Re: what would happen if a large asteroid hit the earth

Hi Tanya, you ask a very interesting and pertinent question. Large asteroids have indeed hit the earth and have lead to the extinction of many species, the most famous being the dinosaurs. We have even recently observed large cometary impacts on the planet Jupiter. However, since we have not (thankfully) actually witnessed an impact on earth, scientists had to extrapolate an asteroid impact based on computer simulations.

If a large extinction size (500 miles or so) asteroid hits the ocean, it will travel many miles into the ocean floor before being vaporized. At the same time an unimaginably large tidal wave (hundreds of miles high) will be generated. Every coastal city on our planet will be submerged. For the continental US, water will reach well into middle America. The entire Eastern, Western, and Southern coastal states will be under hundreds of feet of water. The vaporized water and dust will travel into the stratosphere and circulate to cover the entire planet in a dark cloak. Temperature will fall, global photosynthesis will be disrupted, and remaining plants will die. This could last for many years, and will be the main cause of global extinction.

If the asteroid hit land, a similiar scenario will ensue (minus the tidal wave of course), except that the cloak of dust generated by the impact will be much thicker. Global extinction will follow due to the lack of sunlight reaching earth's surface.

You can see several supercomputer simulations of an asteroid impact if you go to this site.