An asteroid, say, about 5 miles in diameter impacts with the Earth at say, the modest speed of 20 miles per second. Let’s say it hits in the middle of the ocean (Atlantic or Pacific, take your pick). If I remember right, 99% of the Earth’s atmosphere is contained within 10 miles of the surface so this object doesn’t spend but about a half of a second in our atmosphere. When it hits the deep ocean water it will create a fleeting “crater” in the water. This “crater” will last a matter of seconds as the water comes rushing back inward. This sloshing back and forth will create a tsunami about 2.5 miles high. When this wave gets to the continental shelf it will pile up on itself and become even higher.

Billions and billions of tons of water would instantly vaporize at ground zero. The rising steam would create an updraft of gigantic proportions. Air pressure over the impact sight is estimated to drop to 200 millibars. Normal sea level atmospheric pressure is 1000 millibars; the lowest pressure ever recorded in a hurricane is 890 millibars. The air, with such a void to fill, would come rushing in from all sides and create an updraft of immense size and force. This updraft, coupled with the Earth’s rotation, would create a hurricane of such force that it boggles the mind. With wind speeds estimated to be slightly less than the speed of sound (that’s right, I said, “speed of sound”) this unimaginably strong hurricane would reek havoc the likes of which have never been seen. Numerous, somewhat smaller hurricanes would be spawned around ground zero. Even these would be much larger and fiercer than any hurricane in history.

The steam thrown into the air would be enough to blanket the entire Earth with cloud cover. This cloud cover would last many months, possibly years.

It’s safe to assume that they have discovered all Earth-crossing asteroids larger than a mile in diameter. It’s safe to assume that they have the orbits of these objects plotted. It’s also safe to assume that we would have a warning of one hundred or even two hundred years before impact, plenty of time to send up something to knock it off course. But what about smaller objects? There are quite a few Earth-crossers that are small enough so that they won’t  cause Earth-wide extinction but are still massive enough to kill almost everything on a whole continent. All of these may not have been discovered. Another thing to consider is interstellar asteroids. Theoretically their speed of impact could be much greater. Also there is good reason to believe they exist in large numbers. But there’s an awful lot of space out there. Even if interstellar asteroids do exist in large numbers the chances against seeing one, much less being hit by one, are (no pun intended) astronomical.

The real threat is from smaller asteroids, ones that could take out a city, or perhaps a continent. They are a bigger threat because we do not know how many there are, let alone have their orbits plotted. One could hit Earth at any time, without warning. Also consider comets. Asteroids orbit the sun in the same direction as the Earth, counterclockwise when viewed from above the sun’s North Pole. Comets can come from any direction. Consider the Earth’s orbital speed, 18.5 miles per second. Consider a comet on a hyperbolic orbit and thus going slighter faster than the Earth when it’s at the Earth’s distance from the sun. A comet on a hyperbolic orbit would be going 26.1 miles per second when it’s as far from the sun as we are. A head on collision would dissipate much more energy than the asteroid collision I described above.

Most comets are not in hyperbolic orbits. They are in elliptical orbits but occasionally Jupiter will throw one into a hyperbolic orbit. A comet thrown into a hyperbolic orbit will never return because it has achieved escape velocity from the sun. It is extremely unlikely that the Earth would be struck by a comet on a hyperbolic orbit. A comet in an elliptical orbit will be going just slightly faster than the Earth when it is in the Earth’s vicinity, though not quite as fast as one on a hyperbolic orbit. Comets are also a threat because they are so numerous (about five a year have been discovered over the last couple hundred years, most are long period comets) and because a “new” one can arrive in the inner solar system any time, without any warning. I wouldn’t loose any sleep over this though. The odds are that it almost certainly won’t happen in our lifetimes. But give it enough time and it will happen!