Even asteroids that don’t make contact with the Earth can kill one million people.
Reilly2007 (Michael. "Small Asteroids Can Pack a Mighty Punch." EBSCO Publishing Service Selection Page. 22 Dec. 2007. Web. 27 June 2011. . znf)
Asteroids can cause more damage than we expected when they explode in the atmosphere BEWARE the blast from above: small asteroids that explode before they hit the ground may be more dangerous than we thought. Asteroids a few tens of metres in diameter rip through the atmosphere at between 40 and 60 times the speed of sound, and many explode before they hit Earth. Extreme friction and heating can cause these asteroids to flatten into a pancake, which increases drag even more and eventually tears them apart. The resultant "airburst" is thought to be behind the 1908 Tunguska explosion in Siberia, which leveled 2000 square kilometres of forest. Because airbursts spread material over a wide area and there is no impact crater, researchers rely on computer simulations to calculate the size of the asteroids that caused them. Previous calculations for the Tunguska event suggested an asteroid around 50 metres in diameter exploding with a force of between 10 and 20 million tonnes of TNT. Now a computer simulation by Mark Boslough of Sandia National Laboratories in New Mexico shows that a 30-metre asteroid could have been behind the Tunguska blast. That suggests smaller asteroids can do more damage than previously thought - a worry when one considers that objects smaller than 140 metres across are not currently detected as they zip round the solar system. Previous simulations overestimated the size of the bodies responsible for airbursts because they treated them much like a nuclear explosion at a fixed point in the atmosphere, says Boslough. As a result, the damage they caused was thought to be related only to the size and temperature of the blast, and its distance away from Earth's surface. "That neglects something significant, though - momentum," says Boslough. His calculations show that the resulting fireball would continue to rocket towards Earth as it exploded.In the case of Tunguska, this jet didn't quite reach the surface - stalling at an altitude of around 5 kilometres - but a heat and shock wave would have carried on to Earth's surface to do much of the damage. It's becoming clear that previous models aren't right, says Boslough, who presented his results at the annual meeting of the American Geophysical Union in San Francisco this month. "If one of these events hit an area of high population density, it could kill 1 million people."
Exploding Asteroid Tsunamis
Asteroid airbursts over the ocean can cause massive tsunamis that threaten most of the Earth’s population
IRWIN I. SHAPIRO et al in 10,( Harvard-Smithsonian Center for Astrophysics, Chair FAITH VILAS, MMT Observatory at Mt. Hopkins, Arizona, Vice Chair MICHAEL A’HEARN, University of Maryland, College Park, Vice Chair ANDREW F. CHENG, Johns Hopkins University Applied Physics Laboratory FRANK CULBERTSON, JR., Orbital Sciences Corporation DAVID C. JEWITT, University of California, Los Angeles STEPHEN MACKWELL, Lunar and Planetary Institute H. JAY MELOSH, Purdue University JOSEPH H. ROTHENBERG, Universal Space Network, Committee to Review Near-Earth Object Surveys and Hazard Mitigation Strategies Space Studies Board Aeronautics and Space Engineering Board Division on Engineering and Physical Sciences, THE NATIONAL ACADEMIES PRESS, http://www.fas.harvard.edu/~planets/sstewart/reprints/other/4_NEOReportDefending%20Planet%20Earth%20Prepub%202010.pdf)\
More recent work on airburst events over the ocean suggest that this too is an area of uncertainty. Previous investigations have treated these types of airbursts in a fashion similar to nuclear explosions that deliver their energy from a single point. If this treatment were correct, then the resultant blast waves would not produce a tsunami-type of event. However, a recent study suggests that NEOs entering the upper atmosphere and exploding there act more like a linear series of nearly simultaneous explosions. (Boslough and Crawford, 2008) These blast effects are not as localized as those from the single source models, in which the momentum of the object is carried downwards into the atmosphere and produces a shockwave. If the shockwave were sufficiently strong to depress a wide area of the ocean’s surface, the resultant rebound effect of the ocean would create a classic tsunami. Hence the threat from small NEO airbursts over the ocean might present their most significant hazard to humanity given that most of the world’s population is concentrated on or near oceanic coastlines.
Ocean Impact More Likely
Impact over the ocean is more likely, as well as more deadly.
Shiga in 09 [David, “It's behind you!” Staff Writer for New Scientist Editorial, 02624079, 9/26/2009, Vol. 203, Issue 2727, PN]
Of course, two-thirds of Earth's surface is ocean. While our atmosphere is likely to protect us from asteroids smaller than 100 metres across, anything larger hitting the ocean - including chunks of Innoculatus's rubble pile - would cause a giant splash that could smash coastal buildings with high-speed volleys of water. The tremendous damage and loss of life that would ensue if multiple cities around an ocean basin were flooded led NASA scientists in 2003 to rate ocean impacts by asteroids as far more dangerous than those on or over land.
Asteroid strikes in the ocean would cause massive damage across the world – surveillance and defense systems could solve this cheaply.
Los Alamos National Laboratory 98 (“RESEARCHERS MAKING WAVES AT LOS ALAMOS” WASHINGTON, D.C., Jan. 7, 1998 http://carlkop.home.xs4all.nl/strike.html ) JM
Los Alamos National Laboratory researchers are demonstrating the enormous damage of an asteroid strike -- not from an impact on land but from tsunamis caused by an asteroid hitting Earth's oceans. Computer models show how impacts of various sizes will generate waves that could devastate entire coastlines on several continents.A surveillance and defense system could prevent such a disaster. Astrophysicist Jack Hills of the U.S. Department of Energy's Los Alamos National Laboratory presented his findings today at a news conference and a scientific session at the Washington, D.C., meeting of the American Astronomical Society. A tsunami is a fast-moving ocean wave, usually caused by underwater earthquakes or volcanic eruptions, that runs up on a coastline, causing widespread damage. A tsunami retains its destructive energy while it travels enormous distances. When the wave strikes a continental shelf, its speed decreases and its height increases. An asteroid impact would induce a series of waves that could scour thousands of miles of coastline with walls of water and roiling debris. Hills and his colleague Charles Mader use a detailed numerical simulation with a one kilometer spatial resolution and comparative data from historical tsunami events. The Los Alamos model estimates that an asteroid three miles across hitting the mid-Atlantic would produce a tsunami that would swamp the entire upper East Coast of the United States to the Appalachian Mountains. Delaware, Maryland and Virginia would be inundated, including Long Island and all the coastal cities in this region. It would also drown the coasts of France and Portugal. Alternately, Hills' model shows how much of Los Angeles and Waikiki would be lost if the same rock cratered the ocean between Hawaii and the West Coast. Fortunately, Earth is likely to take a hit from an object that large only once every 10 million years. However, the chance of a strike by a relatively small asteroid is two or three thousand times more likely, or once every few thousand years. Objects larger than about 600 feet across are virtually unaffected by the atmosphere and will reach Earth's surface at nearly full velocity to cause a crater on land or sea. Most of the damage from such an impact would come from a tsunami. For example, the Los Alamos model shows that an asteroid about 1,300 feet in diameter would devastate the coasts on both sides of the ocean with a tsunami more than 300 feet high. Asteroids smaller than the threshold 600 feet across lose most of their energy in the atmosphere but can still cause unprecedented damage. A "small" impactor hit near the Tunguska River in central Siberia in 1908. Though it never hit the ground, the shock wave flattened 800 square miles of forest. An impact like Tunguska, which hit with a force a thousand times greater than the Hiroshima bomb, occurs over land every 300 years on average. Hills and Mader have received Laboratory funding for an additional three years of model development. They expect increasingly sophisticated models to predict more extensive coastal damage than previously calculated. And Hills would like to see the research yield a practical plan of defense. "An impact from the smaller asteroids is one disaster that is preventable," Hills said. But to deflect an asteroid on a collision course, first it must be seen ahead of time. Then a nuclear-armed rocket must be ready to intercept it. A nuclear blast in space could either shatter or re-direct the incoming asteroid, Hills said. Currently, there is no such surveillance or defense capability in place. "It's a problem that could be solved for much less than the cost of one hurricane. We could just set it up and be done with it," said Hills.