive observers of the cosmos. Planetary defence makes us active participants in it. We not only live in a time in which we can make intricate maps of the night sky and everything in it, we are also able to rearrange our galactic neighbourhood to make it a more habitable place to live.
The world is besieged by conundrums: the climate crisis, war, poverty, political instability, pandemics, environmental destruction. Earth is a beautiful, troubled place. But, increasingly, it’s one protected from threats originating from beyond the firmament – and for that, we can most certainly be thankful.
Dr Robin George Andrews is the author of How to Kill an Asteroid: The Real Science of Planetary Defence (WW Norton & Company, £19.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply