Updates and very cool Java based sim available at:
Now 4 on the Torino scale (was the object first to reach 2).
Chance of impact on 13 April 2029 2.2% (was originally 0.3%).
It's smaller than they though. Power of impact now estmated as 1400 megatons.
For comparison, here's a few documented historical explosions so you can see what they did:
1 Mount St. Helens = 25 megatons
1 Tunguska = 40 megatons
1 Krakatoa = 150 megatons
At 50 miles radius, anyone indoors is probably safe.
At 100 miles, outdoors is probably safe if you don't mind getting pelted by 1/4" rocks.
Climate change though, that's different. 1816, "The year without a summer" is though to have happened due to 5 "major" volcanic eruption between 1811 and 1815, primarily Mt. Tambora in 1815. It threw 6 times as much ejecta as Krakatoa. With 2004MN4 it depends on what it hits, not so much what it's made of. Unless it happens to be, say, cyanide ice from the rings of Saturn.
A week ago we were missed by less than 25,000 miles.
2004YD5 was luckily only about 5 meters in diameter.
Chicken Little was right.