Okay - depends what counts as significant.
Quoting from Spaceguard FAQ
So - if you mean large enough for mass extinctions - according to Spaceguard, that happens every 50 million years on average.
Probability of a single impact
So plugging in the numbers from that page - and interpreting it as a 50% chance of it happening in the next 50 million years, the chance that we are hit by an asteroid as large as that before 2100 is under 0.0001%.
For an impact of 2 km or more, large enough to have a global impact, such as an "impact winter" then the FAQ says those happen once or twice per million years.
So - the chance of one of those by 2100 is a little under 0.01%.
Another thing to bear in mind though is that we have already identified 90% of all potential impactors of diameter 1 km or more in the inner solar system - and by 2020 NASA has been tasked with plotting the trajectories of 90% of potential impactors of diameter of 140 meters or more.
So, since we haven't yet found a giant impactor with a chance of hitting Earth by 2100, the chance that there is one amongst the remaining 10% of large asteroids not yet mapped is even smaller. So make that more like a 0.00001% chance (I've added an extra 0 there though you might not have noticed).
Then - within a few years as we continue to map down to 140 meters - that 0.00001% chance will be reduced hugely - will be almost impossible that there are any unknown 10 km impactors in the inner solar system which could hit Earth by 2100. Is already pretty unlikely that there are.
There is a bit of an unknown factor there though - the probability of an impactor coming from the Oort cloud or the Kuiper belt - a "giant comet".
This is already included in the calculation of overall risk - as that's based on analysis of cratering records. But Spaceguard can't find these, not by 2020 anyway, as they are just too far away to spot.
As a result, Spaceguard can't reduce the probability of them any further. It's hard to estimate how many impacts there are in this category - but probably only a few percent of the impacts are from long period giant comets.
Another way to look at this is to look at the actual impacts we've had.
So, the Dinosaurs impact was 66 million years ago, and the crater is 180 km in diameter,
Then a more recent impact is this impact in Russia
Popigai crater
35 million years ago, thought to be created by an impactor either 5 km in diameter or 8 km in diameter depending on the type of asteroid.
Here is a list of identified giant impact craters on the Earth
List of impact craters on Earth
Most of the larger ones are ancient but are a few younger ones, including Karakul 52 km in diameter and 25 million years old, and Chesapeake Bay impact crater originally 40 km in diameter and 35 million years old.
You might perhaps think - that it's been 66 million years since the dinosaurs impact - so we are "overdue" for a new one so it might happen right now (to the nearest million years or so).
But- statistics don't work like that - not unless there are other factors involved. It is no more likely that we will be hit by another asteroid like that now, than it was immediately after it happened. And - we have been hit by quite a few asteroids since then that were nearly as big.
So - chances are - that it's not us that will need to deal with this size of impact. But rather - whatever technological species have evolved on our planet a few tens of millions of years into the future.
BTW the movies may give a false impression of the size of a "giant asteroid". Perhaps you have an image of something like this?
But asteroids as large as that only hit the Earth in the first few hundred million years soon after the formation of the Moon in the late heavy bombardment, when the solar system was still settling down and full of half formed planets and embryo planets and assorted debris from their formation. That's also when the large dark patches were created on the Moon.
There is pretty close to zero chance of us getting hit by one of those now., as there is no impact crater large enough in the geological record.
The largest one we know about is a 300 km crater in S. Africa which formed about 2 billion years, and impactor rather similar in size, 5 to 10 km in diameter Vredefort crater. There have probably never been any significantly larger impactors since the Hadean period 4 billion years ago.
Except that is - several hundred million years from now - there is a tiny chance that resonances with Jupiter could shift the tiny planet Mercury out of its orbit to such an extent that it escapes from the solar system, and it might hit Earth or Venus on the way out. Apart from that, not likely to get an impact like this any more.
Here is the Earth with a 10 km asteroid added, to scale - it is too small to see at this resolution.
To have a go at spotting it in the full scale image go here
Killer Asteroids - Impact: Earth
So - that's what the fuss is about. And - it would indeed be a major thing if something like that hit the Earth. But chance is absolutely tiny that it will happen before 2100.
If we keep technology - then surely by then we will have mapped out the entire solar system right out to the Oort cloud.
If we still have technology millions of years into the future, chances are we'd know about the impactor not just for years in advance, but for thousands, even tens or hundreds of thousands of years in advance - that there was a possibility of it hitting the Earth. A
Also - is important to realise I think - that the dinosaurs of course didn't have technology - or spaceguard - and that humans do - that makes a big difference.
Even with, say, a couple of years of warning - even with a few months of warning with people world wide working hard to do something about it - we could do things such as build underground shelters to survive the initial impact, evacuate the zone of the impact crater itself - indeed move almost everyone to the other side of the world, stock up on food - and build underground seed banks (which we already have one in Norway) to survive the impact - and do many things to help us to survive.
The "impact winter" following would be something we could do something about also, including for instance growing crops with artificial light, or in greenhouses or both. And - the "impact winter" doesn't last that long - just a few years - then we can get back to norm
But - is really hardly worth spending a lot of time thinking about such things - with such low probabilities.
This calculation of probabilities is based on things such as cratering record so covers both asteroids and comets. So, comets don't add to the overall risk as they are already taken account of.
Where they are significant is when you try to reduce the risk further.
Spaceguard covers short period comets such as Halley's comet.
But in the case of long period comets, SpaceGuard can't do anything currently to reduce the chances of an undetected comet approaching Earth, especially one approaching the inner solar system for the first time since the origin of the solar system, born in the Oort cloud.
The Oort cloud is just too far away for it to be practical to survey 10 km scale objects with our present day or near future telescopes or spacecraft.
They are also potentially more dangerous for the same mass because typically they are twice as fast and the effect goes up as the square of the velocity, so four times more destructive for the same mass.
We can get a rough idea of how that would go from the example of the SlidingSpring comet. C/2013 A1
It's now thought to be around 700 meters in diameter. We have known about it since 3rd January 2013 and it is due to encounter Mars on 19 October 2014. The original calculated probability of hitting Mars was 1 in 1250 back in March 2013. It's now known that the nucleus will miss Mars.
So - similarly if we had a giant comet heading for Earth - we'd spot it perhaps a year in advance - and would have an initial probability like that of impact that would gradually be refined as it gets closer. We would know pretty much for sure a few months before it's closest approach.
It's likely to be some time before we can begin to map Oort cloud comets.
But - important to realize, I think, that even in the case of an Oort cloud or very long period comet, we would still have many months to prepare and either destroy the comet, deflect it, or to refine calculations of the impact point and apply mitigation strategies here on the Earth.
For more about this see Page on spaceguardcentre.com
Smaller asteroids would be pretty devastating, of course. And that's where Spaceguard come in. They aim to plot 90% of the asteroids of over 140 meters by 2020. This is definitely worth doing as we know the smaller ones happen frequently.
There's a significant risk of an impactor of say 50 meters or larger - indeed already had one like that last century, the Tunguska meteorite.
They estimate that impacts like that happen every thousand years or so. And - they are too small for Spaceguard to find so far, that will need to be a future refininement.
This impact knocked down trees over a large area of Siberia. Local effects only - made no difference outside the impact region.
It would be devastating if it landed on a city. Though chance of that happening is of course small with most of the Earth still relatively uninhabited - seas, deserts and so on.
It would also be locally devastating if it landed in an ocean and caused a Tsunami.
As for intermediate ones - here are some figures for the frequency and risk - Impact event frequency and risk
Take for instance the 140 m or larger ones that Spaceguard aim to find by 2020 - then they happen on average every 15,000 years or so and though not global in impact, would be pretty devastating "locally". So chance of one of those by the end of the century - roughly 2.5% - but by 2020 then that will be greatly reduced to roughly 0.25% - either that or we will discover one of them - and then will be thinking about how to divert it before it hits Earth.
These impact probability figures - are approximate - and change.
If we do find one on an impact trajectory with Earth - then there are things we can do to divert the asteroids. Lots of suggestions, the main thing would be to decide which of the many ideas to use.
It depends how much advance warning you have. If you have decades of warning, then just a tiny nudge could make a difference. For instance, with enough warning, it might be enough to simply dust one of its poles with white or black "paint" to alter its trajectory through the Yarkovsky effect.
For more about this see Asteroid impact avoidance
Quoting from Spaceguard FAQ
"WHAT SIZE OF NEOs ARE DANGEROUS?
The Earth’s atmosphere protects us from most NEOs smaller than a modest office building (40 m diameter, or impact energy of about 3 megatons). From this size up to about 1 km diameter, an impacting NEO can do tremendous damage on a local scale. Above an energy of a million megatons (diameter about 2 km), an impact will produce severe environmental damage on a global scale. The probable consequence would be an “impact winter” with loss of crops worldwide and subsequent starvation and disease. Still larger impacts can cause mass extinctions, like the one that ended the age of the dinosaurs 65 million years ago (15 km diameter and about 100 million megatons)."
So - if you mean large enough for mass extinctions - according to Spaceguard, that happens every 50 million years on average.
Probability of a single impact
So plugging in the numbers from that page - and interpreting it as a 50% chance of it happening in the next 50 million years, the chance that we are hit by an asteroid as large as that before 2100 is under 0.0001%.
For an impact of 2 km or more, large enough to have a global impact, such as an "impact winter" then the FAQ says those happen once or twice per million years.
So - the chance of one of those by 2100 is a little under 0.01%.
Another thing to bear in mind though is that we have already identified 90% of all potential impactors of diameter 1 km or more in the inner solar system - and by 2020 NASA has been tasked with plotting the trajectories of 90% of potential impactors of diameter of 140 meters or more.
So, since we haven't yet found a giant impactor with a chance of hitting Earth by 2100, the chance that there is one amongst the remaining 10% of large asteroids not yet mapped is even smaller. So make that more like a 0.00001% chance (I've added an extra 0 there though you might not have noticed).
Then - within a few years as we continue to map down to 140 meters - that 0.00001% chance will be reduced hugely - will be almost impossible that there are any unknown 10 km impactors in the inner solar system which could hit Earth by 2100. Is already pretty unlikely that there are.
There is a bit of an unknown factor there though - the probability of an impactor coming from the Oort cloud or the Kuiper belt - a "giant comet".
This is already included in the calculation of overall risk - as that's based on analysis of cratering records. But Spaceguard can't find these, not by 2020 anyway, as they are just too far away to spot.
As a result, Spaceguard can't reduce the probability of them any further. It's hard to estimate how many impacts there are in this category - but probably only a few percent of the impacts are from long period giant comets.
Another way to look at this is to look at the actual impacts we've had.
So, the Dinosaurs impact was 66 million years ago, and the crater is 180 km in diameter,
Then a more recent impact is this impact in Russia
Popigai crater
35 million years ago, thought to be created by an impactor either 5 km in diameter or 8 km in diameter depending on the type of asteroid.
Here is a list of identified giant impact craters on the Earth
List of impact craters on Earth
Most of the larger ones are ancient but are a few younger ones, including Karakul 52 km in diameter and 25 million years old, and Chesapeake Bay impact crater originally 40 km in diameter and 35 million years old.
You might perhaps think - that it's been 66 million years since the dinosaurs impact - so we are "overdue" for a new one so it might happen right now (to the nearest million years or so).
But- statistics don't work like that - not unless there are other factors involved. It is no more likely that we will be hit by another asteroid like that now, than it was immediately after it happened. And - we have been hit by quite a few asteroids since then that were nearly as big.
So - chances are - that it's not us that will need to deal with this size of impact. But rather - whatever technological species have evolved on our planet a few tens of millions of years into the future.
BTW the movies may give a false impression of the size of a "giant asteroid". Perhaps you have an image of something like this?
But asteroids as large as that only hit the Earth in the first few hundred million years soon after the formation of the Moon in the late heavy bombardment, when the solar system was still settling down and full of half formed planets and embryo planets and assorted debris from their formation. That's also when the large dark patches were created on the Moon.
There is pretty close to zero chance of us getting hit by one of those now., as there is no impact crater large enough in the geological record.
The largest one we know about is a 300 km crater in S. Africa which formed about 2 billion years, and impactor rather similar in size, 5 to 10 km in diameter Vredefort crater. There have probably never been any significantly larger impactors since the Hadean period 4 billion years ago.
Except that is - several hundred million years from now - there is a tiny chance that resonances with Jupiter could shift the tiny planet Mercury out of its orbit to such an extent that it escapes from the solar system, and it might hit Earth or Venus on the way out. Apart from that, not likely to get an impact like this any more.
Here is the Earth with a 10 km asteroid added, to scale - it is too small to see at this resolution.
Killer Asteroids - Impact: Earth
So - that's what the fuss is about. And - it would indeed be a major thing if something like that hit the Earth. But chance is absolutely tiny that it will happen before 2100.
WHAT ABOUT FUTURE TECHNOLOGICAL SPECIES - AFTER ALL - IT IS PRETTY MUCH INEVITABLE TO HAPPEN SOME TIME
If we keep technology - then surely by then we will have mapped out the entire solar system right out to the Oort cloud.
If we still have technology millions of years into the future, chances are we'd know about the impactor not just for years in advance, but for thousands, even tens or hundreds of thousands of years in advance - that there was a possibility of it hitting the Earth. A
Also - is important to realise I think - that the dinosaurs of course didn't have technology - or spaceguard - and that humans do - that makes a big difference.
Even with, say, a couple of years of warning - even with a few months of warning with people world wide working hard to do something about it - we could do things such as build underground shelters to survive the initial impact, evacuate the zone of the impact crater itself - indeed move almost everyone to the other side of the world, stock up on food - and build underground seed banks (which we already have one in Norway) to survive the impact - and do many things to help us to survive.
The "impact winter" following would be something we could do something about also, including for instance growing crops with artificial light, or in greenhouses or both. And - the "impact winter" doesn't last that long - just a few years - then we can get back to norm
But - is really hardly worth spending a lot of time thinking about such things - with such low probabilities.
COMETS
This calculation of probabilities is based on things such as cratering record so covers both asteroids and comets. So, comets don't add to the overall risk as they are already taken account of.
Where they are significant is when you try to reduce the risk further.
Spaceguard covers short period comets such as Halley's comet.
But in the case of long period comets, SpaceGuard can't do anything currently to reduce the chances of an undetected comet approaching Earth, especially one approaching the inner solar system for the first time since the origin of the solar system, born in the Oort cloud.
The Oort cloud is just too far away for it to be practical to survey 10 km scale objects with our present day or near future telescopes or spacecraft.
They are also potentially more dangerous for the same mass because typically they are twice as fast and the effect goes up as the square of the velocity, so four times more destructive for the same mass.
We can get a rough idea of how that would go from the example of the SlidingSpring comet. C/2013 A1
It's now thought to be around 700 meters in diameter. We have known about it since 3rd January 2013 and it is due to encounter Mars on 19 October 2014. The original calculated probability of hitting Mars was 1 in 1250 back in March 2013. It's now known that the nucleus will miss Mars.
So - similarly if we had a giant comet heading for Earth - we'd spot it perhaps a year in advance - and would have an initial probability like that of impact that would gradually be refined as it gets closer. We would know pretty much for sure a few months before it's closest approach.
It's likely to be some time before we can begin to map Oort cloud comets.
But - important to realize, I think, that even in the case of an Oort cloud or very long period comet, we would still have many months to prepare and either destroy the comet, deflect it, or to refine calculations of the impact point and apply mitigation strategies here on the Earth.
For more about this see Page on spaceguardcentre.com
SMALLER ASTEROIDS
Smaller asteroids would be pretty devastating, of course. And that's where Spaceguard come in. They aim to plot 90% of the asteroids of over 140 meters by 2020. This is definitely worth doing as we know the smaller ones happen frequently.
There's a significant risk of an impactor of say 50 meters or larger - indeed already had one like that last century, the Tunguska meteorite.
They estimate that impacts like that happen every thousand years or so. And - they are too small for Spaceguard to find so far, that will need to be a future refininement.
This impact knocked down trees over a large area of Siberia. Local effects only - made no difference outside the impact region.
It would be devastating if it landed on a city. Though chance of that happening is of course small with most of the Earth still relatively uninhabited - seas, deserts and so on.
It would also be locally devastating if it landed in an ocean and caused a Tsunami.
As for intermediate ones - here are some figures for the frequency and risk - Impact event frequency and risk
Take for instance the 140 m or larger ones that Spaceguard aim to find by 2020 - then they happen on average every 15,000 years or so and though not global in impact, would be pretty devastating "locally". So chance of one of those by the end of the century - roughly 2.5% - but by 2020 then that will be greatly reduced to roughly 0.25% - either that or we will discover one of them - and then will be thinking about how to divert it before it hits Earth.
These impact probability figures - are approximate - and change.
If we do find one on an impact trajectory with Earth - then there are things we can do to divert the asteroids. Lots of suggestions, the main thing would be to decide which of the many ideas to use.
It depends how much advance warning you have. If you have decades of warning, then just a tiny nudge could make a difference. For instance, with enough warning, it might be enough to simply dust one of its poles with white or black "paint" to alter its trajectory through the Yarkovsky effect.
For more about this see Asteroid impact avoidance
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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