what would the governments and nasa?

If we have five years, then it would be possible to land something on the surface, and nudge it out of the way. With enough time, even really big asteroids can be nudged far enough to avoid the Earth, and eventually fall into the Sun.

One thing we can *never* do is try to blow the thing up. All that would do is create thousands of smaller asteroids still aimed at the Earth, but now they will be radioactive.

Nasa has only been charged with the mandate to find and identify the nature of near Earth satellites so the don't have anyone working anything else. However they did collects documents about the 1960 era Project Orion which was canceled because of the nuclear arms treaty. To this day, a Project Orion is the only viable interplanetary spacecraft of any notable size. The scaled down Mars mission version could probably still be launched quickly if the mothballed Saturn V still works. A full scale 300 men version could probably be ground launched at the expense of some fallout and several consecutive EMP bursts. However unless an evacuation to a space habitat was the only option, a Project Orion or the modernized Medusa spacecraft version would probably not be considered.

The ESA has proposed a kinetic strike space craft which would be a golf cart size space craft with ion drive designed to hit the asteroid enough to nudge it off course. However this would only work if it's not too big and we can intercept it fairly far away. Also, the asteroid would have to be a solid iron nickel core asteroid, if it's a looser material, it would just break apart, it might even reform before colliding.

The US air force has a high power continuous wave laser mounted on a jet airliner to shoot down missiles. A intermittent small scale version of that could be mounted on a ion drive space craft and selective shots could vaporize enough material to nudge the asteroid off course.

If they ignore the nuclear arms treaty a standoff nuclear blast could vapourize material on one side of the asteroid to nudge it off course. As it doesn't explode on the surface, it just showers one side with gamma rays and therefore is unlikely to break up a rubble asteroid.

Problem with the laser and nuke is that asteroids tend to tumble so there's no telling where it would be nudged to and it could just spiral in.

A proposal to launch mirrors to shine more sunlight on one side exists but positioning that mirror constantly against the solar wind would be difficult.

There's a proposal to paint the asteroid white so the effect of sunshine on it's speed is greater but that would be like painting Mount Everest.

A couple of astronauts have proposed a gravitational tractor where an ion drive space craft is positioned ahead or behind so the gravitational force nudges it off course. But this only works with small asteroids and if we have a lot of time. You would need a project Orion sized craft to affect something larger than the KT asteroid.

We would need several decades to address an asteroid larger than the KT extinction asteroid.

If we only had 3 to 5 years, the governments would secretly stash valuables on their submarines and park them and their leaders under the North pole where it's least likely to hit. They'd wait till the two days of 750 degrees heat from the re-entering ejaculate has cooled off and then try to gather survivors in coastal areas for the impending ice age.

It would be a good idea to scoop up a cold war soviet union Whiskey class sub for $500,000 and equip it with a closed cycle stirling engine or closed cycle diesel generator so that you could sit under the ice cap for a while too. If you're really rich, you can buy the 80 million dollar luxury Phoenix 1000 submersible but you'll have to add the Stirling engine option at an additional cost and hope the manufacturers don't take off without you.

If an asteroid that big was coming this way, and it was to hit in a short time (let's say less than 10 years), then yes, we'd be screwed.

Luckily, there is nothing like that coming for at least a hundred years.

Check it out here:

http://neo.jpl.nasa.gov/risk/

The key is the last column (Torino scale).

If something like that was coming our way within a hundred years, there would be a high number in that column (say, bigger than 7).

We get nervous whenever there is a number of 4 or bigger. (It did happen, back in 2004, with Apophis, but better observations showed that it was going to miss us).

The Torino scale does not extend beyond 100 years.

If the lead time is more than 10 years, then governments would probably scramble to find a way to throw the asteroid off its collision course. But for the moment, we (humans) have nothing that would do that.

We would need to design and make a spacecraft that could travel fast enough to the asteroid and tweak its trajectory so that it would miss Earth or just prevent it from hitting Earth. If this fails, the government would use a Nuclear bomb against it

Surly they would try to do something about it like move it out of it course towards the earth, but if that was impossible, and still headed our way we would be screwed.We would all going about our daily routines (because we wouldn't be told about it), and hours after it hit, earth would be void of life for hundreds of years until living conditions on earth are possible again.

yes, it would make the atmosphere really dusty and virtually impossible to breath...