ENVIRONMENTAL
���������������EFFECTS ON EARTH

The atmosphere protects the surface of the Earth from most Near Earth Objects, which burn up or explode at high altitudes. Whether they impact on the surface depends on a number of factors: their size, their composition, their velocity, and the angle of their approach.

Unlike some bodies in the Solar System, the Earth carries relatively few scars from past impacts. In the short term, erosion by wind and weather and, in the long term, tectonic plate movement, remove the traces. Only recently have we been able to detect some of the major impact craters, ranging from Chicxulub in Mexico with a diameter of 180 kilometres (caused by the explosion which marked the end of the dinosaurs), to Ries in Germany with a diameter of 25 kilometres; and to assess the magnitude of the environmental effects which must have been caused.We also have immediate experience of minor events. For example the explosion of an object of around 5 metres diameter at 20 kilometres altitude over the Yukon on 18 January 2000 caused a loud bang, a flash of light, a shower of fragments, and an electromagnetic pulse which caused a temporary loss of power transmission over the area. Further examples of impacts are given at Annex B.

The main effects of impacts are blast waves, tsunamis (or ocean waves), injection of material into the atmosphere, and electromagnetic changes near the surface. We now look briefly at each. Depending on its size, a particular Near Earth Object can have one or more of these effects.The table overleaf summarises the range of effects for objects of different diameter; and the graph on page 17 shows their average frequency of impact.

Blast waves
An asteroid colliding with the Earth is travelling at a speed between 15 and 30 kilometres per second when it arrives at the top of the Earth�s atmosphere. A comet is much faster, up to about 75 kilometres per second. For comparison Concorde travels at about 0.6 kilometres per second.The asteroid or comet generates powerful shock waves as it enters the atmosphere, which lead to enormous heating of both the Earth�s atmosphere and the object, which might be destroyed or vaporised.

Whether or not an object reaches the surface, its energy is released in an explosion which causes a blast wave. This wave represents an abrupt change in pressure that generates a high speed wind, and it is this wind and the debris it carries which cause most destruction. The area affected by winds of much greater than hurricane force can be calculated for air bursts, and it has been found that � as for nuclear weapon explosions � the area of devastation for a given energy of explosion varies according to altitude. The air burst caused by the impact of an object of around 50 metres in diameter at Tunguska in Siberia in 1908 flattened some 2,000 square kilometres of forest. Had it struck an urban area, there would have been an enormous death toll. Obviously the size of the area affected by an air burst depends on the composition and mass of the asteroid and on its path to the Earth. The 1.2 kilometre diameter Barringer crater in Arizona was created by a similar-sized iron asteroid.

For relatively small surface impact events, blast wave damage is comparable to that caused by air bursts. However, as the size of the object and therefore the energy of the impact increases, the explosion becomes so vast that some of the atmosphere above the impact site is blown away from the Earth. In this way the coupling of the energy into the blast wave is reduced. As a result blast waves from large objects, like the one that caused the Chicxulub event, are not expected to devastate directly more than a few per cent of the Earth�s surface, but the area of devastation on the ground would be the size of a large country. The immediate consequence of the blast wave associated with these large events is local � not global scale � damage. Nonetheless it could cause many deaths and great material damage. It could also have other consequences which could enhance the death toll many times. For these really large objects, the ejection of material into the atmosphere described below has a far greater effect in global terms.

Tsunamis
Some two-thirds of the Earth�s surface is covered by the oceans so that the chances of an impact are greater there than on land. As in the case of impact on land, a �crater� is produced in the water but such craters are unstable and rapidly refill. A 30 kilometre crater penetrates through all but the deepest ocean depths.These flows create a series of deep water waves, so-called tsunamis, which propagate outwards from the point of impact. Such waves are also set in motion by earthquakes and underwater land slips.They can travel far around the Earth with devastating effects. For example the 1960 earthquake in Chile caused a tsunami in Japan, some 17,000 kilometres away, that killed at least 114 people. The maximum height above sea level in Hawaii for the same event was 15 metres, with 61 deaths.

Tsunamis travel as fast as aircraft, and their destructive effects can be enormous.The destruction is caused by the amplification in the height of the waves as the waves approach the shoreline. The inflow and outflow of the water mentioned above causes huge damage to property as well as risk to life. The evidence of the effects of large tsunamis, in terms of relocated rocks, is found widely, the most extreme example being in Hawaii where unconsolidated coral is found at 326 metres above sea level. Asteroid impacts are capable of producing tsunamis much larger than that associated with the 1960 earthquake, and may occur anywhere in the oceans.Tsunami from the Chicxulub impact deposited material widely and often far inland; recognition of such deposits in Haiti,Texas and Florida helped to confirm the nature and location of the event.The tsunami generated by the Eltanin impact about 2 million years ago is shown on the two maps on this page.

Objects that are small, or small fragments of larger objects that break up in the atmosphere, do not usually reach the Earth�s surface. But for objects in the range of between 200 to 1,000 metres, tsunamis may be the most devastating of all the consequences of an impact, because so much of the Earth�s population lives near coasts. Some studies have indicated that an impact anywhere in the Atlantic of an object 400 metres in diameter would seriously affect coasts on both sides of the ocean by tsunamis as much as 10 metres or more in height at the shore line.

Injection of material into the atmosphere
The clay layer marking the huge Chicxulub event contains large numbers of particles which were melted at the time. They are the size of small raindrops. Such drops remain in the atmosphere only for a day or two, and are not therefore important for reducing sunlight. But the energy they radiate as they cool would be capable of igniting fires from any combustible material. Such fires would generate soot and poison the air with pyrotoxins.

Smaller particles would stay aloft longer, possibly for months - or, for very big impacts, years - and the large number of such particles could cause reduction of sunlight. But if large quantities of water from the oceans were also injected into the atmosphere, then the formation of ice crystals on the particles could help to sweep the dust from the skies. A cooling effect at the surface of the Earth - the so called nuclear winter effect - would be caused in the event of a large number of such particles, and that would correspond to a very large explosion, say of an object around 1 kilometre in diameter. The effects on the Earth of such a winter could be devastating for all forms of life, including humans. Other chemical changes would also follow impacts.Temperatures behind shock waves are such that nitrogen burns in the oxygen to create nitrogen oxides of various kinds.These oxides are a source of acid rain, and also remove ozone from the atmosphere. The effective screen to solar ultra-violet radiation would therefore be weakened. It would take time for the ozone layer to recover.

Electromagnetic effects in the upper atmosphere
Disturbances in the ionosphere from the atmospheric detonation of a nuclear weapon have been detected at distances as large as 3,000 kilometres from the explosion. Although this explosion was at a low altitude, shock waves occurred at altitudes as great as between 100 and 200 kilometres. Because impacts of Near Earth Objects are of much higher energy than explosions of nuclear weapons, the electromagnetic effects will be correspondingly greater, leading to large scale heating and high intensity electromagnetic disturbances. Even from the frequent air bursts that occur from the impacts of small objects (around 10 times per year), modest disturbances in radio communications are frequently noted and power line failures occur. More severe electromagnetic effects may disrupt other electrical installations.

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