Current Affairs

How often do we brush with asteroid annihilation?

Almost 2000 'Potentially Hazardous Asteroids' were discovered in the last year. What's the risk one will hit the earth?

Based in Cambridge, Massachusetts, the Minor Planet Centre (MPC) forms part of the Harvard-Smithsonian Centre for Astrophysics, and works with NASA to identify and catalogue any and all ‘Near Earth Asteroids’ – essentially, any chunk of rock and ice flying through space. One such object, the two-metre-wide Asteroid 2018 LA, collided with our planet in June.

Google ‘Asteroid 2018 LA’, and you’ll see news stories decrying the fact that NASA discovered this asteroid just hours before impact. What you won’t get from the headlines, however, is the fact that this was also only the third instance ever of an object discovered to be on an impact trajectory with earth.

As an astronomer at the MPC, Peter Veres is one of the researchers responsible for keeping tabs on potential dangers whirling about in space. He works daily to process asteroid observations from around the world, as well as helping to develop new software in collaboration with NASA. It is by the combined efforts of this global observation network that asteroid threats to the earth are documented and monitored. In short, if Planet Earth does get in the way of a chunk of rock the size of a small moon, Veres would, hopefully, be among the very first people on the planet to know about it.

‘A 100-metre body hits Earth every few thousand years,’ explains Veres. ‘This would certainly leave an impact crater. A kilometre-sized impactor hits the Earth roughly every half a million years and could devastate an area hundreds of kilometres in radius, as well as affecting our climate for some time. When it comes to dinosaur-killer-class asteroids that cause global extinction events, that’s an asteroid of 10-kilometres across, or more.’

So, just how likely are we to meet the same fate as the Tyrannosaurus Rex? Is it simply a matter of time before an asteroid with our name on it arrives? And how much warning would we have? Veres weighs up our chances…

‘Near misses’ happen all the time

‘Around 18,500 “Near-Earth Asteroids” have been discovered so far. More than 2,000 were discovered in 2017. Only a few of these spark media attention. However, encounters you might class as “near misses” are not rare; for instance, within the last year, 62 objects came closer to the Earth than the Moon – a distance of 239,000 miles. That is incredibly close, considering asteroids are thought to travel between 25,000 mph to 160,000 mph.’

There are no imminent threats – that we know of

‘There is a category of so-called “Potentially Hazardous Asteroids” (PHA). We define these as objects larger than 140 metres, with an orbit that brings it closer than 7.5 million kilometres to Earth. So far, we have discovered almost 2,000 PHAs. However, none of these are currently on a direct intercept course with Earth, and we can predict where they will be in the future. The danger comes from the undiscovered asteroids and the smaller objects, which are the most numerous.’

But it’s not ‘if’, it’s ‘when’ and ‘how big’

‘Earth impact probability can be derived from studying past impacts. We know Earth has been hit many times, and will be in the future. Asteroid fragments that are a millimetre to a centimetre in diameter bombard the Earth constantly, and we observe those as shooting stars. Statistically, a 20-metre asteroid hits the Earth once per 50 years. The Chelyabinsk meteoroid was about 20-metres wide, and its explosion in 2013 injured more than 1,000 people. Generally, objects of this size do not reach the surface, as the atmosphere slows them down and they explode high in the atmosphere.’

The chance of Armageddon is low, but not zero

‘A kilometre-wide object could cause disaster on a continental scale. An asteroid of a few kilometres is enough to cause a global disaster. Fortunately, we believe that about 95 per cent of Near-Earth Objects larger than one kilometre have been discovered, and pose no threat. NASA’s current goal is to discover 90 per cent of objects larger than 100-metres in the next decade. Our best defence is to discover all potentially hazardous objects and be able to compute their position hundreds of years into the future. The early discovery of a potential impactor will give us decades to get ready.’