Short summary - the risk from larger asteroids of 1 km upwards, large enough to have some global effects is more or less retired as a result of astronomical searches for the last few decades. There is still a risk from smaller asteroids, but in most cases we’d be able to deflect them if necessary, or evacuate the area. The chance of dying from an asteroid is tiny, far less than the risk of dying of lighning.

20 - 40 METER ASTEROIDS ARE TINY - JUST LARGE ENOUGH TO GET THROUGH THE EARTH’S ATMOSPHERE

An asteroid 20 meters in diameter hits every 80 years or so. Since only 1% of the Earth is urban that means around 8,000 years between impacts on an urban area.

2012 DA 14 - this is an artist’s impression of a similar sized asteroid that did a close flyby in 2013 - closer than the Moon - and was the first time astronomers were able to observe such a small asteroid so close. Now we detect them routinely. It is an example of how good we have got at detecting even rather small asteroids.

An asteroid this size hits us less than once every 80 years and since 99% of Earth’s surface is ocean, desert, ice fields, or rural areas with very few inhabitants, the chance of it hitting an urban area is very tiny indeed. It is something that might happen every few thousand years that a 40 meter diameter asteroid hits an urban area on Earth.

If it was headed for us - well it wouldn’t be a big deal either. Even if it was going to hit an urban area, even a city - with a day of warning you can evacuate people from the impact zone. It would be panicky yes, but there is no reason for people to die, if we get plenty of warning.

But most likely, if an object like this is hazardous at all, it’s like Chelyabinsk where you warn people to stay away from windows to watch out for flying glass.

Most won’t need to do anything else. A few people in some remote area, reindeer herders, bird spotters, nomadic people, may have to be evacuated from the impact zone. If there is enough warning, a day or two, keen astronomers would probably charter flights to the impact site to watch it land from the air!

From most directions we'd get a day or two of warning especially with the new “ADAM” advanced warning system which automatically sends messages to astronomers before anyone has looked at the images with their own eyes. Since 2015 we’ve also had ATLAS, two telescopes a hundred miles apart to give them a kind of binary vision to judge distances to any asteroids about to hit Earth, scanning the half of the sky visible to them every two nights.

But right now it can still approach from the direction of the sun without warning. It’s not something to worry about personally - you are far far more likely to be hit by lightning than an asteroid.

LARGER ONES, UP TO ONE KILOMETER

With larger ones, up to around one kilometer. there are no global effects at all. But can be devastating locally. An asteroid of a few hundred meters in diameter would be devastating to a city and indeed an entire small country and produce a tsunami if it hit the sea (though a normal sized one of a few meters at most, not the huge ones of a really large asteroid).

These are much rarer, tens of thousands of years, and most again will hit in remote desert areas or in the sea. The main risk for tsunamis are for ones that hit very close to the shore - because they produce a kind of tsunami that almost cancels itself out at source in the deep sea (makes a hole in the sea that immediately fills in again pulling back much of the effect of the initial tsunami before it gets far)

We'd be likely to have months but most likely many years of warning. Bennu, for example, has a tiny chance of impact in 2175. We can deflect it easily with current technology if we need to. That’s plenty of time to do something about it (Debunked: NASA can’t deflect asteroid Bennu).

Cities are often evacuated for hurricanes with only a few days warning. Even if we can’t deflect an asteroid, if we have enough warning we can evacuate the impact zone.

ONE KILOMETER AND LARGER - SOME GLOBAL EFFECTS

From around one to ten kilometers you get increasingly global effects.

There are only three known that have even the slightest chance of impact. None currently considered to be of any public concern, though they monitor them closely.

1950 DA, diameter 1.3 km, has no risk of hitting us until 2880, when it has one chance in 8,300. This means we have more than 860 years to study it and do something about it. It’s 99.988% certain that it misses us even then.

These are only just large enough to have some global effects.

There was a slightly larger object in the table called 2010 AU118 at 1.7 km, but it was removed from the table on 3rd October 2018, after more observations showed that it would miss.

The asteroid search programs have found 894 asteroids of one kilometer or larger, and find a new one every one to two months, with four discovered so far in 2018. A recent estimate suggests a total population of 920±10 which makes the survey so far 97% complete with 15 - 35 still to find, though you get slightly varying estimates for the total.

They expect to find at least 99% of the one kilometer asteroids by some time in the 2020s. None of these 894 asteroids found so far has a significant chance of hitting us by 2100.

Most likely none of the remaining 3% (or whatever it is) have any chance of hitting us in the next few centuries. The ones they haven’t found yet are likely behind the sun. They are in orbits that are almost the exact length of Earth’s year. As soon as they drift around into view then all of our asteroid surveys will pick them up giving us probably decades of warning before they can fly past Earth.

The largest one not found yet is statistically most likely to be at most 3 kilometres.

TEN KILOMETERS AND LARGER - ALL FOUND

None of these of any danger to Earth for now or for centuries. The comet Swift Tuttle (26 km) has one chance in a million of an impact in 4479. The only other one that seems to have a chance of hitting Earth is 433 Eros (16.84 km) that could hit us some time after a million years from now, 5% chance, but is not likely to before 100,000 years from now.

We know that the list of 10 kilometre asteroids is complete. The JPL table lists them as:

1036 Ganymed (1924 TD) 37.675 km MOID .343497 au (not likely to hit Earth ever, in an orbit likely to be ejected from solar system or hit the sun within 10 million years, see paper)

3552 Don Quixote (1983 SA) 19.0km MOID .333524 au

433 Eros (1898 DQ) 16.84 km MOID .149341 au (has a 50% chance of becoming Earth crosser in a million years, very unlikely within 100,000 years, if that happens has a small chance of hitting Earth eventually, Mathematicians Say Asteroid May Hit Earth in a Million Years, paper)

4954 Eric (1990 SQ) 10.8 km MOID .194159 au

Definitions:
MOID (minimum orbit intersection distance) is the minimum distance between its orbit and Earth’s orbit. It doesn’t mean it ever gets that close to Earth, for instance an Earth trojan orbiting a third of an orbit ahead or behind Earth is very close to Earth’s orbit but never comes close to Earth.

A PHA (Potentially Hazardous Asteroid) is one that gets within 0.02 au, or about 3 million kilometers. It means that there is perhaps as much as a few percent chance of it hitting Earth eventually but it is still most likely to hit the sun, be ejected or hit Jupiter.

There are also four comets

109P/Swift-Tuttle 26 km MOID .000892135 au (PHA, one chance of a million of impact in 4479)

C/2001 OG108 (LONEOS) 13.6 km MOID .300836 au

C/1991 L3 (Levy) 11.6 km MOID .0749881 au

1P/Halley 11.0 km MOID .0637815 au

There are also two more just short of 10 km in size:

1627 Ivar (1929 SH) 9.12 km

1866 Sisyphus (1972 XA) 8.48 km

Then there’s a big jump to the next one

53319 (1999 JM8) 7 km

To see the list, The JPL table shows 25 measured objects with the diameter measured at 5 km or larger.

It’s complete down to 10 km and is probably complete down to 5 km as they expect the largest undiscovered object to be around 3 km, but they haven’t necessarily measured all the objects at around 5 km exactly.

For expanded version with orbit diagrams:

COMET SWIFT TUTTLE AND LONG PERIOD COMETS

There is one object in that list, comet Swift Tuttle, that could hit us in the fourth millennium. The chance is very tiny one chance in a million of this. Just about impossible, a million times more likely to miss, 99.9999% certain it misses.

But it would be devastating, boil the seas, it's a biggy. Not had an impact like that for well over 3 billion years. However, we have plenty of time in that very remote chance, - with whatever technology we have over 2000 years from now continuing as a technological civilization if we can do that, then it should be no problem to do something about it on that remote eventuality.

That leaves long period comets. They are rare at present, only one for every hundred NEO’s. The closest any has come is Lexell's comet in the eighteenth century and it passed at a vast distance of six times the distance to the moon. It's possible we spot one of those headed for Earth, though unlikely, we'd see it years before it gets here and it would with a near certainty miss the very tiny target of Earth.

Though you'd know a large comet has a chance of impact years in advance, even a few months before impact by a comet, then you have at most a 50:50 prediction of impact because of the effects of its jets that make the comet tail. The flip side of this is that if we can visit it and control its jets ourselves, which parts evaporate and which are stable, we can deflect it away from Earth even with only a few months notice. Though we don’t have that technology yet, surely we will soon, long before 4479. It's not currently a priority because long period comets are so rare.

DON’T WORRY ABOUT “POTENTIALLY HAZARDOUS ASTEROIDS”

This confuses some people including journalists. If it says “Potentially Hazardous Asteroid” in the press release - the thing is that most don’t think on such long time scales as astronomers. That classification means there is a chance it could hit Earth some time in the next 20 MILLION YEARS.

And it’s not a certainty on that timescale either. Only a few percent of the PHAs will ever hit Earth. Others hit Venus, Mercury, some Mars, most though hit the Sun. Others hit Jupiter or outer planets and a significant number get ejected from the solar system.

At present none of the PHAs that they are tracking is at significant risk of hitting Earth this century.

COMPARISON WITH LIGHTNING STRIKES

A MISS IS NOT A HIT

If the announcement says it flies past Earth then it is harmless. Doesn't matter if it misses by a hundred kilometers or a million kilometers, a miss is a miss.

An analogy that helps some people. If you travel to work and your car crosses a railway track on a bridge -well sometimes you may happen to cross the track at the very moment a train passes beneath. You may pass within meters of a train. That sounds like it should be scary. But it isn’t. You know that you are on a bridge and the train is on the track beneath the bridge. It makes no difference ifyou are just meters above it, or it is kilometers away when you cross the bridge, you still can’t hit it.

Well asteroids are like that, and more so. If they have refined the prediction enough to know that it misses Earth then that means it’s a miss. Just like the train, there is no way it can hit Earth.

If you get one of these announcements it means they have worked out its orbit to tell you so precisely that it passes by at say a fifth of the distance to the Moon. If there was any risk of it hitting Earth the announcement would say so. And it would be really major news what with the current interest in asteroids.

So in summary, if a news story says

“A potentially hazardous asteroid will pass by safely at a distance of …”

This means

“An asteroid that may have a tiny chance of hitting Earth some time in the next 20 million years is going to pass by safely and we know it won’t hit Earth this time (like the train example).”

They typically check the orbit forward for 100 or 200 years so normally you can also say it won’t hit Earth for at least a century.

You can check for yourself anyway.

COLOUR CODED ALERT LEVELS

These are easy to check.

It is colour coded so now you only need to check the colour of the first row. It does not matter what it says. If the row is white, green or blue, then there is no known asteroid that is a significant risk. So the story is fake.

If it is yellow there's a slght chance, but it is likely to soon change to green, white or blue. If it is orange there's a higher warning level and if it is red then it is a definite impact (though it could still be a harmless impact into a desert, ice field or ocean, indeed that's the most likely outocome at this level).

But if it ever goes orange or red - there is so much interest in asteroid impact that it will surely be headline news on the TV and in all respectable news media. You won't have to hunt around in the red top tabloids to find out about it. I will explain the warning levels in a bit more detail later on, but that’s enough to get started.

If you search the list there and your object isn’t in it- this means that they have searched forward for 100 years and it never comes close enough to even enter into that table at the white or blue level.

More details here - this last section is my intro to it:

I cover ATLAS here:

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