A new study on comet and asteroid impact craters has found that the Earth is just as likely to be struck now as it was hundreds of millions of years ago. 

The 50,000 year old Meteor Crater in Arizona. The visitors centre at the top provides a sense of scale. (National Map Seamless Viewer/US Geological Service)

Almost 200 impact craters have been discovered dotting the Earth’s surface, including the most famous one in the Yucatan Peninsula of Mexico where an asteroid smashed into our planet 65 million years ago and wiped out the dinosaurs. Previously, it was thought that comet and asteroid impacts came in waves every 13 to 50 million years.

One explanation for this behaviour is that the motion of our Solar System through the Galaxy alters the gravitational influence of nearby stars on the Oort cloud, a vast swarm of icy bodies thought to exist on the edge of the Solar System. This could cause an object to be tugged free from this massive arsenal of comets and be sent hurtling towards Earth.

Another more far-fetched explanation is that an undetected solar neighbour, known as Nemesis, is responsible. A highly elliptical orbit would bring Nemesis close enough to the Oort cloud every few million years to dislodge comets.

However, Coryn Bailer-Jones of the Max Planck Institute for Astronomy has suggested that these patterns in impact rates don’t exist at all and are merely a result of the statistics used. Instead, his models show that the Earth is just as likely to be hit by a comet or asteroid as it was 250 million years ago and it is even possible that the rate of bombardment is increasing.

Bailer-Jones comes to these conclusions by using what is known as “Bayesian” statistics instead of traditional statistics used by previous scientists. Traditional statistics involves comparing a hypothesis to a “default” situation to see how likely it is. Bayesian statistics on the other hand, compares several different scenarios in order to see which is the most likely.

Bailer-Jones only included craters less than 250 million years old and at least 5 kilometres in diameter, to eliminate bias. The reason we see fewer older craters is not because there were less impacts, but because they have succumbed to the detrimental effects of erosion. We are also less likely to see smaller craters for the same reason.

While there is no satisfactory explanation as to why the impact rates might be increasing, these results do show that the theoretical Nemesis is no longer required to explain cycles of impact rates.