How Dangerous are Earth-Crossing Objects?

Spacewatch and other near-Earth object search programs demonstrate that the Earth is surrounded by a swarm of asteroids and comets that threaten us with collision and world-wide destruction. The danger from near-Earth objects has sparked research into the probability of occurrence of damaging impacts as well as the possibility of deflecting potential impactors before they strike the Earth.

The extent of the damage even a small impactor can cause is exemplified by the asteroid or comet fragment which exploded in the air over Tunguska in Siberia in June of 1908 with a force equivalent to between ten and twenty megatons of TNT. (Such an explosion in the air in which the impactor does not reach the ground intact is called an airburst or airblast.) The resulting blast wave leveled hundreds of square kilometers of forest. The area was sparsely inhabited so only two people are reported to have been killed: Vasiliy son of Okhchen died from wounds sustained after being hurled against a tree by the blast, and the aged hunter Lyuburman of Shanyagir died from shock.

The Tunguska object was probably a stony body about 50-70 meters (around 200 feet) in diameter. An object of this size could easily destroy a large metropolitan center. This nearly happened with Tunguska; a difference in arrival time of a few hours might have seen populous St. Peterburg or another European city destroyed. In fact, at about the same time as the Tunguska object exploded, a small object struck near the city of Kiev. The coincidence in time leads some scientists to speculate that the Kiev object may be a fragment of the Tunguska impactor, or at least, a fragment of the same parent object as the Tunguska impactor.

Smaller scale airbursts over populated areas have caused minor damage. For example, an airburst over Madrid, Spain in 1896 smashed windows and leveled a wall. There are many reports of airbursts causing tremors and minor damage in inhabited areas. John Lewis's book Rain of Iron and Ice lists a couple of dozen such incidents over the past century. A small airburst which occurred over El Paso, Texas, USA on October 9, 1997 caused no apparent damage but did alarm residents. Another which occurred July 7, 1999 over New Zealand was captured on videotape. Fortunately, most airbursts occur over the oceans, so no damage to human habitations results.

What size impactor makes it through the atmosphere to the lower atmosphere or the ground with enough remaining velocity to produce a damaging airburst or crater-forming impact? It turns out that the Earth's atmosphere is ineffective in preventing ground impact damage for stony meteorites greater than 200 meters (about 650 feet) in diameter. For iron meteorites that impact at greater than 20 km/sec (12.5 mi/sec), the critical diameter is about 40-60 meters (130-200 feet). Stony bodies greater than 60 meters and less than 200 meters can cause significant airburst damage as at Tunguska.

The greatest danger from an ocean impact occurs when the incoming body does not disintegrate in the atmosphere but instead strikes the water relatively intact. The impact raises a tsunami which, if the object is large enough, can devastate coastal areas hundreds of miles away. Tsunamis of unknown origin are usually attributed to earthquakes and volcanos, but it is likely that some -- including the largest and most damaging -- result from cosmic impacts. An asteroid of sufficient size to raise a tsunami with an average height of 100 meters along the entire coast of the ocean strikes once every few thousand years on average.

Stony bodies less than 200 meters in diameter do not produce tsunamis, while those larger than 200 meters can produce catastrophic tsunamis. Water waves generated by such an impactor are two-dimensional disturbances that fall off in height only inversely with distance from the point of impact. The average runup in height of a tsunami as it reaches the continental shelf is more than an order of magnitude. An impact anywhere in the Atlantic of a stony asteroid more than 400m (1,300 feet) in diameter would devastate coasts on both sides of the ocean. Tsunami runups would exceed 60m (200 feet).

Frequently it is asserted than there have been no recorded deaths caused by meteorite strikes. In fact, as John Lewis points out in his book Rain of Iron and Ice, there have been a number of injuries and deaths attributed to meteorite impacts throughout history. See the table listing some instances of such injurious impacts taken from Lewis's book. Walter Branch offers another list of meteorites that have struck man-made objects, humans, and animals .

The well-known Richter scale is often used to gauge the severity of an earthquake. The recently developed Torino Scale measures the potential damage from a cosmic impact on a scale on 0 (no damage) to 10 (an impact event capable of causing a global climatic catastrophe). The Torino scale was developed by Richard P. Binzell of MIT.

The idea of deflecting impactors before they strike the Earth goes back at least to Lord Byron, who in 1822 wrote:

Who knows whether, when a comet shall approach this globe to destroy it, as it often has been and will be destroyed, men will not tear rocks from their foundations by means of steam, and hurl mountains, as the giants are said to have done, against the flaming mass? - and then we shall have traditions of Titans again, and of wars with Heaven.

A few ideas for deflecting a threatening near-Earth comet or asteroid include:

All of these methods -- and many more which have been proposed -- rely on sufficiently early detection of the threat from a particular near-Earth object. That is why the NEO search programs are so important. If we don't know a threatening object is coming, we can't prepare to deflect it. If we don't deflect the NEO, the impact may destroy out civilization. A sufficiently large impactor we extinguish us and most life on Earth. We could go the way of the dinosaurs without even knowing what hit us.

Collision Probabilities


Detection of, Dangers of, and Defenses against, Impacts


Impact-Generated Tsunamis


Impacts on Earth


Impacts that weren't


Modeling Impacts


Near Collisions


Tunguska and similar events


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Last modified by pib on July 6, 2003.