If an asteroid hits Earth today, how big must it be in order to wipe out all life?

Around 66 million years ago, a 10km-wide asteroid led to the extinction of the dinosaurs. But plenty of life survived. Could an asteroid kill everything?

It is likely that some form of life would survive almost any size of asteroid. There are species such as tardigrades that are hardy enough to survive even in the frozen vacuum of space. A team of researchers from Oxford and Harvard Universities has estimated that these water-dwelling ‘water bears’ would be unaffected by any astronomical event short of impact from an asteroid large enough to boil away all the oceans of the world.

To achieve this would require an asteroid of mass (2×1018 kg), the researchers wrote in a 2017 article in Scientific Reports, which might require a diameter over 200km. No known object of this size intersects Earth’s path, and the chance of an unknown impact from an asteroid of that size in the whole of Earth’s remaining lifespan is below 0.01%.

The researchers also considered whether a nearby supernova could boil

away our oceans, but calculated that the explosion would need to be within a distance of 0.14 light years, while the nearest star is some 4 light years away.

Even if the tardigrades die, there may be some ‘extremophile’ which survives those particular apocalyptic circumstances. Whether life on Earth could regenerate from such survivors would depend on ongoing conditions.

Of course smaller impacts would still cause major disasters. An asteroid of diameter 15km struck 2 billion years ago in what is now South Africa, creating the Vredefort Crater. Scientists think that complex life was emerging just as the Vredefort asteroid struck, and that the disaster delayed evolution by more than a billion years. A repeat of such a major impact today would be unlikely to send humans fully extinct directly, but the vast majority would perish from hunger, cold, air pollution and/or the collapse of law.