Future — Near Earth Objects

Most bodies in the solar system with a visible solid surface exhibit craters. On Earth, we see very few because geological processes such as weathering and erosion soon destroy the obvious evidence. On bodies with no atmosphere, such as Mercury or the Moon, craters are everywhere. There is strong evidence of a period of intense cratering in the solar system that ended about 3.9 billion years ago. Since that time, cratering appears to have continued at a much slower and fairly uniform rate. The craters were caused by the impacts of comets and asteroids. Most asteroids follow sensibly circular orbits between the planets Mars and Jupiter, but all of these asteroids are perturbed, occasionally by each other and more regularly and dramatically by Jupiter. As a result, some find themselves in orbits that cross that of Mars or even Earth. Comets, on the other hand, follow highly elongated orbits that often come close to Earth or other major bodies. These orbits are greatly affected if they come anywhere near Jupiter. Over the eons every moon and planet finds itself in the wrong place in its orbit at the wrong time, many times, and suffers the insult of a major impact.

Current technology permits us to discover and track nearly all asteroids or short-period comets larger than 1 km diameter that are potential Earth-impactors. These objects are readily detected with moderate-size ground-based telescopes. Most of what we now know about the population of Earth-crossing asteroids (ECAs) has been derived over the past two decades (1980's and 1990's) from studies carried out by a few dedicated observing teams using small ground-based telescopes. Currently several new ECAs are discovered each month. At this rate, however, it will require more than a century to approach a complete survey, even for the larger objects.

Every few centuries the Earth is struck by an NEO large enough to cause thousands of deaths, or hundreds of thousands of deaths if it were to strike in an urban area. On time scales of millennia, impacts large enough to cause damage comparable to the greatest known natural disasters may be expected to occur. Indeed, during our lifetime, there is a small but non-zero chance (very roughly 1 in 10,000) that the Earth will be struck by an object large enough to destroy food crops on a global scale and possibly end civilization, as we know it.

NEO dia. Yield in MTs Crater dia. (km) Ave years between impact Effects
  <10     Upper atmosphere detonation of "stones" (stony asteroids) and comets; only "irons" (iron asteroids) <3%, penetrate to surface.
75m 10 to 100 1.5   Irons make craters (Barringer Crater); Stones produce air bursts (Tunguska). Land impacts could destroy area the size of a city.
160m 100 to 1k 3 4k Irons and stones produce ground-bursts; comets produce air bursts Ocean impacts produce significant tsunamis. Land impacts destroy area the size of large urban area.
350m 1k to 10k 6 4k Impacts on land produce craters; ocean-wide tsunamis are produced by ocean impacts. Land impacts destroy area the size of a small state.
700m 10k to 100k 12 63k Tsunamis reach hemispheric scales, exceed damage from land impacts. Land impacts destroy area the size of a moderate state.
1.7km 100k to 1m 30 250k Both land and ocean impacts raise enough dust to affect climate, freeze crops. Ocean impacts generate global scale tsunamis. Global destruction of ozone. Land impacts destroy area the size of a large state. A 30-kilometre crater penetrates through all but the deepest ocean depths.
3km 1m to 10m 60 1m Both land and ocean impacts raise dust, change climate. Impact ejecta are global, triggering widespread fires. Land impacts destroy area size of a large nation.
7km 10m to 100m 125 10m Prolonged climate effects, global conflagration, and probable mass extinction. Direct destruction approaches continental scale.
16km 100m to 1b 250 100m Large mass extinction (for example K/T or Cretaceous-Tertiary geological boundary).
  > 1b     Threatens survival of all advanced life forms.
  • Big splash theory says meteors hit regularly — see here.
  • NEO effects are described here.
  • The environmental consequences are discussed here.
  • Calculate the impact effect here.

Meteors and Bolides.

The Earth experiences a constant barrage of impacts, adding an estimated mass of 105 kg per day. Objects give up most of their kinetic energy in the atmosphere and dissipate (as meteors) or likely fragment due to aerodynamic stress or explode (as bolides) if they encounter a column of atmosphere equal to or greater than their mass. Impacts having Hiroshima scale energies (~0.015 MT) occur annually and dissipate in the atmosphere. (Such events are routinely detected by surveillance satellites. Recently declassified results are reported by Tagliaferri et al. 1994.) The upper size limit for the Earth's atmospheric shield is in the range of 30-50 m (depending on object strength and density) for a typical entry velocity of 20 km s-1 (Chyba 1993). While dozens of impact events yield recoverable meteorite fragments each year, there is no authenticated human fatality from such meteorite falls, though a famous car-conking incident occurred in Peekskill, NY in October 1992. With the exception of rare (about 5%) events involving solid iron objects, objects smaller than 20-50m produce no substantial effects on the surface or to the environment of the Earth.

Locally or Regionally Destructive Events.

If a 30-50 m meteoroid is able to penetrate to within ~10 km (or strike) the surface of the Earth, the kinetic energy imparted to the surface by the atmospheric shock wave or by direct impact can cause severe local damage in a manner analogous to a nuclear bomb, but without the coincident radiation or radioactive fallout. Civil defence studies (e.g. Glassone and Dolan 1977) suggest that damage scales by the energy to the 2/3 power. The 1908 Tunguska air burst event and Meteor Crater Arizona provide important calibration. Tunguska involved a weak or modest strength >50 m impactor having an energy of 10-20 MT resulting in the devastation of >1000 km2 of Siberian forest. The 1 km Meteor Crater formed 50,000 years ago as the result of a smaller (~30 m) but higher density (iron) object reaching the surface. The average flux rate suggests a Tunguska-sized impactor strikes the Earth on average every 2-3 centuries, corresponding to a 30 to 50% chance for such an event occurring somewhere on Earth during the next century. The largest impactor for which there is a ~1% chance of occurrence during the next century is in the size range of 250m (1,000 MT). Such an impact would cause regional environmental devastation through the formation a 3-5 km crater on land or a massive tsunami if off shore.

Events Having Global Environmental Consequences.

The transition from increasingly severe regional environmental damage to global environmental damage likely resides in the energy range of 105-106 MT, corresponding to the impact of a 1-2 km asteroid or comet. Impacts within this size range occur with an average frequency of once per hundred-thousand to one-million years. Thus, there is a 1:1,000 to 1:10,000 chance of such an event during the next century. As shown below, Chapman and Morrison (1994) consider that globally catastrophic effects conceivably may result from impact energies as low as 104 megatons, which are events that have a higher than 1:1,000 chance of occurring during the next century. The enormously more energetic Cretaceous-Tertiary mass extinction (Alvarez et al. 1980) appears to be a 50-100 million year event, corresponding to a one-in-a-million chance during the next century. Significant temporal variations in the flux may occur due to "comet showers" or the break-up of a large Earth-crossing asteroid.

Estimated thresholds (shaded region) for impact energies and probabilities capable of triggering a global catastrophe, where an estimate of global fatalities is shown at the right. The lower threshold corresponds to a 1:1,000 chance of astronomically induced environmental effects during the next century. The dashed line indicates the potential local effects of tsunamis, whose century-timescale hazard chance may exceed 1%. Figure is from Chapman and Morrison (1994). (View large)

Toon et al. (1994) considered the global environmental perturbations that are likely to result from impacts and reached conclusions compatible with mean estimate from Chapman and Morrison (1994) in that global environmental consequences result from impact energies exceeding 105 MT, corresponding to the same 1:1,000 to 1:10,000 chance for such an event during the next century. The environmental effects considered by Toon et al. (1994) included those arising from the effects of dust and water injected into the stratosphere and the depletion of ozone due to the formation of NO by the entering impactor and the re-entry of impact ejecta. Toon et al. predict environmental effects (manifested in part by significant global temperature changes) would persist for time periods ranging from months to years depending on the energy of the impactor and the precise circumstances of its trajectory, impact location, etc.


Earth's atmosphere protects us from the multitude of small debris, the size of grains of sand or pebbles, thousands of which pelt our planet every day. The meteors in our night sky are visible evidence of bodies of this type burning up high in the atmosphere. In fact, up to a diameter of about 10 meters (33 feet), most stony meteoroids are destroyed in the atmosphere in a terminal explosion.

A 10-meter (33-foot) body typically has the kinetic energy of about five Hiroshima fission bombs, however, and the shock wave it creates can do considerable damage even if nothing but comparatively small fragments survive to reach the ground. Many fragments of a 10-meter (33 foot) iron meteoroid will reach the ground — the only well studied example showed fragments covered an area of about 1 to 2 square kilometres (0.4 to .8 square miles) within which there were 102 craters greater than 1 meter (39 inches) in diameter, the largest of them 26.5 meters (87 feet), and about 100 more smaller craters. Such an event occurs about once per decade somewhere on Earth.

It is the falls larger than 10 meters (33 feet) that start to become really worrisome. The 1908 Tunguska event was a stony meteorite about 60 metres that exploded in the atmosphere at an estimated 8 Km altitude with the force of 12 megatons. The famous meteor crater in northern Arizona (right), some 1.2 kilometres (4,000 feet) in diameter and 183 meters (600 feet) deep, was created 50,000 years ago by a nickel-iron meteorite perhaps 60 meters (197 feet) in diameter. It probably survived nearly intact until impact; an explosion equivalent to some 15 megatons. Falls of this class occur once or twice every 1,000 years.

Michael Baillie (Queen's University, Belfast) believes that modern civilisation would collapse after the impact of a 500-meter-wide object. "The trouble is," he says, "that a significantly smaller impact could still do the trick, especially if over an ocean… Civilisation is a thin veneer. Take away all air travel, restrict global food supplies, demonstrate that the military and governments are ineffective, and demonstrate that coastal zones should be avoided, and where would we all be? That is not to mention the problems when all the dead sea life washes up."

Falls up to about 1 km diameter, an impacting NEO can do tremendous damage on a local scale and it is now estimated that there are about 300,000 near-Earth asteroids over 100 meters in diameter, and about 2000 over 1 kilometre in diameter.

If an asteroid of size 200 meters hit the ocean (which covers 70% of the Earth), the tsunami it would create would inflict catastrophic destruction of coastal cities and substantial worldwide human casualties along coastlines. Recently modeled, an ocean impact could produce huge ozone holes that would effect plant and plankton and the whole food chain. If an asteroid of size 1 kilometre hit Earth, it would cause a dust cloud which would block out sunlight for at least a year and lead to a deep worldwide winter, exhausting food supplies.

Above a million megatons (diameter about 2 km), an impact will produce severe environmental damage on a global scale. The probable consequence would be a "nuclear winter" with loss of crops worldwide and subsequent starvation and disease. Still larger impacts can cause mass extinctions, like the one that ended the age of the dinosaurs 65 million years ago (15 km diameter and about 100 million megatons).

It doesn't have to be a big NEO to cause a lot of destruction – the kinetic energy of a 10-metre projectile traveling at a typical atmospheric entry velocity of 20 km/s is about 100 kilotons, a 30-metre projectile about 20 megatons; most of these will explode high up in the atmosphere as they enter it. Small objects intercept Earth every decade. Bodies about 100-metre diameter and larger hit, on average, several times per millennium. The kinetic energy of a 100-m diameter body is similar to the explosive energy of about 100 megatons.

The effects of a NEO strike is thought to be similar to that of a nuclear weapon, hence the equivalent tonnage of TNT given in the tables. But there are differences:

  • The NEO is a solid object. Where it hits the earth it will have the added effect of damaging, or puncturing, the earth's crust. The angle of attack will have an important part to play here. The collision of object and earth will produce earthquakes and may move some of the tectonic plates and effect fault lines – this is in addition to a ground burst nuclear weapon effect. If it explodes in the air it will be like an air burst nuclear weapon.
  • The NEO has to pass through the atmosphere. If it comes straight down the effects may be minimal, but if it has a low angle of attack it may pass through hundreds or thousands of miles of the atmosphere. This could produce super heating or even explosions along its path before it eventually heats up enough to detonate; a path of destruction could be left before the object explodes.
  • Depending upon the shape and size the NEO could break up (or explode and break up) in the atmosphere. This could produce multiple hits many miles apart.
  • After the collision with the earth, very large particles could be ejected into space to re-enter the atmosphere (like ICBMs) and cause other NEO events elsewhere.

"The impact of an asteroid or comet several kilometres across heaps environmental insult after insult on the world," said Dr. Daniel Durda, a senior research scientist at Southwest Research Institute. "One aspect of the devastation wrought by large impacts is the potential for global wildfires ignited by material ejected from the crater reentering the atmosphere in the hours after the impact."

Large impacts can blast thousands of cubic kilometres of vaporized impactor and target sediments into the atmosphere and above, expanding into space and enveloping the entire planet. These high-energy, vapour-rich materials reenter the atmosphere and heat up air temperatures to the point that vegetation on the ground below can spontaneously burst into flame."

Reentry of a vast quantity of ejecta

the so-called 'Broiler Effect', the radiant heating of the earth's surface caused by the massed reentry of a vast quantity of ejecta from Chicxulub crater. Think of the Leonid meteor shower magnified billions of times. The effect was first modeled by Melosh, Schneider, Zahnle and Latham ('Ignition of global wildfires at the Cretaceous/Tertiary boundary' in: Nature, volume 343, pages 251-254, 18 January 1990) and has been recently refined by Croskell ('Ejecta dispersal and infra-red pulse generation by the Chicxulub impact' in: Proceedings of the 30th Annual Lunar & Planetary Science Conference, #1746, 1999).

Let me quote Croskell (of the T.H. Huxley School, Imperial College, London):

'Material is evacuated from the crater, launched on ballistic trajectories and re-enters the Earth's atmosphere at velocities up to 11km/s (escape velocity). The small particles of ejecta accelerated to these high speeds (mostly sub-mm) are decelerated in the middle atmosphere by molecular drag. As they rapidly slow to terminal velocity (close to 0km/s), the vast majority of their kinetic energy is transformed to heat, enough to cause the larger particles to melt. The ejecta cools, emitting radiation in the near and mid infra-red range. Half of this energy is directed downwards and although water vapour and carbon dioxide absorb certain wavelengths, around 30% of this energy can potentially reach the ground.'

Dr. Croskell used a computer model of the ejecta cloud enveloping the earth (representing a mass of rock particles of 4 x 1015kg — 4 trillion tonnes) to calculate the average heat energy reaching the ground at different locations on earth. He confirmed the earlier model of Melosh et al. (1990), showing that the heat pulse reached higher levels within a few thousand kilometres of Chicxulub (e.g. 2500kW/m2 — kilowatts per square meter — in New Mexico) and decreased significantly on the far side of the earth (e.g. 40kW/m2 in New Zealand). The more remote the pulse, however, the longer it would last (e.g. about 20 minutes in New Mexico vs nearly 2 hours in New Zealand). Given that a domestic oven broiler produces about ten kilowatts per square meter, it is clear that the term 'Broiler Effect' is an understatement NOT an exaggeration. Studies of the effects of nuclear explosions on forests indicate that, even on the far side of the earth, K/T heat levels would have been more than enough to spontaneously ignite wildfires (and nicely sear a lot of dinosaurs). Only thick layers of cloud would have shielded the ground below.

So it should come as no great surprise that large quantities of soot, containing all the characteristic chemical compounds of burned plant material, should be found in the K/T boundary layer all over the world (e.g. see Wolbach et al. 1988. Nature, vol. 334, pages 665-669, or Kruge et al. 1994. Geochimica et Cosmochimica Acta, vol. 58, pages 1393-1397). It should come as no great surprise that so many of the terrestrial survivors of the K/T impact would have been in, or been able to quickly reach, some form of shelter from searing heat above.

Meteor Strike Scenaros

700 m: land impact, with little deceleration

At approximately 8:15 this evening, Asteroid 2004 Cyrus struck the Earth 5 km west of Paris, France. Since the discovery of its trajectory and impending impact about six months ago, the governments of Western Europe have done their best to relocate the populations of France, Belgium, Luxembourg and the Netherlands.The force of the impact, equal to that of 50,000,000,000 tons of TNT, annihilated the city of Paris in an instant. The shock wave from the explosion flattened large areas of London, Brussels, Antwerp andAmsterdam, while the intense heat ignited firestorms across much of France. The cloud of smoke and dust from the explosion and fires is expected to alter the climate of the Earth for a period of months or years, causing crop failures and possible mass starvation.

Credit: http://homepages.wmich.edu/~korista/bang.html

700 m: ocean impact, with little deceleration

At approximately 11.45 this morning GMT (night time in Japan) a meteorite landed 500 miles south of Tokyo in the Pacific Ocean. Giant tsunamis have destroyed all coastal and island areas for thousands of miles. Damage is devastating for the islands of Japan, the Korean peninsula and much of Northern China. Death tolls are expected to reach as high as 100 million. Specific damage reports are as of yet unavailable.

Credit: http://homepages.wmich.edu/~korista/bang.html


Credit: http://worldif.economist.com/article/12/what-if-an-asteroid-heads-for-earth-taking-the-hit

The likelihood of a NEO strike

The likelihood of a NEO strike is pretty scarce – but oddly enough, you get better odds for getting hit with a meteor than you do for winning the lottery!

This picture shows the 140 known impact sites on the Earth (there are many hundreds awaiting verification). Whilst these look numerous, they have occurred over hundreds of millions of years. There are noticeable groups in the eastern USA and Northern Europe. (large)

Most bolides have an explosive force in the kilotons range, but are high enough in the atmosphere to not cause damage on Earth, just creating a very bright flash and a loud bang. Data collected by one site — the Air Force Technical Applications Centre at the Patrick Air Force Base in Florida — between 1960 and 1972 was made public. It characterized 20 explosions on 10 different dates (some dates had multiple hits). Most of the detections were within only about 5000km (3000 miles) range. However, two bolides delivered the energy of over one megaton of TNT, which is the same as a large nuclear warhead, and over 50 times the power of the nuclear bomb dropped on Hiroshima.

On March 23, 1989, an asteroid with a kinetic energy of over 1000 one-megaton hydrogen bombs (i.e., about 50,000 times more powerful than the bomb dropped on Hiroshima) was recorded to have passed very close to Earth, discovered using new technology equipment recently placed. Named 1989FC, this asteroid was detected only well after its point of closest approach, and we found out it had passed so close only after calculating backwards its orbital path after realizing its nearness. This was a key event that brought near Earth asteroids into the political arena.

Excerpts from Dr V.A. Gostin, School of Earth & Environmental Science, University of Adelaide:

In recent years a more accurate time scale has become available through the study of tree-rings of long-lived trees (dendrochronology). This has enabled scientists to reconstruct the growth patterns (and hence climate) of swamp oaks in Ireland and Germany, going back more than 7,000 years for that region. The bristlecone pine of California has similarly produced a climatic record reaching back an incredible 11,000 years. In their 1988 study of the Irish swamp oaks, Baillie and Munro of Queen's University, Belfast, focused on those natural events that produced the narrowest rings in many trees from different sites. Their results revealed six major periods of climatic stress: 3195BC, 2345BC, 1628BC, 1159BC, 207BC, and the latest around AD540 (Baillie, 1999; 2002). These were clearly times when there was a marked reduction in summer growth due to major atmospheric disturbance lasting five to ten years and sometimes longer.

In 1990, British astrophysicists, Bailey, Clube and Napier, published their book The Origin of Comets where they indicated that the Earth must have had periodic encounters with large comets, and that one such close encounter probably occurred during the last two thousand years of recorded history, with a suggested event occurring sometime between AD 400-600, possibly from the breakup of the comet Beila. In 1995, Duncan Steel presented further research dealing with Earth’s hazardous interaction with comets and asteroids, including the possible astronomical connection with megaliths like Stonehenge. Additional support for this interpretation has come from the discovery of cosmic dust in Swedish and Irish bogs corresponding it time to most of the tree ring events (New Scientist 14.9.02).

Comets have relatively small masses with diameters of a few kilometres (rarely 100km), made up of solid lumpy cores surrounded by various frozen gasses, ice, and complex organic compounds that evaporate during the comets passage through the inner solar system. These gasses produce a huge coma or nebulous envelope from 10,000 to 100,000km in diameter, followed by two tails that may be millions of kilometres long. The curved tail consists of dust grains and meteoroids, while a straight blue tail is made up of charged particles aligned with the flow of the solar wind (Steel, 1995: 277-279). The Earth’s encounter with cometary tails would have resulted in swarms of cometary debris raining down as meteor storms, and megaton class air bursts, like Tunguska, caused by large cometary fragments exploding in the atmosphere. Cometary debris exploding in the atmosphere or impacting into oceans could provide the dry fog or dust-veil associated with early historical records. A comet entering the Earth's magnetosphere would also trigger massive auroral displays with charged particles spiraling down magnetic lines of force. The comet would appear like a second sun, dominating the sky for months, rotating like a wheel with legs (swastika), spitting out white curved fountains, that were eventually swept back into an enormous tail that extended from horizon to horizon (Baillie, 2002).

In comparison to large volcanic eruptions, or the rare meteorite impacts that occur with little warning, comets can be seen months in advance, giving the observant humans clear signs of impending disasters. Thus the Chinese abandoned their capital in AD 534, but did not escape the terrible famines of 536-538. Indeed, many of the myths and recorded histories concerning such heavenly events, bear striking similarities with each other as recorded by the ancient scribes (Baillie 1999: 207).

Quoting the Celtic myths around the terrible events of 536-545 AD, Baillie concludes that a close encounter with a comet satisfies all the details recorded in the myths. Thus a comet can "come up in the west, it can be as bright as the sun, it can be red from the evening to morning, it can have a long mane of hair, it can appear to have three layers of hair, it can give rise to an auroral display, it can spin and look like a swastika, it can give rise to terrible showers of hail-stones, it can deliver terrible blows (if large bits of it impact the atmosphere), it can cause the Sun and Moon to go dim " (as its dust fills the Earth's atmosphere) (Baillie 2002). Such phenomena would have been most awe-inspiring and frightening to our ancestors, and any meteorites recovered from the ground could have been considered as sacred stones. Baillie recounts many ancient stories where the comet was seen as the Divine Archer — Apollo; as Archangel Michael and the evil dragon; as the Celestial Dragon (T'ien Lung) in Chinese myths; as Lugh — the Celtic comet-deity; and even heralding the death of King Arthur and Merlin, and the onset of the Dark Ages. It is intriguing to note that in the Arthurian legend "the logic also is that he probably is… not dead, but will return." (Baillie 2002).

The likely effects of cometary encounters on agriculture-dependent societies are serious. They probably resulted in severe winters and no summers for several years, with the ensuing famines, pestilence, and the onset of a veritable "dark age". Such events could have affected, or brought about the demise of kings, Egyptian Pharaohs, and Chinese Emperors. Thus the climatic events of 2354 to 2345 BC may be linked to the end of the Old Kingdom in Egypt. The 1628 to 1623 BC events may be linked to the end of the Chinese Xia dynasty, the catastrophe recorded in the Irish Annals, the Biblical plagues of Egypt and the Exodus. Similar historical catastrophes have been recorded for the 1159 to 1141 BC event, strongly recorded in Irish swamp oaks, and in acid layers in both the Antarctic and Greenland ice cores. This marked the end of the Shang dynasty, and coincided with famines at the end of King David's reign, as well as widespread destructions and burnings around the eastern Mediterranean.

The worldwide significance of the 2354-2345BC events was highlighted in the Second SIS Cambridge Conference held in July 1997 called "Natural Catastrophes During Bronze Age Civilizations — Archaeological, geological, astronomical & cultural perspectives". In the conference proceedings, Benny Peiser (Abstract O-7) showed that his survey of some 500 excavation reports and research papers indicated a pattern of abrupt glacial, eustatic, lacustrine, fluvial, pedological and geomorphic changes at around 4250 +/- 250 cal BP in many areas around the world. In addition, the majority of sites and cities of the first urban civilizations in Asia, Africa and Europe appear to have collapsed at around the same time. Most sites in Greece (~260), Iberia (~70), Mesopotamia (~30), the Indian subcontinent (~230), China (~20), Persia (~50) and other areas, show signs of natural calamities and/or rapid abandonment. Peiser concludes that close cometary encounters provide the best explanation for such global catastrophic effects, and associated ecological and social disasters.

To conclude, it is clear that the progress of civilization has been intermittent and that humans have had to face many hazardous natural events that have celestial origins: from the periodic pulsations of our Sun's magnetic activity, to the oscillations and precession of our orbit, and to the millennial close encounters with large comets. All have severely affected our human lives and well-being. We should avoid believing that the present-day celestial peace is set to continue. Rather, we have become aware of just how precarious and precious is our continuing existence on this planet.

Confirmed NEO impact sites by continent

Impact sites in Europe (large)

Impact sites in Asia (large)

Impact sites in North America (large)

Impact sites in South America (large)

Impact sites in Africa (large)

Impact sites in Australia (large)

New evidence of more recent strikes

At the southern end of Madagascar lie four enormous wedge-shaped sediment deposits, called chevrons, that are composed of material from the ocean floor. Each covers twice the area of Manhattan with sediment as deep as the Chrysler Building is high.

On close inspection, the chevron deposits contain deep ocean microfossils that are fused with a medley of metals typically formed by cosmic impacts. And all of them point in the same direction — toward the middle of the Indian Ocean where a newly discovered crater, 18 miles in diameter, lies 12,500 feet below the surface.

The explanation is obvious to some scientists. A large asteroid or comet, the kind that could kill a quarter of the world’s population, smashed into the Indian Ocean 4,800 years ago, producing a tsunami at least 600 feet high, about 13 times as big as the one that inundated Indonesia in December 2004. The wave carried the huge deposits of sediment to land.

About 900 miles southeast from the Madagascar chevrons, in deep ocean, is Burckle crater, which Dr. Abbott discovered last year. Although its sediments have not been directly sampled, cores from the area contain high levels of nickel and magnetic components associated with impact ejecta. Burckle crater has not been dated, but Dr. Abbott estimates that it is 4,500 to 5,000 years old.

But they might have more trouble believing one of the scientists, Bruce Masse, an environmental archaeologist at the Los Alamos National Laboratory in New Mexico. He thinks he can say precisely when the comet fell: on the morning of May 10, 2807 B.C.

Dr. Masse analyzed 175 flood myths from around the world, and tried to relate them to known and accurately dated natural events like solar eclipses and volcanic eruptions. Among other evidence, he said, 14 flood myths specifically mention a full solar eclipse, which could have been the one that occurred in May 2807 B.C.

Half the myths talk of a torrential downpour, Dr. Masse said. A third talk of a tsunami. Worldwide they describe hurricane force winds and darkness during the storm. All of these could come from a mega-tsunami.

Most astronomers doubt that any large comets or asteroids have crashed into the Earth in the last 10,000 years. But the self-described "band of misfits" that make up the two-year-old Holocene Impact Working Group say that astronomers simply have not known how or where to look for evidence of such impacts along the world’s shorelines and in the deep ocean.

Scientists in the working group say the evidence for such impacts during the last 10,000 years, known as the Holocene epoch, is strong enough to overturn current estimates of how often the Earth suffers a violent impact on the order of a 10-megaton explosion.

Instead of once in 500,000 to one million years, as astronomers now calculate, catastrophic impacts could happen every 1,000 years.

More about this here.

Ad hoc group called the Holocene Impact Working Group (HIWG) is a consortium of researchers and research groups from several countries that was created in early 2005 as follow-up the ICSU-sponsored Workshop on Comets/Asteroid Hazard held in the Canary Islands in December of 2004. The group includes the researchers and research teams from different field of geoscience who believe that Holocene impacts were more frequent in the recent past than the accepted view and that these impacts have played a significant role in past environmental change and biological and cultural/cognitive evolution. Evidence already collected by the group suggests that the large impacts on the Earth by comets and asteroids have taken place more recently and with greater frequency that presently argued by most NEO planetary scientists. The hypothesized oceanic/glacial impacts that are currently under study include the large comet impact over the Canadian ice shield some 13,000 years ago that triggered the beginning of the Younger Dryas climatic ordeal at 12,900 BP, the Burckle-Madagascar impact at round 4800-5000 BP, that may be associated with the Great (Noah's) Flood and the boundary change from middle to late Holocene around 4800 BP, the Gulf of Carpentaria impacts that are associated with "years without summers" climatic event 535-545 AD, and Mahuika crater just south of New Zealand that may be related to the beginning of the Little Ice Age at around 1450 AD. The focus of the current group activity is further search for physical, anthropological and archeological evidence in support of these and other impact events.

Map of confirmed (red), perspective for verification (magenta) and proposed for further study (blue) impact structures on the Earth. Size of circles is proportional to the crater diameter. Altogether almost 800 structures are shown. (large)
Source — Expert Database on the Earth Impact Structures (EDEIS), Tsunami Laboratory, ICMMG SD RAS, Novosibirsk, 2006

NEOs coming our way

There is a surprising amount. Check the Minor Planet Center for:

Or the Near Earth Object Program:

NEOs are given a risk on the Torino Scale and an automatic risk table is generated here.

NASA, also, gives each asteroid a "condition code" from zero to nine of how certain it is about its predicted orbital path. A zero means there is "good certainty" about it, while nine means it is highly uncertain, with numbers in between on a sliding scale.

Recent encounters

Mar 2002 — In March, another asteroid, 2002 EM7, passed within 463,000km. This asteroid also was not found until after its flyby of Earth.

14 Jun 2002 — Near Earth Asteroid (NEA) 2002 MN passed the Earth at a distance of only 120,000km, one of the closest asteroid fly-bys on record. Based on its brightness, 2002 MN has a nominal diameter of about 100m, large enough to penetrate through the atmosphere to the surface if it struck the Earth. It flew past the Earth on 14 June, but it remained unnoticed by astronomers until the 17th.

Mar 2004 — The closest recent flyby listed by the Minor Planet Centre is 2004 FU162, a small asteroid about 6m (20ft) across which came within about 6,500km (4,000 miles) of our planet in March 2004.

18 Aug 2002 — The space rock, 800 metres (half a mile) across and designated 2002 NY40, made its closest approach to the Earth on Sunday 18th before heading off in the direction of the Sun. The nearest the asteroid came was within 530,000 kilometres (330,000 miles) of the Earth — slightly further away than the Moon.

3 Jul 2006 — A 600m-wide (2,000ft) asteroid whizzed past the Earth on Monday, 3rd under the close scrutiny of astronomers. The mountain-sized object had been classed as a "potentially hazardous asteroid", but scientists said it posed no danger to Earth. The asteroid 2004 XP14, as it is has been designated, was visible through good amateur telescopes. Its closest approach to Earth, above the west coast of North America, occurred at 0444 GMT. At this time calculations suggested it was about 432,709km (268,873 miles) from the Earth, only 1.1 times the planet's distance from the Moon.

29 Jan 2008 — An asteroid some 250m (820ft) across has swept past the Earth.The asteroid — which carries the rather dull designation 2007 TU24 — passed by at a distance of 538,000km (334,000 miles), just outside the Moon's orbit. The moment of closest approach for 2007 TU24 was 0833 GMT, Tuesday, 29th.

7 Oct 2008 — 2008 TC3: Stunned astronomers watched a car-sized asteroid explode into a brilliant meteor shower as it crashed into Earth's atmosphere with force of one or two kilotonnes of TNT. They then went to a Sudanese desert to pick up the pieces of the asteroid, for which they had only 13 hours warning of impact.

2 Mar 2009: An asteroid which may be as big as a 10-storey building has passed close by the Earth, astronomers say. The object, known as 2009 DD45, thought to be 21-47m (68-152ft) across, raced by our planet at 1344 GMT on Monday 2 Mar 2009. The gap was just 72,000km (44,750 miles); a fifth of the distance between our planet and the Moon. The rock is of a similar size to the object that many scientists say exploded over Siberia in 1908 with the force of 1,000 atomic bombs. The object was first reported on Saturday (3 days prior) by the Siding Spring Survey, a near-Earth object search programme in Australia.

6 Nov 2009 — On Friday 6th at 2132 UT asteroid 2009 VA barely missed Earth when it flew just 14,000 km above the planet’s surface. For comparison, Earth’s diameter is 12,756.1 km. That near miss was well inside the “Clarke Belt” of geosynchronous satellites.(35,786 km/22,236 mi). If it had hit, the 6-meter wide space rock would have disintegrated in the atmosphere as a spectacular fireball, causing no significant damage to the ground. But the fact that there was so little warning is troubling. 2009 VA was discovered just 15 hours before closest approach.

13 Jan 2010 — An asteroid, called 2010 AL30, 30 to 50 feet across will pass by the Earth at just more than one-third the distance between the Earth and the moon on Wednesday 12th.

4 Feb 2011 — Sky monitors did spot one small asteroid, called 2011 CQ1, less than a day before it buzzed Earth at the smallest distance ever recorded. The meter-size rock flew over the Pacific at an altitude of about 5,500 kilometres — about one-seventieth the distance between Earth and the moon and well below the orbit of some high-flying satellites.

27 Jun 2011 — The discovery of 2011 MD on Wednesday 23rd goes to show that we need to get better at identifying potential asteroid threats. The moment of closest approach will occur on Monday, June 27 at 13:00 EDT somewhere over the South Atlantic Ocean. The asteroid is 20m wide and will pass at a close distance of only 12,000 kilometres (7,500 miles), 32 times closer than the moon, and closer than geosynchronous satellites.

8 Nov 2011 — An asteroid that is 400m (1,300ft) wide will pass by the Earth on Tuesday, closer to it even than the Moon. It poses no danger to the Earth and it will be invisible to the naked eye. Asteroid 2005 YU55's closest approach, at a distance of 325,000km (202,000mi), will be at 23:28 GMT. It is the closest the asteroid has been in 200 years. It is also the largest space rock fly-by the Earth has seen since 1976; the next visit by such a large asteroid will be in 2028. The aircraft-carrier-sized asteroid is incredibly darkly coloured in visible wavelengths and nearly spherical, lazily spinning about once every 20 hours as it races through our neighbourhood of the Solar System. It will trace a path across the whole sky through to Thursday. "This is the closest approach by an asteroid that large that we've ever known about in advance," said Lance Benner of Nasa's Jet Propulsion Laboratory.

15 Feb 2013 (0320hrs) — Russian Meteor. The Russian meteor explosion over the city of Chelyabinsk, on Friday, injured more than 1,500 people and blew out windows across the region (around 7,000 buildings) in a massive blast captured on cameras by frightened witnesses. Friday afternoon, NASA scientists estimated the meteor was space rock about 50 feet (15 meters) and sparked a blast equivalent of a 300 kiloton explosion. The energy estimate was later increased to 470 kilotons. But late Friday, NASA revised its estimates on the size and power of the devastating meteor explosion. The meteor's size is now thought to be slightly larger — about 20m wide — with the power of the blast estimate of about 500 kilotons, 30 kilotons higher than before, NASA officials said in a statement. The meteor entered Earth's atmosphere and blew apart over Chelyabinsk at 10:20 p.m. EST on Feb. 14 (03:20:26 GMT on Feb. 15). The meteor briefly outshined the sun during the event.

15 Feb 2013 (1924hrs) — Asteroid 2012 DA14. The 150 foot wide (45 meters) near Earth asteroid 2012 DA14 cruised within 17,200 miles (27,000 kilometres) of Earth at 2:24 p.m. EST (1924 GMT) today, coming closer than many communications satellites circling our planet. Traveling a breakneck 28,100 km/hr (that’s nearly five miles a second!), the flyby marked the closest approach by such a large asteroid that astronomers have ever known about in advance. (2012 DA14 itself was just discovered in February 2012.

07 Sep 2014 (1818hrs) — Asteroid 2014 RC. A small asteroid about the size of a house is passing Earth, US space agency Nasa says. At its closest point, the asteroid 2014 RC passed over New Zealand at 18:18 GMT on Sunday. It is about 18m (60ft) wide. Nasa says it is about 40,000km (25,000 miles) away, and posed no danger to Earth. However, a meteorite that landed near the Nicaraguan capital Managua on Sunday could have come from the asteroid, experts there said. The object caused an explosion and earth tremor, leaving a crater 12m (39ft) across and 5m deep near the city's airport. Nicaraguan volcanologist Humberto Garcia said: "It could have come off that asteroid because it is normal for that to occur. We have to study it more because it could be ice or rock." The asteroid that flew past Earth was first discovered on 31 August and, at its closest approach, was about one-tenth of the distance from the centre of Earth to the Moon, Nasa said in a statement. It is expected to orbit near Earth again in the future.

23 Sep 2015 — Asteroid. A 57-metre rock had a condition code of seven — but its estimated 18.5m mile flyby and relative small size gave it much room for manoeuvre.

24 Sep 2015 — Asteroid 2012 TT5. The most unsettling pass was a monster 270-metre space rock called 2012 TT5. NASA's Asteroid Watch Twitter account tweeted: "As was previously known & expected, asteroid 2012 TT5 safely passed Earth at (9:40 am BST) by 5 million miles." The cruise-ship-sized rock 2012 TT5 has a condition code of six, meaning there was quite high uncertainty about its position.

27 Sep 2015 — Asteriod 39m long.

29 Sep 2015 — Asteroids. Two asteroids whistled past
1. One of up to 190 metres long — the length of eight train carriages — and
2. Another cruise ship-sized space rock of about 280 metres flew by at a relatively safe 14.7million miles.

30 Sep 2015 — Asteroid 2015 SZ2. Most asteroid passes classed as "near-Earth" by Nasa fly by at millions of miles away, but the space rock 2015 SZ2 (31m long) is estimated to be on an orbit of just 309,000 miles — only 1.3 times the distrance from Earth to the Moon (238,000 miles).