First, this is a highly improbable scenario. Only likely in movies I think. But going with your question, the only way that I've seen suggested that could work with an asteroid so big so quickly is to use a 1 gigaton nuclear bomb. Russians and Americans experts together came up with a plan for a 1 gigaton nuclear weapon which could vaporize a 1 km asteroid and deflect a larger asteroid, and you can do it less than ten years before the impact (not sure about six weeks though). A new use for nuclear weapons: hunting rogue asteroids
But you'd want to test it first. And could you get together a new nuclear bomb, more powerful than any previously built and tested, within six weeks?
First, an impact this big is very low probability. Chance is 1 in a million for it to happen in a century, roughly every 100 million years for something this large.
And then - it would do many flybys of Earth almost certainly before hitting, as Earth is such a tiny target. And we haven't had anything that big doing near miss flybys of Earth - we'd notice it easily.
An asteroid this big is roughly the size of Mars's two moons Phobos and Deimos.
There is an object that big that could hit Earth, but it's not in the right orbit right now to hit us. 433 Eros is the second largest Near Earth Asteroid 34.4×11.2×11.2 km. This is a bit bigger even than the dinosaurs meteorite - and it has a 50% chance of hitting Earth in the next 100 million to billion years.
But it's not a problem for us right now, it can only hit us if its orbit changes quite a bit first.
A Jupiter family comet could be this big, but I think we'd know about those also. There's a chance of missing a 1 km Jupiter family comet, but not sure there is much chance of missing a 10 km one. At any rate we can spot 10 km Jupiter trojans easily.
And a comet on its first flyby of the inner solar system or not yet perturbed by Jupiter would almost certainly be in the wrong inclination, and not even interesect the ecliptic at Earth's orbit.
So, it doesn't seem like something to worry about. We need to think about the smaller asteroids. Especially, of the order of 100 meters or so, as by far the most likely to hit Earth in the near future and still large enough to cause significant damage. And while mapping those out, we can find the larger ones as well, and if we detect them long in advance, can then deflect them easily especially if they do several close flybys of Earth (or any other planet) first.
That was also the conclusion of the authors of the original study, see Page on caltech.edu
That was back in 1994. But when it comes to gigaton nuclear weapon deflections, I think it's pretty much the same situation today. We could do it in principle, given plenty of budget and a suitable timescale. But the chance it is ever needed is so very tiny, and cost so high, how could you motivate spending so much on something so unlikely to happen? While as we continue to get better at detecting asteroids, if we detect an asteroid due to impact Earth, even a tiny one of 100 meters or less, we have all the motivation we need then to do something about it.
See also
Robert Walker's answer to Is NASA presently capable of fending off a collision if a meteor or any other celestial body were to strike the earth?
Robert Walker's answer to If a 1km sized rock was, for certain, to hit Earth in 6 months, what could we do about it?
But you'd want to test it first. And could you get together a new nuclear bomb, more powerful than any previously built and tested, within six weeks?
First, an impact this big is very low probability. Chance is 1 in a million for it to happen in a century, roughly every 100 million years for something this large.
And then - it would do many flybys of Earth almost certainly before hitting, as Earth is such a tiny target. And we haven't had anything that big doing near miss flybys of Earth - we'd notice it easily.
An asteroid this big is roughly the size of Mars's two moons Phobos and Deimos.
There is an object that big that could hit Earth, but it's not in the right orbit right now to hit us. 433 Eros is the second largest Near Earth Asteroid 34.4×11.2×11.2 km. This is a bit bigger even than the dinosaurs meteorite - and it has a 50% chance of hitting Earth in the next 100 million to billion years.
But it's not a problem for us right now, it can only hit us if its orbit changes quite a bit first.
A Jupiter family comet could be this big, but I think we'd know about those also. There's a chance of missing a 1 km Jupiter family comet, but not sure there is much chance of missing a 10 km one. At any rate we can spot 10 km Jupiter trojans easily.
And a comet on its first flyby of the inner solar system or not yet perturbed by Jupiter would almost certainly be in the wrong inclination, and not even interesect the ecliptic at Earth's orbit.
So, it doesn't seem like something to worry about. We need to think about the smaller asteroids. Especially, of the order of 100 meters or so, as by far the most likely to hit Earth in the near future and still large enough to cause significant damage. And while mapping those out, we can find the larger ones as well, and if we detect them long in advance, can then deflect them easily especially if they do several close flybys of Earth (or any other planet) first.
That was also the conclusion of the authors of the original study, see Page on caltech.edu
"Finally, we note that, although further study of the feasibility of diverting asteroids may be warranted, we do not believe it is appropriate to conduct engineering designs of systems at this stage because of the low probability of impact of hazardous asteroids, the high cost in the face of a low risk factor, and the rapid changes that are to be expected in defense systems technology"
(see also Page on washington.edu which briefly mentions it)
That was back in 1994. But when it comes to gigaton nuclear weapon deflections, I think it's pretty much the same situation today. We could do it in principle, given plenty of budget and a suitable timescale. But the chance it is ever needed is so very tiny, and cost so high, how could you motivate spending so much on something so unlikely to happen? While as we continue to get better at detecting asteroids, if we detect an asteroid due to impact Earth, even a tiny one of 100 meters or less, we have all the motivation we need then to do something about it.
See also
Robert Walker's answer to Is NASA presently capable of fending off a collision if a meteor or any other celestial body were to strike the earth?
Robert Walker's answer to If a 1km sized rock was, for certain, to hit Earth in 6 months, what could we do about it?
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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