Shoot Down an Asteroid

Would it be possible to stop an asteroid from hitting the earth by firing bullets at it? --Myself

I'll be analyzing one of my own thoughts this time. I'm sure literally everyone^[who?] has wondered about this. For the purposes of this post, I'll assume that we're talking about the asteroid 4942 Munroe. It's big enough to be interesting (6 to 10 kilometers across[1], so I'll go with 8 kilometers), but not big enough to actually destroy the earth. Asteroids typically travel at 25 kilometers per second.[2] Asteroid densities vary widely[3], but let's assume that 4942 Munroe has a density of 4 grams per cubic centimeter. From this, I can calculate that the momentum of 4942 Munroe is 1.28*10^19 kilogram-meters per second.

Now it's time to think about the guns. An AK-47 fires 600 rounds per minute at a velocity of 715 meters per second.[4] Each round weighs 7.9 grams.[5] If one AK-47 is fired straight at the asteroid, the asteroid will lose 715*0.0079*600=3,389.1 kilogram-meters per second every minute. At this rate it would take slightly over 7 billion years to bring 4942 Munroe to a halt. Chances are that there will be no earth to hit by then anyway.

Luckily for humanity, it's possible to do better, but it'll take some thinking. The ideal gun for this sort of task would have massive bullets with high velocity and a high rate of fire. The heavy artillery of World War I and World War II had massive ammunition and often that ammunition had a high velocity. The railway gun Krupp K5 had a shell mass of 255 kilograms and a muzzle velocity of 1120 meters per second. The low firing rate (0.25 rounds per minute) might be a problem, but let's see. 1120*255*0.25=71,400 kilogram-meters per second lost per minute. This would bring the time down to 341 million years, which is actually a lot better than the AK47 (but still impractical). Some cannons are better. The immense V-3 cannon has a far higher rate of fire and muzzle velocity, but only a slightly lower shell size. A V-3 cannon being fired at the asteroid would cause it to lose momentum at a rate of 1,050,000 kilogram-meters per minute, which brings the time down to under 24 million years. The anti-aircraft gun GAU-8 Avenger (link to WP) is slightly superior (13 million years). None of these are going to cut it though.

More modern weapons can do still better. Consider railguns, which are moving from experimental devices into ones with practical uses. The US Navy plans in 2016 to test a railgun with a firing rate of 10 rounds per minute, a muzzle velocity of 2500 meters per second, and a bullet mass of 10.5 kilograms[6] Even that is not enough: the stopping time would be nearly 100 million years. The last thing that I'll try is this Assuming a muzzle velocity of 1000 meters per second, it would take 2.4 million years for one of those to stop 4942 Munroe. Giving one to every person on earth would stop the asteroid in just under three hours, but that wouldn't be practical.

I guess we're doomed if a huge asteroid comes our way. Or not?