There is a 1 in 300 chance of the asteroid 2004 MN4 hitting the Earth in the year 2029. It has a rating of 2 on the 10-point Torino Impact Hazard Scale used by astronomers to predict asteroid or comet impacts. No previous object has received a rating above 1.
The object is roughly 1320 feet long, and would strike the Earth with 1600 megatons of energy. The exact date of impact is Friday, April 13, 2029.
Once more information is gathered by astronomers, the risk level could either increase or decrease.
If the large asteroid hits an ocean, a tsunami would be created, wiping out coastal areas, and extending very far inland wherever it hits.
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The size does not have the capability to destroy all of mankind, but only wipe out a lot of the population, depending on where it hits.
"The worst case scenario is a hit in The Great Forest of the World, Siberia. The entire forest, millions of square miles of dense wood, would burn, sending up soot and ash that would block the world for weeks. This is called Impact Winter."
If the asteroid hits in the Pacific Ocean, a tsunami would wipe out Japan, New Zealand, Australia, The west coast of South America up to the Andes, and the west coast of America and Mexico up to the Rocky Mountains.
Link to Info: http://www.cnn.com/2004/TECH/space/12/24/asteroid.ap/index.html