Well first, if by "threaten Earth" you mean make humans extinct, instantly, no humans left to rebuild civilization - well no such asteroid has hit Earth for nearly four billion years.
And the top speed - well it depends on its orbit - but would be kilometers per second. But the top speed isn't really the main thing that decides whether we find it or not. What matters is its size, and the direction it approaches Earth from. For the larger objects, we can see them so far away and most likely decades in advance, that the speed of impact is kind of irrelevant, most likely many orbits before it hits us.
Especially as we have nearly finished mapping all the objects of 1 km or larger most likely to hit Earth.
So first of all, this is not really possible, hasn't happened for four billion years.
If you see a picture of the entire Earth and you can also see the impactor, you know it is too large. A 10 km impactor, on an image of the Earth 1600 pixels wide will be a little over 1 pixel in width. Even a 100 km impactor would only be 10 pixels and it is billions of years since one of those hit Earth.
This is possible, but there is only one chance in a million of it happening this century.
This means that when we detect an object this big that's due to hit Earth, there's a 99.9999% chance that we have at least a century to do something about it. And it is most likely that we have at least a million years and probably tens of millions of years to deflect it.
Then, turtles, small mammals, birds, many creatures survived the asteroid impact at the end of the dinosaur era, and that one was as big as they get pretty much. Dinosaurs couldn't because they didn't have our technology, but we have boats, planes, submarines, can build fireproof shelters, can travel anywhere in the world. We would survive, some of us.
Such an asteroid would be easily visible way across the solar system. Right out to about Jupiter we could spot a 10 km or larger asteroid easily. But the 100 km size asteroid that could really threaten Earth, could be spotted much further away.
Anyway they have pretty much finished the survey of the 1 km or larger objects already in orbits similar to Earth. They hope to finish the survey of similar objects in comet type orbits in the next decade. By then we'll either have found a potential impactor, or know that there is almost negligible chance of one hitting us in the near future. And that's just with existing facilities. They are also focusing attention on smaller asteroids from 30 meters upwards, and large asteroids right out to the Kuiper belt.
To search for asteroids you need a sensitive wide field camera. Often Schmidt telescopes as those can give a wide field of view with pin sharp star images right across the field of view.
Teams currently searching for asteriods include
For more about this and many other surveys going on: Surveys, Astrometric Follow-up & Population Statistics
The ones that are difficult to spot at present are the smaller ones, especially if they come at us from the sun, like the Russian Chelyabinsk meteor. Which briefly became brighter than the sun in the sky.
It would have been very damaging if it had hit in a city.
Since these are much more common than the larger ones, I expect the first accurate prediction to be of one of these. Actually a very small asteroid has already been observed, 19 hours before impact,
2008 TC3
See also my What are the chances of Asteroid 2012 TT5 hitting the Earth on September 24, 2015?
And the top speed - well it depends on its orbit - but would be kilometers per second. But the top speed isn't really the main thing that decides whether we find it or not. What matters is its size, and the direction it approaches Earth from. For the larger objects, we can see them so far away and most likely decades in advance, that the speed of impact is kind of irrelevant, most likely many orbits before it hits us.
Especially as we have nearly finished mapping all the objects of 1 km or larger most likely to hit Earth.
So first of all, this is not really possible, hasn't happened for four billion years.
If you see a picture of the entire Earth and you can also see the impactor, you know it is too large. A 10 km impactor, on an image of the Earth 1600 pixels wide will be a little over 1 pixel in width. Even a 100 km impactor would only be 10 pixels and it is billions of years since one of those hit Earth.
This is possible, but there is only one chance in a million of it happening this century.
This means that when we detect an object this big that's due to hit Earth, there's a 99.9999% chance that we have at least a century to do something about it. And it is most likely that we have at least a million years and probably tens of millions of years to deflect it.
Then, turtles, small mammals, birds, many creatures survived the asteroid impact at the end of the dinosaur era, and that one was as big as they get pretty much. Dinosaurs couldn't because they didn't have our technology, but we have boats, planes, submarines, can build fireproof shelters, can travel anywhere in the world. We would survive, some of us.
Such an asteroid would be easily visible way across the solar system. Right out to about Jupiter we could spot a 10 km or larger asteroid easily. But the 100 km size asteroid that could really threaten Earth, could be spotted much further away.
Anyway they have pretty much finished the survey of the 1 km or larger objects already in orbits similar to Earth. They hope to finish the survey of similar objects in comet type orbits in the next decade. By then we'll either have found a potential impactor, or know that there is almost negligible chance of one hitting us in the near future. And that's just with existing facilities. They are also focusing attention on smaller asteroids from 30 meters upwards, and large asteroids right out to the Kuiper belt.
To search for asteroids you need a sensitive wide field camera. Often Schmidt telescopes as those can give a wide field of view with pin sharp star images right across the field of view.
Teams currently searching for asteriods include
- The Spacewatch Project, University of Arizona - originally used for detection, now for follow up as other surveys got more sensitive
- Catalina Sky Survey (CSS), University of Arizona - focused on detecting Near Earth Asteroids. High detection rate, also excels at finding small objects close to Earth.
- Pan-STARRS, University of Hawaii - faintest limiting magnitude, of about 22 under good conditions. Also able to survey close to the sun.
- NEOWISE - detects them in the near infrared with a 0.4 meter telescope on board the Wide-field Infrared Survey Explorer
For more about this and many other surveys going on: Surveys, Astrometric Follow-up & Population Statistics
The ones that are difficult to spot at present are the smaller ones, especially if they come at us from the sun, like the Russian Chelyabinsk meteor. Which briefly became brighter than the sun in the sky.
It would have been very damaging if it had hit in a city.
Six meter diameter hole in Chebarkul Lake made by a fragment of the meteorite. Photo credit: Chebarkul town head Andrey Orlov. See: Meteorite hits Russian Urals: Fireball explosion wreaks havoc, up to 1,200 injured (PHOTOS, VIDEO)
Since these are much more common than the larger ones, I expect the first accurate prediction to be of one of these. Actually a very small asteroid has already been observed, 19 hours before impact,
2008 TC3
80 tonnes 4.1 meters in diameter, hit in the Sudan desert and about 600 meteorite fragments were recovered. It was detected only 19 hours before impact, but was the first ever successful prediction of a meteorite impact.
See also my What are the chances of Asteroid 2012 TT5 hitting the Earth on September 24, 2015?
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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