ohn September 4, an asteroid was spotted that was curving towards Earth. Astronomers quickly determined that it would impact the planet within 10 hours. The Philippine 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 here reading this, it’s because that meteor was only a meter long. Far too small to do any damage, the asteroid instead ignited harmlessly 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 huge jump in size for one to become a threat.
An asteroid just 20m long exploding in mid-air can blow out windows and knock people off their feet. A 50 m space rock can destroy a town and cause widespread infrastructure damage, injuries and deaths many kilometers from the location of the air blast. And an asteroid 140 m long would make its way to earth, cut a hole in the face of the planet and instantly destroy a sprawling metropolis.
For billions of years the 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 defense, 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 they do this is by spying on the heavens and scanning the night sky for asteroids that might be headed our way. In the next few years, two next-generation telescopes are coming online that will find almost all the space rocks that have eluded 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 defense 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-wasting dimensions. In 2022, Nasa conducted the first planetary defense experiment in history. As part of the Double Asteroid Redirection Testor Dart mission, it crashed an unmanned spacecraft into a (harmless) asteroid to see if it could deflect it. Dart passed this test – a dress rehearsal for a true global emergency – with flags flying, suggesting that an asteroid large enough to vaporize a metropolis could be knocked out of Earth’s path, we would rush to hit it with force and to meet precision.
There’s a big caveat to this technique, though: we can’t deflect asteroids if we don’t know where they are. This is why planetary defense is a team effort. While space agencies are building spacecraft and developing technology to deflect (or destroy) incoming asteroids, others have their eyes on the skies, looking for any near-Earth asteroids that might endanger us.
Right now, Earth’s continued safety relies on optical astronomy: telescopes that look for sunlight glinting off yet-to-be-discovered space rocks. Many observatories perform all kinds of astronomical quests; finding asteroids is something that happens opportunistically during those surveys. Some telescopes, including a select few that through Nasais solely dedicated to finding errant asteroids.
This method of rock detection in space has been quite effective, especially for the heavier asteroids. Nasa estimates that most of the planet killers – asteroids a kilometer or more in length – that zoom close to Earth’s orbit around the sun have been found. (The asteroid that quickly ended the reign of the non-avian dinosaurs 66 million years ago was 10 km long and fits easily into the planet-killer category.) It’s also suspected to be just under half of that 140 m near-Earth city noticed -killer-sized asteroids. (And luckily, none of them are on a collision course with Earth.)
But this crop of asteroid-seeking surveys is insufficient to protect the planet. There are about 14,000 near-Earth asteroids with city-destroying potential still out there. And only a handful of near-Earth asteroids 50m long have been identified; Nasa reckons there are hundreds of thousands of space rocks destroyed by the town hiding nearby. Astronomers have been clamoring for a better tool to scan the stars to find these asteroids before they hit us. Fortunately, they are about to get two.
The first is Nasa’s Near-Earth Object Surveyoror NEO Surveyor, mission. It is essentially a sniper that will be hidden in outer space. Within 10 years of launch, it will find 90% or more of those city-killing asteroids that have yet to be found by conventional means.
This planetary defense mission went through development hell, and had 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’s a separate, dedicated mission with its own funding stream – and Nasa recently gave the green light to start building it. Its secret sauce comes from the fact that, instead of using reflected starlight to find asteroids, it looks for their heat signatures.
Using visible light to spy on asteroids allows astronomers to see moving objects and get an estimate of their size. But there’s a problem with this method: a small asteroid that has a shiny, rocky coating reflects as much light as a larger asteroid that has a dull, charcoal-like coating. This means that it’s difficult to determine the size of an asteroid using reflected light, which is problematic if you’re trying to work out whether you’ve got a town junkie or a city killer coming your way.
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 would not advise – you will struggle to see anything. The same is true of Earth’s telescopes: if they are pointed at the sun, many asteroids will be invisible, like lit matches before a raging bonfire.
NEO Surveyor circumvents both problems. Sitting far from Earth and covered with a solar shield, it will be one of the coldest objects ever built. And that makes his infrared eye extremely sensitive to any heat sources, including that of city-killer asteroids heated by the sun. It will be so noticeable that even asteroids hidden by the sun’s glare will quickly appear on its scope.
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It will be launched sometime in the next five years. And when it does, it will already have a ground-based partner doing its own near-Earth asteroid count: the Vera C Rubin Observatoryunder 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 carit will see and chronicle anything moving in the dark skies above, from distant exploding stars to interstellar comets.
It will also create a detailed inventory of almost everything in the solar system, including the myriad objects that fly 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, starting in 2025, Rubin will double that number. In other words, it is 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 still has to deal with bad weather and an increasing number of reflective artificial satellites that block its view. But, together 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 we should know by the 2040s whether Earth is in danger of being hit by a city-killer-sized asteroid within the next century.
If we did discover we were in the line of fire, it would be terrifying. But at least we can do something about it: space agencies can launch a mission to deflect it — either hit it with a Dart-like spacecraft, or aggressively irradiate one end of it with a nuclear explosion – or blow it to small pieces, or at least (and once the impact zone is more precisely known) plan to get those harmed to a place of safety. And if none of these asteroids are found to be headed our way for the foreseeable future, humanity can breathe a collective sigh of relief, and have one less existential risk to worry about.
For most of our species’ history, we had no dominion over space. It was something that affected us, not the other way around. Even after we set up space stations in orbit around the planet, after we visited the moon with astronauts, and after we sent spacecraft into interstellar space, we remained passive observers of the cosmos. Planetary defense makes us active participants in it. Not only do we live in a time where we can make intricate maps of the night sky and everything in it, we are also able to rearrange our galactic environment to make it a more habitable place to live.
The world is besieged by mysteries: the climate crisis, war, poverty, political instability, pandemics, environmental destruction. The earth is a beautiful, troubled place. But it is increasingly one that is protected from threats emanating from beyond the firmament – and for that we can certainly be thankful.
Dr. Robin George Andrews is the author of How to Kill an Asteroid: The Real Science of Planetary Defense (WW Norton & Company, £19.99). To support the Guardian and Observer, order your copy from guardianbookshop.com. Delivery charges may apply
