It's true that we couldn't deflect asteroids as large as Texas with only 18 days notice. We don't have any spacecraft ready to fly anyway on a moment's notice like that, and there isn't much they could do if we did. But luckily we don't have to, because that scenario is impossible, totally unrealistic, as so often with movies.

In actuality we have already found all the Near Earth Object (NEO) asteroids larger than 1 km, and Pan-Starrs can see a 300 meter diameter asteroid right out to the outer edge of the asteroid belt.

Even in a worst possible case scenario, a 10 km comet would be spotted at least several years before impact (Siding Spring which is much less than 1 km in diameter was spotted nearly two years in advance).

An asteroid the size of Texas simply can't happen (for all practical purposes) - no asteroid that large has ever hit Earth, Mars, Mercury, moons of Mars, our Moon and what we hae of the history of Venus since its global resurfacing due to volcanism a few hundred million yeas ago - for over three billion years. We can tell this from the cratering records. There are craters from impacts this large, such as the ones on the Moon, but they are all more than three billion years old.

They just don't happen any more. When they run the models to try to figure out how this works, they find that Jupiter is able to protect us from the very largest asteroids - either they hit Jupiter, are ejected, hit the sun or break into many smaller pieces before they get to Earth. The only exceptions are the ones in the asteroid belt, and all the big ones there are in reasonably stable orbits long term also, so are not a concern (any that weren't would most likely have got cleared out billions of years ago).

Which is just as well as there is no way we could deflect an asteroid the size of Texas with only 18 days warning. Well if we had anything that big headed our way, we’d spot it right out to Neptune’s orbit, so at least ten years before flyby. But the chance of it hitting Earth, given that no such object has ever hit any of the surfaces we are able to study from Mars inwards for over three billion years - it’s so low there’s not much point in even thinking about it. Jupiter is so much larger a target.

What happens is that Jupiter deflects and breaks up the larger objects or they hit the Sun or Jupiter and they are unlikely even to be in the same plane as Earth or to cross the plane at Earth’s distance from the Sun. It doesn’t do such a good job of the 10 km and smaller ones as there are so many of them and it also adds to them by breaking up larger comets.

For the ones that can happen, then the sooner you can spot it the better. If an asteroid is due to hit us a decade later, then we need only cms per second of delta v. With even a 10 km asteroid, that's within the range of what we could hope to achieve with the technology we have, through various methods.

If it does a flyby of Earth first before impact, as is usually the case, then we just need it to miss a small keyhole in space, perhaps a few hundred meters in diameter. That makes the required delta v, if we can spot it a decade before the flyby only microns per second. In that case, even just painting it white could so change its trajectory that it misses through changing the amount of the Yarkovsky effect - painting here doesn't mean giving it a good coating of paint, but dusting it with some white or light powder to increase its surface albedo significantly (as most asteroids are pretty dark, so easy to change in this way).

Only 1 in 146 of the asteroid flybys are by long period comets. A ten kilometer comet with several years of warning is a pretty impossible thing to detect. But this is extremely unlikely.

It was already a 1 in a million chance of a 10 km assteroid hitting us this century. Now that we've found all the 10 km NEOs, it's a one in a 146 million chance. But a new comet coming in from the outer solar system would be likely to be perturbed by Jupiter first before it has a chance of hitting Earth, or at least to do a flyby of Earth.

The thing is that we have probably found just about all the Jupiter crossing short period 10 km asteroids also. We can spot 10 km asteroids easily in the population of Jupiter trojans. So you are talking about a long period comet that hasn't yet been perturbed into a short period. It probably does a flyby of Earth, heads off, and doesn't come back again for many decades at least.

Earth is a tiny target. And close flybys by comets are extremely rare. We get a flyby at the end of March by a comet fragment, and that's the closest one recorded in history since 1770. It passes at nine times the distance to the Moon.

We get many flybys of NEOs, and some of them so close they pass between the Earth and the Moon. Sometimes they come within Geostationary orbit, only a tenth of the way to the Moon.

So, you can see how rare large comets are going to be and how unlikely that we would find one headed straight for us with only six months to spare before impact. Far more likely that we have several decades from discovery to impact.

Which then takes us into that scenario where the required delta v is cms / second, or even microns per second if it does a flyby first, as it probably would.

Those are the giant asteroids. As for smaller ones, 1 km in diameter, well we have found 90% of those. We will find 90% of those left by the 2020s. By which time either we find one headed our way - or much more likely find that none are headed our way. At which point then the risk from those would be negligible also, and again same thing, if one is headed our way likely to have lots of warning.

With even smaller ones, 100 meters in diameter or so, then the objective is to find 90% of those as well by the 2020s but it's not yet clear how we will do it. Because they re so numerous and many still to find, and quite hard to spot when far from Earth, especially smaller ones down to 20 meters

There is a solution though, if someone can find a way to fund the $450 million B612 telescope. That could achieve the goal in 6.5 years and extend the mapping down even to 20 meters diameter asteroids.

Meanwhile, we can still miss small asteroids, and if small enough we can even miss them on the day of impact as the Chelyabinsk meteorite showed. As a result of the Pan Starrs and Linear searches, we simply can't miss big asteroids as large as 10 km in diameter on such a short timescale.

And as a result of Pan-STARRS the impact threat is now equally divided between the large ones and small ones in terms of expected numbers of casualties averaged out per year.

With a bit more of a search we can almost eliminate the large asteroid threat, and should get there by the late 2020s.

And - no way that any of these asteroids could make humans extinct. Many creatures survived the CP boundary dinosaur asteroid including turtles, aligators and crocodiles, flying dinosaurs (which became birds), dawn redwood, small mammals. Humans are great generalists and particularly now that we have technology, I don't think asteroid impacts are an existential risk for us. Though they could be devastating for us. They also aren't the most likely of hazards at all. We don't have the equivalent of a Pompei destroyed by asteroids anywhere in our history. There are a few accounts of people being killed by asteroids. But only individuals not hamlets or cities.

But asteroids can be deflected, so it's a unique natural risk there, can be predicted to the minute, and then can also be prevented.

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

available online for free, also from Amazon kindle as:

Giant Asteroid Is Headed Your Way? : How We Can Detect and Deflect Them (Amazon)