66 million years ago, an object the size of Mount Everest crashed down into the sea near modern-day Mexico and radically changed the world.
It sent a cloud of dust and molten rock into the air that blocked out the sun, which killed loads of plants, which big herbivores like Triceratops needed, which large carnivores like T. Rex needed. The end result: tiny, furry creatures called mammals started to dominate the world instead of giant reptiles. (Incidentally, there is good evidence to suggest that they were on the way out anyways because of a changing climate and the asteroid was just the killer blow).
That wasn’t the first time, and it probably isn’t the last.
We’ve had plenty of smaller impacts since then. Most memorably, in 2013, an object the size of a house (20m) entered the atmosphere above Chelyabinsk, Russia. Due to the high speed and shallow angle, the meteor exploded while it was still 30km above the ground. It still injured about 1000 people and broke most of the windows in town, but it could have been much worse.
Why didn’t we see it coming?
For one, it was relatively small as far as asteroids go. This one was 20m across, so even if it had hit directly, it still would have turned into an airburst and never hit the ground. But they can get as big as tens of kilometres and wipe out continents.
For another, its radiant (where it appeared to be coming from) was pretty close to the sun, meaning any telescopes that could have spotted it would have been blinded by the light of the sun.
Even if we didn’t see the Chelyabinsk meteor coming, we are keeping an eye on the sky. The UN has two working groups that meet regularly on the topic of potentially dangerous asteroids: the International Asteroid Warning Group (IAWC) and the Space Missions Planning Advisory Group (SMPAG).
The IAWC keeps an eye out for anything that might pose a danger and maintains a network and plan of what to do if it finds any “city-killers”.
We’re pretty sure we know where 90% of the objects bigger than 1km are and we’ll soon get a handle on 90% of objects as small as 140m. That still leaves a pretty scary 10% unknown, but it’s definitely better than burying our heads in the sand.
But it’s not just governments keeping an eye on the sky. There also non-profits like the B612 foundation, founded by former astronauts Ed Lu, Rusty Schweickart, and their space science friends. They have a fantastic motto: “Finding asteroids before they find us”.
They’re planning a mission called Sentinel, which would orbit closer to the Sun than Earth, keeping the sun to its back and looking back towards the Earth. It’s a good idea, but the money might be drying up. It’s notoriously expensive to launch a space mission, and without Nasa’s help, it might be even harder.
The plan
So what do we do if we find an asteroid on a collision course with Earth?
Send Bruce Willis.
Or the Space Missions Planning Advisory Group and Nasa’s remotely operated satellite.
If we know long enough in advance, the most realistic and safe way to redirect an asteroid is actually just to loiter next to it for long enough so that the gravity of the satellite starts to move the asteroid. No nukes that turn one dangerous asteroid into thousands of smaller ones, no lasers that would require sending thousands of laser-carrying satellites into orbit (dual-use anyone?).
With the success of Rosetta, there’s proof we can send missions to asteroids and comets, so the next step is to test out a satellite that can pick up some mass from an asteroid and use it as a gravity tractor. The current Nasa plan is to launch just such a satellite in 2021.
So while there’s a terrifyingly real (if small) possibility of an imminent apocalyptic event, I take solace in the fact that there are smart people working on it. We have a plan.
Part of the plan is actually not to worry too much unless we need to.
(Below is from the minutes of the October 2016 meeting of the SMPAG)
ANNEX II: Recommended criteria and thresholds for impact response actions
After data collected and analyzed has been adequately verified and validated, given the circumstances of the actual, real-world scenario:
1) IAWN shall warn of predicted impacts exceeding a probability of 1% for all objects characterized to be greater than 10 meters in size, or roughly equivalent to absolute magnitude of 28 if only brightness data can be collected.
2) Terrestrial preparedness planning should begin when warned of a possible impact:
– Predicted to be within 20 years,
– Probability of impact is assessed to be greater than 10%, and
– Object is characterized to be greater than 20 meters in size, or roughly equivalent to absolute magnitude of 27 if only brightness data can be collected.
3) SMPAG should start mission option(s) planning when warned of a possible impact:
– Predicted to be within 50 years,
– Probability is assessed to be greater than 1%, and
– Object is characterized to be greater than 50 meters in size, or roughly equivalent to absolute magnitude of 26 if only brightness data can be collected.
Jon Farrow 
