AActually, the impact of a comet that big would be far worse than the film portrayed. The moving scene of the father and daughter facing their end together on the beach would not be possible. They would have died long before the tsunami arrived.Using the Earth Impact Effects Program developed by Robert Marcus, H. Jay Melosh, and Gareth Collins and entering typical values for a comet this size, here are some reasonable results:Inputs: Assuming you were 69 miles (about 100 km) from the impact of an icy comet 2.5 miles wide at typical angle (45 degrees) and velocity (51 km/sec) for comet and allowing for atmospheric slowing. Impact is in about 300 meters of water over typical rock.Effects are as follows:The comet's energy before atmospheric entry is about 1 million Megatons of TNT. That is the equivalent of one trillion tons of TNT explosive. (This is half a billion times more powerful than the largest hydrogen bomb ever detonated on Earth.)When the comet hits the ocean, it creates a water crater 34.8 miles wide and an initial seafloor (rock) crater 20 miles in diameter and an incredible 7 miles deep. This complex crater would melt or vaporize 50.3 cubic miles of rock. About half of this rock mass would remain in the crater, while the rest would be ejected. The crater would finally stabilize at 31.6 miles in diameter and just less than 1 kilometer deep.Tsunamis would undoubtedly occur, but they would not be your most immediate worry!First there is heat. The impact creates a fireball 43.3 miles wide that looks 159 times larger than the sun. Only 1.39 seconds after impact, the thermal radiation reaches you. For the next 15 minutes, you would be subjected to heat hot enough that your clothing would ignite and much of your body would suffer third degree burns. Paper, grass, trees, and wood would all ignite.Assuming you were shielded from the thermal radiation, you would next have to worry about seismic effects. About 20 seconds after the impact, you feel an earthquake of magnitude 9.2 on the Richter Scale. This powerful of a quake would damage almost all structures and collapse many, even substantial engineered structures. Most masonry and wood frame structures, such as houses, would be destroyed. Serious damage would overtake dams, dikes, and embankments with water being thrown from the banks of canals, rivers, lakes, etc. Large landslides would also result.Somehow surviving the seismic mayhem amid the burning heat, you find the ejecta arriving next. A little more than two minutes after impact, rock fragments of a punishing average diameter of 13.3 inches would rain down from the unfriendly sky, eventually burying your site in a layer almost 32 feet thick.Next, about five minutes after the blast, as you somehow toughed out the heat, the shaking, and the avalanche from the sky, an air blast from the impact would reach you. At 40.7 bars or 578 pounds per square inch, this blast would take the form of a mighty wind with a maximum velocity of 3,580 miles per hour. The sound would damage or destroy your hearing. Around you in the heat, the shaking, and the raining debris, the titanic blast would collapse almost any remaining buildings and bridges. Cars and trucks would be tossed and bent out of shape. Trees would be flattened.Certainly there wouldn't be much left of your location by the time the first tsunami came. The Earth Impact Effects Program does not calculate tsunamis, but clearly water would rush into the huge crater and spring back outward against itself as it contended with the molten rock and vaporized water. This would create not just one but a series of enormous tsunamis--perhaps doing justice to the ones portrayed in the movie. (Think of the effects of the tsunamis in the Indian Ocean in 2004 or in Japan in 2011 multiplied many times over.)Life on Earth would not be destroyed, nor would all people. But things would go very hard for human civilization for a while with massive loss of life, economic collapse, ecological devastation, and short-term climate cooling as dust from the impact blocked sunlight around the globe. This cooling would add the additional specter of global famine as growing cycles for food crops everywhere were disrupted.All in all, it's really quite a good thing that big objects like this don't fall on our planet very often!
