If we are unlucky, the 20 meters Chelyabinsk meteorite still could not be detected if it came from that direction though we’d see it at least a day or two in advance in any other direction. However if it was a bit larger we’d have seen it several months before. It was in our night sky, just too faint to see.
So I think that would be around the limit. A Chelyabinsk type meteorite impacting at an angle like that is an air burst.
The Chelyabinsk meteorite came in at a shallow angle 16 degrees, half megaton.
An impact at that angle even over a city would mainly cause broken glass and such like - could hurt people near to windows as for Chelyabinsk.
However as you can see it is just chance that it came in at a shallow angle. If shifted a bit more to the right in that diagram it could hit the Earth more directly at a steep angle. That would have been much more dangerous for humans. It was about a one megaton explosion, slightly less, harmless as an air burst. But if it happened near the ground it could be devastating.
“For a 1-megaton explosion the optimum burst height is about 1700 meters (a mile) and widespread structural damage occurs for any blast below about 5000 m (3 mi)
I'm a little surprised that something only 20 meters across could be so dangerous, it did have a high relative velocity though, 15 km /sec. Well above average for Earth meteorites though not as high as short period comets
There’s a webinar about it here:
I was interested also to hear that it's really hard to model whether or not you get tsunami for ocean impacts.
I’m in the middle of writing an article about this topic here
How did we miss the Chelyabinsk asteroid? by Robert Walker on Debunking Doomsday
We could detect most asteroids this small even from the direction of the sun well in advance if we had a space telescope operating between Earth and the Sun, close to Venus’s orbit and there’s a plan to send such a telescope into orbit - it just needs funding. $450 million would do it. It would take less than a decade to spot many of the asteroids.
The risk of hitting a populated area is very low because first of all, most of the Earth’s surface is water, and then of the land area, most is uninhabited or sparsely inhabited. Then many of the impacts would be at shallow angles because you have to hit Earth pretty much dead center to get a steep impact.
So - the next asteroid to hit Earth of this size will probably not kill anyone, but it could be hazardous, similar to Chelyabinsk and it could be much worse.
This is by far the most likely size of asteroid to hit us as the smaller ones are much more numerous. We are probably hit by asteroids this size roughly every 80 years I’ve heard as a ballpark figure. Which does not mean that we are safe for the next 80 years. We could be hit tomorrow or not be hit by another one for centuries.
If we can find them well in advance, there are many ways we can deflect them. If we can warn of them a day or two in advance that is still time to evacuate the impact zone. ~Detection is the key and this is definitely an addressable problem, it mainly needs funding. Astronomers know what to do to find them.
The risk though shouldn’t be over played. It’s more likely that you are killed by a tornado or by lightning and traffic accidents and health issues are far more important statistically. But it is a preventable type of accident, can warn to the minute when it will happen and can actually deflect it given enough time. It’s surely well worth finding the funding to detect and deflect them.
Giant Asteroid Headed Your Way? - How We Can Detect And Deflect Them
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