Yes we would find out right away. In fact there have been a few cases where there is a low probability of an impact in the future. And those were rather blown out of proportion in news stories.
CURRENT IMPACT RISKS
It is easy to keep up to date with the latest news by visiting this page:
Current Impact Risks.
This is the first place to go if you see one of those stories. It is automatically updated with all the new observations. Look to see if any of the entries in the table are orange or red. As of writing (December 2015), this has never yet happened.
If any of them go red, a collision is certain, but how much of a threat depends on the number.
If the number is orange, then there is a significant possibility of a major impact, but it is not yet confirmed. With again the severity depending on the number.
If any entry is yellow, then it merits attention by astronomers, but there is at least a 99% chance that it will miss (for levels 3 and 4) and it may be very unlikely to hit (for level 2). It may be of significance to the public if the potential impact is less than a decade away - but since it is so improbable, the chances are that it will soon be reassigned to level 0.
The Near-Earth Asteroid 2004 MN4 briefly reached level 4 in 2004, so it went right up to the highest level in the yellow section, setting a record. But it was soon reassigned first to level 1 and then by 2006, to level 0. See 99942 Apophis (on wikipedia) for the details (2004 MN4 was a provisional name, and it was renamed to 99942 Apophis).
This should be no great surprise as, after all, there was a 99% chance of it missing Earth even when the news first broke.
If (as is usual) all the entries are coloured blue (very small objects), white or green then there is no confirmed impact threat for the next 100 years. It's quite common for an asteroid to reach level 1, green briefly. This means that it will do a close flyby of Earth, with a collision extremely unlikely.
If you follow the news stories about asteroids, you might wonder why the probabilities of hitting Earth keep changing.
It is a bit like a tricky shot in snooker. When the player lines up a shot - it is very hard, for anyone to be sure, right away, that it is going to work. As the ball starts to roll, you get a better and better idea of its direction and speed, and before it has traveled very far, an expert player may begin to have a decent idea of whether it is going to work.
It's much the same for asteroids. When the first few observations are made, it has hardly moved across the sky since its discovery, so as for the snooker player you just haven't seen it travel far enough to be sure where it is going.
Also they might not have a good estimate of how far away it is in the early stages. If you look at something in the sky with just one eye, you can't tell if it is right above you or far away. A detailed RC plane just tens of meters above you, could seem as large as a full scale aircraft.
That's typical of these events. Later on as you make more observations you find out that it is less likely to hit Earth. Then, for all the large ones so far, finally you find it has no chance at all of hitting us.
For another example, 2013 TV135 became headline news when it went up to level 1 in the Torino scale (green), briefly, with a 1 in 63,000 chance of hitting Earth. This is the level where a flyby is expected but an impact is extremely unlikely. Even so, this lead to headlines such as Massive asteroid that could hit earth in 2032 with force 50 times greater than biggest nuclear bomb.
However, to put it in perspective, it had a 99.998% chance of missing, even on the day the news broke. It was no surprise to astronomers when it was eventually found to miss Earth. In the end, they predicted a miss over three quarters of the distance to the sun (0.76 AU) !
Another large object, 2002 NT7, two kilometers in diameter, large enough to have potentially global effects, had a risk of 1 in a million of hitting Earth in February 2019. That was a 99.9999% chance it would miss. Again, no surprise to astronomers, it is now known for sure that it will miss. More about these misses here:Near-Earth object (Wikipedia)
The largest impact we could have realistically is one of order ten kilometers in diameter or so. We can tell from looking at the cratering record that nothing as large as, say, 100 kms has hit the inner solar system for more than three billion years - seems we are protected by Jupiter.
With our technology many people would survive. Small mammals, turtles, birds survived the Dinosaur era ending impact which was about the same size. With our technology we can certainly, many of us, survive an impact that turtles and birds can survive.
In that case - yes the chance of an impact is tiny, a one in a million chance every century. Reduced now to one chance in ten million for the next century, as we have found all the ten kilometer asteroids in the NEO population right out to Jupiter, and none of those is headed for Earth.
So yes, not only would we be told, we actually have been told, in the case of examples like the two kilometers diameter 2002 NT7,. It started off with a one in a million chance of impacting Earth - and that's how it would start off if we found one actually headed our way.
As time goes on then amateur astronomers and professionals track it, refine its orbit more and more. Usually before long they prove that it won't impact.
But suppose we found one that is headed our way - then in that case the probability would go up gradually, as they refine the orbit, until eventually they would prove that it does impact. So then it would edge up the Torino scale into the red region. Then we'd look into ways of deflecting it.
Normally this would play out over some time. But if we spot it just a few weeks before impact, or even just a few days before impact, it could play out very quickly. Indeed we have already had a practice run of this, a small asteroid that was predicted to hit the Sudan desert in October 2008.
They informed aircraft of the expected impact, and the crew of a KLM jetliner, on the route from Johannesburg to Amsterdam, spotted the fireball as a flash on the horizon, from nearly 1400 km away. It was also spotted on spy satellites, with the US government saying it detonated at an altitude of about 37 km with energy radiated equivalent to 0.1 kilotons of TNT.
(Details from NASA and from Emily Lakdawalla's blog post on Planetary.org).
If it is found too late to deflect, they'd look into ways of mitigating its effects, e.g. by moving people out of the target zone - surely would do that. And building up supplies of food, making firestorm shelters etc etc.
Our priority right now is to detect the asteroids. If we can complete detection, then we can know about potentially hazardous asteroids many decades in advance, and then it gets relatively easy to deflect them through just gentle nudges, especially if they do flybys of Earth in between.
This survey could be completed very quickly if we were, for instance, able to find the $450 million needed for the B612 telescope. We can find that funding for wars and for nuclear weapons. Why can't we find it to defend the Earth?
For more about this: Giant Asteroid Headed Your Way? - How We Can Detect And Deflect Them
CURRENT IMPACT RISKS
It is easy to keep up to date with the latest news by visiting this page:
Current Impact Risks.
This is the first place to go if you see one of those stories. It is automatically updated with all the new observations. Look to see if any of the entries in the table are orange or red. As of writing (December 2015), this has never yet happened.
If any of them go red, a collision is certain, but how much of a threat depends on the number.
If the number is orange, then there is a significant possibility of a major impact, but it is not yet confirmed. With again the severity depending on the number.
If any entry is yellow, then it merits attention by astronomers, but there is at least a 99% chance that it will miss (for levels 3 and 4) and it may be very unlikely to hit (for level 2). It may be of significance to the public if the potential impact is less than a decade away - but since it is so improbable, the chances are that it will soon be reassigned to level 0.
The Near-Earth Asteroid 2004 MN4 briefly reached level 4 in 2004, so it went right up to the highest level in the yellow section, setting a record. But it was soon reassigned first to level 1 and then by 2006, to level 0. See 99942 Apophis (on wikipedia) for the details (2004 MN4 was a provisional name, and it was renamed to 99942 Apophis).
This should be no great surprise as, after all, there was a 99% chance of it missing Earth even when the news first broke.
If (as is usual) all the entries are coloured blue (very small objects), white or green then there is no confirmed impact threat for the next 100 years. It's quite common for an asteroid to reach level 1, green briefly. This means that it will do a close flyby of Earth, with a collision extremely unlikely.
HOW PROBABILITIES CHANGE AFTER DISCOVERY
If you follow the news stories about asteroids, you might wonder why the probabilities of hitting Earth keep changing.
It is a bit like a tricky shot in snooker. When the player lines up a shot - it is very hard, for anyone to be sure, right away, that it is going to work. As the ball starts to roll, you get a better and better idea of its direction and speed, and before it has traveled very far, an expert player may begin to have a decent idea of whether it is going to work.
It's much the same for asteroids. When the first few observations are made, it has hardly moved across the sky since its discovery, so as for the snooker player you just haven't seen it travel far enough to be sure where it is going.
Also they might not have a good estimate of how far away it is in the early stages. If you look at something in the sky with just one eye, you can't tell if it is right above you or far away. A detailed RC plane just tens of meters above you, could seem as large as a full scale aircraft.
That's typical of these events. Later on as you make more observations you find out that it is less likely to hit Earth. Then, for all the large ones so far, finally you find it has no chance at all of hitting us.
For another example, 2013 TV135 became headline news when it went up to level 1 in the Torino scale (green), briefly, with a 1 in 63,000 chance of hitting Earth. This is the level where a flyby is expected but an impact is extremely unlikely. Even so, this lead to headlines such as Massive asteroid that could hit earth in 2032 with force 50 times greater than biggest nuclear bomb.
However, to put it in perspective, it had a 99.998% chance of missing, even on the day the news broke. It was no surprise to astronomers when it was eventually found to miss Earth. In the end, they predicted a miss over three quarters of the distance to the sun (0.76 AU) !
Another large object, 2002 NT7, two kilometers in diameter, large enough to have potentially global effects, had a risk of 1 in a million of hitting Earth in February 2019. That was a 99.9999% chance it would miss. Again, no surprise to astronomers, it is now known for sure that it will miss. More about these misses here:Near-Earth object (Wikipedia)
FOR A REALLY BIG IMPACT
The largest impact we could have realistically is one of order ten kilometers in diameter or so. We can tell from looking at the cratering record that nothing as large as, say, 100 kms has hit the inner solar system for more than three billion years - seems we are protected by Jupiter.
With our technology many people would survive. Small mammals, turtles, birds survived the Dinosaur era ending impact which was about the same size. With our technology we can certainly, many of us, survive an impact that turtles and birds can survive.
In that case - yes the chance of an impact is tiny, a one in a million chance every century. Reduced now to one chance in ten million for the next century, as we have found all the ten kilometer asteroids in the NEO population right out to Jupiter, and none of those is headed for Earth.
So yes, not only would we be told, we actually have been told, in the case of examples like the two kilometers diameter 2002 NT7,. It started off with a one in a million chance of impacting Earth - and that's how it would start off if we found one actually headed our way.
As time goes on then amateur astronomers and professionals track it, refine its orbit more and more. Usually before long they prove that it won't impact.
But suppose we found one that is headed our way - then in that case the probability would go up gradually, as they refine the orbit, until eventually they would prove that it does impact. So then it would edge up the Torino scale into the red region. Then we'd look into ways of deflecting it.
Normally this would play out over some time. But if we spot it just a few weeks before impact, or even just a few days before impact, it could play out very quickly. Indeed we have already had a practice run of this, a small asteroid that was predicted to hit the Sudan desert in October 2008.
They informed aircraft of the expected impact, and the crew of a KLM jetliner, on the route from Johannesburg to Amsterdam, spotted the fireball as a flash on the horizon, from nearly 1400 km away. It was also spotted on spy satellites, with the US government saying it detonated at an altitude of about 37 km with energy radiated equivalent to 0.1 kilotons of TNT.
(Details from NASA and from Emily Lakdawalla's blog post on Planetary.org).
If it is found too late to deflect, they'd look into ways of mitigating its effects, e.g. by moving people out of the target zone - surely would do that. And building up supplies of food, making firestorm shelters etc etc.
Our priority right now is to detect the asteroids. If we can complete detection, then we can know about potentially hazardous asteroids many decades in advance, and then it gets relatively easy to deflect them through just gentle nudges, especially if they do flybys of Earth in between.
This survey could be completed very quickly if we were, for instance, able to find the $450 million needed for the B612 telescope. We can find that funding for wars and for nuclear weapons. Why can't we find it to defend the Earth?
For more about this: Giant Asteroid Headed Your Way? - How We Can Detect And Deflect Them
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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