Encounters with Extraterrestrial Objects

The Earth constantly passes through a widely dispersed (but in aggregate quite massive) amount of universal debris (McSween 1999). Common sizes of these mete-oroids range from microscopic particles to bodies with diameters <10 m. As a result, the planet is constantly showered with microscopic dust, and even the bits with diameter 1 mm, large enough to leave behind a light path as they self-destruct in the atmosphere (meteors), come every 30 s. This constant infall (about 5 t per day) poses virtually no risk to the evolution of life or to the functioning of modern civilization because these objects disintegrate during their passage through the

Closeups of large asteroids. The Earth's collision with asteroids of this size would almost certainly destroy civilization. Left, composite image of Ida (~52 km long); right, Gaspra (illuminated portion ~18 km long). Galileo spacecraft images (1993 and 1991). From NASA (2006).

Closeups of large asteroids. The Earth's collision with asteroids of this size would almost certainly destroy civilization. Left, composite image of Ida (~52 km long); right, Gaspra (illuminated portion ~18 km long). Galileo spacecraft images (1993 and 1991). From NASA (2006).

atmosphere, and only dust or small fragments reach the ground. But the planet's orbit is also repeatedly crossed by much larger objects, above all by stony asteroids with diameters >10 m and as large as tens of kilometers across (fig. 2.4), and by comets.

The risk of encounters with extraterrestrial bodies was first recognized during the 1940s. It began to receive greater attention during the 1980s, but until the early 1990s no systematic effort was made to comprehensively identify such objects, assess the frequencies of their encounters with the Earth, and devise possible defensive measures. Known Earth-crossing asteroids numbered 236 at the beginning of 1992 (compared to 20 in 1900), the year in which NASA proposed the Spaceguard Survey (Morrison 1992), whose goal is to identify 90% of all near-Earth asteroids (NEAs) by the year 2008. NASA funded and coordinated monitoring began in 1995, and ten years later the U.S. House of Representatives approved the Near-Earth Object Survey Act, which directs NASA to expand its detection and tracking program. These actions have been accompanied by publications assessing the threat (Chapman and Morrison 1994; Gehrels 1994; J. S. Lewis 1995; 2000; Atkinson, Tickell, and Williams 2000).

The progress in discovering new near-Earth objects (NEOs) has been rapid (NASA 2007). By the end of 1995 the total number of known objects was 386; by

Cumulative discoveries of near-Earth asteroids, 1980-2007. From NASA (2007).

Cumulative discoveries of near-Earth asteroids, 1980-2007. From NASA (2007).

the end of 2000, 1,254; and by June 2007, more than 4,100, of which nearly 880 were bodies with diameters >1 km (fig. 2.5). As the findings accumulate, there has been an expected decline in annual discoveries of NEAs with diameters >1 km, and the search has been asymptotically approaching the total number of such NEAs. Consequently, we are now much better able to assess the size-dependent impact frequencies and to quantify the probabilities of encounters whose consequences range from local damage through regional devastation to a global catastrophe.

There are perhaps as many as 109 asteroids orbiting the sun in a broad and constantly replenished belt between Mars and Jupiter as well as a similar number of comets moving in more distant orbits within the Opik-Oort cloud beyond Pluto. Gravitational attraction of nearby planets constantly displaces a small portion of these bodies (remnant debris from the time of the solar system's formation 4.6 Ga ago) into elliptical orbits that move them toward the inner solar system and into the vicinity of the Earth. Several million near-Earth objects cross the Earth's orbit, and at least 1,000 of them have diameters >1 km. Because of their high impact

What Extraterrestrial Disaster

Oblique aerial view of Meteor Crater in Arizona. USGS photo by David J. Roddy.

Oblique aerial view of Meteor Crater in Arizona. USGS photo by David J. Roddy.

velocities, even small NEOs have kinetic energy equivalent to that of a small nuclear bomb; larger bodies can bring regional devastation, and the largest can cause a global catastrophe.

Craters provide the most obvious evidence of major past impacts (fig. 2.6) (Grieve 1987; Pilkington and Grieve 1992). More than 150 of these structures have been identified so far, but it must be kept in mind that most impacts have been lost in the ocean, and the evidence of most of the older terrestrial impacts has been erased by tectonic and geomorphic processes. The largest known crater, the now buried Chicxulub structure in Yucatan with diameter 300 km (Sharpton et al. 1993), was created 65 Ma ago by an asteroid whose impact has been credited with the great extinction at the Cretaceous-Tertiary (K-T) boundary (Alvarez et al. 1980). The most recent impact of an NEO with diameter >1 km took place less than 1 million years ago in Kazakhstan (NRCanada 2007). Asteroids and short-period comets make up about 90% of NEOs; the remaining risk is posed by intermediate and long-period comets that cross the planet's orbit only once in several decades. The frequency of NEO impacts declines exponentially with the increasing size of the impacting objects, and their kinetic energy determines the extent of damage (fig. 2.7).

log impact energy (Mt TNT)

log impact energy (Mt TNT)

3 lo

diameter (km)

i i 111 mi riir 10

Size, impact frequency, and impact energy of near-Earth asteroids. All four axes are logarithmic; the band indicates the range of uncertainty regarding the numbers and impact intervals of objects with diameter <1 km. Based on NASA (2003), Bland and Artemieva (2003), and Chapman (2004).

diameter (km)

Size, impact frequency, and impact energy of near-Earth asteroids. All four axes are logarithmic; the band indicates the range of uncertainty regarding the numbers and impact intervals of objects with diameter <1 km. Based on NASA (2003), Bland and Artemieva (2003), and Chapman (2004).

Roughly once a year the Earth encounters an extraterrestrial body whose size is 5 m across and whose air burst releases nearly 21 TJ, equivalent to 5 kt TNT (explosive power of 11 TNT is equal to 4.18 GJ). This makes it about one-third as powerful as the Hiroshima bomb; there is no definite number for the explosive yield of that bomb, but the most authoritative source (Malik 1985) puts it at 15 (±3) kt TNT. Only if this body's center of disintegration were right above the U.S. Capitol during the President's State of the Union speech would the effect be felt globally. But the probability of such an encounter is vanishingly small, at least 8 OM smaller than that of a similar object's disintegrating at any time above any densely populated area.

Stony objects with diameters 10 m are intercepted by the Earth's atmosphere every decade, and their entry (at speeds ~20 km/s) discharges energy equivalent to about 100 kt TNT, roughly seven times the energy released by the Hiroshima bomb. As these bolides disintegrate during atmospheric deceleration, a fireball and a shock wave are the only phenomena felt on the ground within a radius of 102 km around their entry point. Brown et al. (2002) used data from a satellite designed to detect nuclear explosions in order to identify light records of bolide detonations (objects 1-10 m in size) in the atmosphere. From these observations they concluded that on average the Earth is struck by an object with diameter 50 m (equivalent to 10 Mt TNT) every 1,000 years.

The probability of such an impact is thus about 5% (uncertainty band of about 3%-12%) during the next 50 years, and its effects would be similar to those caused by the famous Tunguska meteor of June 30, 1908. Atmospheric disintegration of that stony object released energy equivalent to 12-20 Mt TNT, produced a shock wave that flattened trees over an area of about 2150 km2 but killed nobody in that unpopulated region of central Siberia (Dolgov 1984). If a similar object were to disintegrate over a densely populated urban area, it could cause great damage. Its explosion about 15 km above the ground would release energy equivalent to at least 800 Hiroshima bombs and result in 105 casualties and S1011 of material damage. But the chances of such an event are roughly 2 OM smaller than the probability of hitting an unpopulated or thinly inhabited region because densely populated areas cover only about 1% of the planet's surface.

As was clearly demonstrated by the contrast of casualties in Hiroshima and Nagasaki, the actual destruction would depend on the physical configuration of the affected area. Hiroshima, with a bowl-like setting that acted as a natural concentrator of the blast, had about 40% more fatalities and more destruction from a 15-kt blast than did Nagasaki from a 21-kt explosion (CCM 1981). Another complicating factor is that a Tunguska-like blast may not be a point-source event (similar to a nuclear bomb) but rather a plume-forming event (similar to a line of explosive charges) and hence could be caused by much less powerful objects (NASA 2003).

Asteroids with diameters >100 m reach the atmosphere once every 2,000-3,000 years, and their energy (equivalent to >60 Mt TNT) is as large as the yield of the largest tested thermonuclear devices. Hills and Goda (1993) calculated that stony objects with diameters up to ~150 m will release most of their energy in the atmosphere and will not hit the surface and create impact craters (however, heavier metallic objects of that diameter might penetrate). Stony objects with diameter

>150 m hit the Earth once every 5,000 years, and their terrestrial impacts create only local effects, small craters with adjacent areas covered by ejecta. Using as a reference point a stony body that produces only air blast to 220 m diameter, Bland and Artemieva (2003) estimated that bodies with a larger diameter would hit the Earth once in 170,000 years. There is broad consensus that the threshold size for an impact producing a global effect is a body with diameter at least 1 km and possibly closer to 2 km.

Toon et al. (1997) concluded that only bodies with kinetic energies equivalent to at least 100 Gt TNT (diameters >1.8 km) would cause global damage beyond the historic experience, and objects with diameters between 850 m and 1.4 km (energy equivalents of 10-100 Gt TNT) would cause globally significant atmospheric water vapor injection and ozone loss but would not inject enough submicrometer par-ticulates into the stratosphere to have major, longer-term climatic effects. A 1-km body (density 2.5-3.3 g/cm3, velocity 20-22 km/s) colliding with the Earth would release energy equivalent to about 62-105 Gt TNT, almost 1 OM more than the energy that would have been expended by an all-out thermonuclear war between the two superpowers in 1980 (Sakharov 1983). A 3-km asteroid would liberate energy equivalent to about 2 Tt TNT, possibly enough to terminate modern civilization regardless of where the asteroid hit (fig. 2.8).

The consequences of a collision with a 1-km body would depend greatly on the impact site. Odds are roughly 7 : 3 that the object would hit the ocean and damage the land indirectly by generating tsunamis, but a terrestrial impact would create a crater with diameter 10-15 times the object's size and pose an unprecedented threat to the survival of civilization. Such a collision would vaporize and fragment both the projectile and the impacted area, and enormous masses of dust would reach the stratosphere. While the larger dust fractions would rapidly settle, submicrometer-sized particles would remain in the atmosphere for weeks to months.

Simulations using the global circulation model show that ocean heat storage would prevent a global freeze even if the impact were equivalent to the K-T event (with kinetic energy perhaps as high as 1 Pt TNT) but that surface land temperatures would drop by more than 10°C and still be some 6°C lower a year later (Covey et al. 1994; Toon et al. 1997). In addition, hot ejecta would produce significant amounts of nitrogen oxides, whose presence in the stratosphere would degrade (and in extreme cases, largely destroy) the ozone shield that protects the Earth against UV radiation. A 1-km object would have much less effect because it would not

1 rfi

102 103 diameter of object (m)

Expected fatalities from impacts of near-Earth objects. From Morrison (1992).

102 103 diameter of object (m)

Expected fatalities from impacts of near-Earth objects. From Morrison (1992).

generate enough dust to cause temporary planetwide darkness and shut down photosynthesis.

At least 10 Gt of submicrometer-sized dust would be required to make the minimum amount of light unavailable for photosynthesis (Toon et al. 1997), but using the analogy of a ground-level nuclear explosion—which produces about 25 t of submicrometer-sized dust per kt of yield (Turco et al. 1983)—means that a 1-km body would produce only about 1.5 Gt of fine dust, 4 OM less than a K-T-sized object (25 Tt). Moreover, Pope (2002) questioned the assumptions regarding the fine dust fraction in the ejecta produced by the K-T impact. Pope's calculations, coupled with observations of the deposited coarse fraction, indicated that a minor share was laid down as submicrometer-sized dust and that little debris diffused to high southern latitudes. These conclusions invalidate the original attribution of K-T extinction to the shutdown of photosynthesis by submicrometer-sized dust. Pope calculated that the impact released only 0.1% (and perhaps much less) of the total amount as fine dust (but his conclusions were questioned as unrealistic).

In any case, it is impossible to quantify satisfactorily the actual effect because fine dust would not be the only climate-modifying factor. Soot from massive fires ignited by hot ejecta and sulfate aerosols liberated from impacted rocks could each have as much cooling effect on the atmosphere as the fine dust. However, lingering aerosols would also increase the intercept of the outgoing terrestrial radiation and contribute to tropospheric warming. A rapid reversal of ground temperatures could follow once the debris settled, and water vapor and CO2 injected into the stratosphere (from impacted carbonate rocks) would greatly enhance the natural greenhouse gas effect. With positive feedbacks (higher temperatures enhancing evaporation as well as plant respiration and the release of CO2 from the ocean and soils), this bout of global warming could persist for decades.

The only defensible conclusion is that the impact of a 1-km object would most likely not have consequences resembling the aftermath of a thermonuclear war: a drop in ground temperature severe enough to produce a nuclear winter and a temporary cessation of all photosynthesis (Turco et al. 1991). The overall effect on photosynthesis, biodiversity, agricultural production, and human survival would depend critically on the mass of ejecta and their atmospheric perseverance. Specifics are impossible to enumerate, but extensive forest and grassland destruction by fires, a temporary but substantial reduction of precipitation due to the disrupted global water cycle, sharp declines in food production, and extensive interference in industrial, commercial, and transport activities are all easy to imagine. The impact would not bring an abrupt end to modern civilization, but it could be an enormously costly setback.

Earlier estimates put the number of NEOs with diameter >1 km at about 2,000, but Rabinowitz et al. (2000) used improved detection techniques to conclude that there were only about 1,000 such objects, and Stuart (2001) put the total number of kilometer-sized NEAs at just over 1,200 (he also found them less likely to collide with the Earth than previously assumed). If 1,100 were the actual total, then 80% of them had been discovered by June 2007. Certainly the most notable outcome of this effort is the good news that the likelihood of near-term impacts has been decreasing. On a 10-point Torino scale, measuring the severity of the collision threat (Binzel 2000), 0 indicates no hazard (white zone) with effectively no likelihood of collision, and 1 (normal, green zone) indicates an object whose close path near the Earth poses no unusual danger and which will very likely be reassigned to level 0

after additional observations. Levels 3 and 4 indicate close encounters with 1% or greater chance of collision capable of localized or regional destruction; and significant threats of close encounters causing a global catastrophe begin only with level 6.

As of 2007, only two objects, 2004 VD17 and 2004 MN4, were rated 2 and all other NEAs scored 0 on the Torino scale during the twenty-first century. The first of these objects is about 580 m across; the other is 320 m across, and it became the subject of short-lived concern when initial calculations indicated its collision with the Earth on April 13, 2029. That is not going to happen, but there is still a distant possibility of an encounter with MN4 between 2036 and 2056, and VD17 may come close by 2102 (Yeomans, Chesley, and Choclas 2004). By far the highest known probability of an NEO's colliding with the Earth is nearly a millennium away, on March 16, 2880. Analysis by Giorgini et al. (2002) suggests a very close approach by asteroid 29075, an asymmetrical spheroid with mean diameter 1.1 km that was discovered in 1950 (as 1950 DA), lost from view after 17 days, and rediscovered in 2000 (fig. 2.9). The impact probability was put as high as 0.33%, but because of the unknown direction of the asteroid's spin pole, the range of the actual risk may be closer to 0.

While it is very likely that we have already discovered all existing NEAs with diameter >2 km, we can never be quite sure that we know of every large NEA that

A collision that is not going to happen: the orbits of four planets and asteroid 29075 (1950 DA). Based on NASA (2007).

A collision that is not going to happen: the orbits of four planets and asteroid 29075 (1950 DA). Based on NASA (2007).

is already on an Earth-crossing orbit and we will not be able to identify promptly every new addition to this dynamic collection of extraterrestrial objects. Consequently, assessing the risks of collision will always require assumptions regarding the impact frequency of various-sized objects. The general size frequency distribution of NEOs is now fairly well known (see fig. 2.7), but there are different assumptions about the most likely frequency of impacts; the estimates differ by up to 1 OM. For example, Ward and Asphaug (2000) assume that an object with diameter 400 m hits the Earth once every 10,000 years, and with diameter 1 km, once every 100,000 years. By contrast, Brown et al. (2002) would expect a body with diameter 400 m to hit once every 100,000 years, and with diameter 1 km, once every 2 million years; Chapman (2004) would expect an object with diameter 400 m to hit once every 1 million years; and Jewitt (2000), an object with diameter 400 m, once every 400,000 years.

Another important consideration enters at this point: even impacts of bodies with diameter <1 km could have global consequences if they destroyed a core area of a major nation. For example, an object 500 m across would devastate an area of about 70,000 km2; Tokyo and its surrounding prefectures cover less than half that area (~30,000 km2) and are inhabited by about 30 million people. Alternatively, calculations by Ward and Asphaug (2000) show that if an asteroid with diameter 400 m were to hit a 1-km-deep ocean site at 20 km/s, the maximum amplitude of a tsunami generated by this impact would be more than 200 m at a distunce of 100 km and 20 m at a distance of 1000 km. A near-shore impact off eastern Honshu or in the North Sea between London and Amsterdam would instantly obliterate core regions of the world's two leading economies, Japan and the EU, and unlike with tsunami generated by a distant earthquake, there would not be sufficient time for mass population evacuation.

Naturally, the probability for such a site-specific impact (PS) is a small fraction of that for an unspecified location on the Earth (PE): PS = PE(AE/AS). Assuming that an object with diameter 400 m arrives once every 100,000 years (PE = 1-5) then the probability of its destroying the Tokyo area (AS = 310 m2) would be no more than 6-10 (Ae = 5.114 m2), an annual probability of about 1 in 1.66 billion. Ward and Asphaug (2000) calculated specific probabilities of a 5-m-high tsunami wave's hitting Tokyo and New York at, respectively, 4.2% and 2.1% during the next 1,000 years, or roughly 0.2% and 0.1% during the next 50 years. In contrast, Bland and Artemieva (2003) estimated the frequencies of bolides that would most likely cause hazardous tsunami at only about one-fiftieth of the rate reported by Ward and

Asphaug (2000). Chesley and Ward (2006) calculated the overall long-term casualties that would be caused by impacts of objects with diameters 200-400 m at fewer than 200 deaths per year (or fewer than 10,000 total during the next half century).

The highest risk of collision-related fatalities comes from the land impact of smaller, and hence more common, bodies, with a more than 1% chance that such an impact will kill about 100,000 people during the twenty-first century, whereas somewhat larger objects (diameter 150-600 m) will pose the greatest tsunami hazards (Chapman 2004). By contrast, probabilities of encounters with NEOs with diameter >1 km are orders of magnitude smaller. If the average recurrence interval for a 1-km asteroid were 400,000 years, then the probability of impact during the next 50 years would be 0.0125%; bracketing this by uncertainties of 100,000 years to 2 million years gives a range of 0.0025%-0.05%. The minimum size of an asteroid whose impact would have severe global consequences depends not only on its diameter but also on its density and speed (an asteroid traveling at 30 km/s would have 2.25 times more kinetic energy than an equally massive counterpart moving at 20 km/s) and on the impacted area.

If a large asteroid were to enter the ocean, such an impact would generate tsunamis that would hit even distant shores with high-amplitude waves: the impact's location would determine the extent of global fatalities and economic damage. For example, 2.15 Ma ago the Eltanin asteroid, whose diameter may have been as much as 4 km, entered a deep (about 5-km) spot in the Pacific Ocean off southern Chile without forming a seabed crater and without resulting in a mass extinction (Mader 1998). The resulting tsunami (total energy of 200 EJ) would have completely destroyed the South Pacific islands, but the wave height along the coasts of North America and East Asia would have been less than 15 m.

But even if we were to discover all NEAs and determine that none of them is on a collision course with the Earth, we would still face an inherently much more difficult challenge of identifying cometary impactors. These bodies, made of rocky material and volatile ice, account for no more than 10% of all NEOs, but because they have higher encounter velocities (as much as 60 km/s compared to 15-25 km/s for asteroids) their kinetic energy is much higher, and they have been responsible for some 25% of all craters with diameters >20 km (Brandt and Chapman 2004). Fortunately, these more powerful impacts are rarer than the encounters with similar-sized asteroids. The closest approaches by historic comets missed the Earth by 3.7 lunar distance (LD = 384,000 km) in 1491, 5.9 LD in 1770, and 8.9 LD in 1366;

all other misses were >10 LD (NASA 2006). Consequently, probabilities of the Earth's catastrophic encounter with a comet are likely less than 0.001% during the next 50 years, a chance approaching the level of 1 out of 1 million.

Continue reading here: Volcanic Megaeruptions and Collapses

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