This can’t happen with a 15 km asteroid as we have already found all of the NEOs of this size, and none are headed our way. That just leaves comets, tiny chance, 1 in 100 million of a comet that large hitting us this century, and if a comet that large was headed our way, we’d have been tracking it for at least a year and probably two or more years in advance. However it could happen for a 1 kilometer asteroid still, as we have only found 90% of them so far. Should reach 99% in the 2020s. The chance of this is really tiny also, so not something to be worried about. Ordinary things like traffic accidents and health issues are far more important.

But yes, it’s a possible scenario though unlikely. If so, it’s most likely to hit the sea and a one kilometer asteroid is large enough to cause a tsunami so you might have to evacuate coastal villages, towns and cities once you work out where it is going to hit.

If it impacts on land, chances are it lands in a desert, so then, no risk of tsunami but you have to evacuate the region where it will impact. And warn people over a large area to stay away from windows which might break from the shockwave, etc.

If it is going to land on a populated area, very unlikely, then you have to evacuate that area.

A one kilometer asteroid is large enough to have some global effects. So stockpiling food might be an idea but 90 days is not enough warning time to do much of that sort.

There is no chance of deflecting it with so little warning, at present. The problem is that most rockets need a lot of time to prepare them to launch. ICBMs are able to launch at a moments notice, so it is possible to design rockets to hit asteroids with almost no warning. But that would be most likely for much smaller asteroids with so little warning. A 1 km asteroid would not be easy to deflect with so little warning.

If you have many years, especially decades of warning, then it’s easy to deflect. E.g. to deflect by the radius of the Earth, 6,371 km given two decades of warning, you need a delta v of 6,371,000/(365*24*60*60*20) meters per second or around 10 cm / sec.

If it does a flyby of Earth in between as is usually the case, then it will have a gravitational window of perhaps 200 meters diameter, so then you need a delta v of only microns per second.

So the best way to deal with asteroids is to detect them long in advance, if you have a limited budget. That’s why the main focus at present is on detecting asteroids. And if astronomers had the budget to spend half a billion dollars on this - a tiny amount compared to what countries spend on defense, then we could send a space telescope into orbit to find nearly all of even the smaller asteroids down to 20 meters in diameter in less than a decade.

So if a president really cares about the asteroid threat, she or he should support a bill to set aside half a billion dollars to launch a dedicated asteroid hunting space telescope to find them.

Sentinel telescope developed by the B612 foundation.

It would sit inside of Earth close to Venus's orbit giving it a good field of view of NEOs close to the sun. It looks away from the sun to avoid being blinded by it - and it can then see faint NEOs that are in between the Earth and the Sun which is the hardest place to spot them from our current Earth based surveys. It would help fix that blind spot for asteroids that come from the direction of the sun. It looks in infra red because the asteroids are far more obvious in the infrared.

Eventually it would spot just about everything out there that's in the vicinity of the Earth orbit. It would find nearly all potential impactors down to 20 meters diameter well within a decade

Then if we do find one that is headed our way, with say a decade or two decades of warning, it would be easy to find the extra funds needed to divert it so it doesn’t hit Earth. So detection I think is the priority. If we had billions of dollars for asteroid defense, as we have for defense against other nations on Earth, then we could do both.

See also my Giant Asteroid Headed Your Way? - How We Can Detect And Deflect Them