One day we might scan the skies and see it coming: an asteroid heading straight for Earth. Our response would depend on how far away the asteroid was when we saw it, how big it was and where exactly it was going to land. Four strategies are most likely, but many others are being explored to give us the best chance of deflecting a fast-travelling visitor from space. ‘Hide in the basement’ Or, in formal terms, civil defence – the response used for natural disasters such as hurricanes or tornadoes where there is a short-term warning. People would evacuate from the affected area if possible, and then hunker down and take cover. This would be the obvious choice for smaller objects, like those 1 metre across that enter Earth’s atmosphere a few times a year but don’t reach the ground intact. It would also likely be the response to objects as big as the 20-metre asteroid that exploded over the Siberian city of Chelyabinsk in 2013, injuring more than 1500 people. Residents were caught unawares and had not taken cover, so these injuries were mostly caused by flying glass smashed in the impact shockwave. Even a small amount of warning would probably have kept everyone safe. For a relatively small, contained impact like this, the resources needed to prepare the city would be far less than the resources needed to try to deflect the asteroid.
The impact of a larger asteroid would be a catastrophe to a region, a continent or the entire world depending on its size. Even a smaller visitor like the one over Chelyabinsk would be more of a threat if it were on course for a large city – making evacuation harder and risking the loss of far more homes and infrastructure – or were going to land in the ocean and cause a tsunami. In these cases, there might be pressure to prevent the impact if possible.
