Asteroid Impact: How Hazardous It Is?

Asteroid suggested as one of the major resources for mining by space colonization advocates. But remember these heavenly pebble may be hazardous and may wipe us out.

Asteroids are much more complicated bodies than purely large-scale replicas of typical pebbles. But even when speaking of simple pebbles, we should be careful. Pebbles are monolithic pieces of rock. Rocks are assemblages of minerals. Minerals are solid-state chemical com- pounds. If we tried to tell the story of any apparently insignificant pebble we may  find during a walk, we would need a long time. This would be a tale of heating and cooling, of pressure and erosion, of endless displacement, and water and wind. But we do not know any pebble grown in our Earth, that would be capable to tell us the tale of the formation of the Sun and of the planets from the original proto-planetary disk. In the most favorable cases, if we were lucky enough to choose as our pebble some rare zircon, we might be told the history of our newly-born planet up to 4.4 Gyrs ago . But if we want to be told the story up to a more distant past, when the original dust started to settle down and accrete into planetesimals, no rock collected on the Earth can help us. It is now widely recognized that our planet melted completely at the beginning of its existence. All this melted material leading to differentiation took place, with the heaviest elements falling toward the centre, and lighter ma- terial tending to float on the surface. Cooling down and solidification came after this. All the information that the original rocks could have about what was before, was lost at the melting epoch.

Since a long time we know that some small samples of rock that fall from the sky, have been witnesses of the processes that came before the melting of the Earth’s material. These samples are included in some meteorites, objects that belong to the general population of minor bodies of our Solar System. Radiometric dating of these samples tell us the age of our Solar System: 4.66 billions of years. These small celestial bodies have survived, apparently not modified by any major physical evolution, since that time. And we know that most meteorites came originally from the asteroid main belt. Comets are also thought to be primitive bodies, but their structures are fluffy, and hardly survive the passage through the terrestrial atmosphere in the case of an impact with our planet. Asteroids are more compact.

All this might look as an interesting, but now well-established body of knowledge. But asteroids can still amaze us: On October 7, 2008, a small asteroid orbiting along an impact path with the Earth was discovered just a few hours before hitting our planet. It was named 2008 TC₃. The reconstructed path, complemented by a few observations of the fireball produced by the impact with the atmosphere, led to identify the site of fall of the meteorite, in Sudan. The recovered meteorite was made of fragile, carbon-rich material, and its original parent asteroid was classified as a member of the so-called F taxonomic class. It was the first time that pieces of an object previously detected in the open space were later recollected on the ground after crossing the atmosphere.

Asteroids belonging to the F class are relatively rare and present some puzzling properties, which suggest some link with the comets. Extinct comets are known to possibly exist among the asteroid population, particularly among the asteroids that are decoupled from the main belt, and orbit in the region of the terrestrial planets. Conversely, another recent discovery has been that of some so-called main belt comets, bodies which were originally classified as normal asteroids in the outer regions of the main belt, around 3.3 Astronomical Units from the Sun, but have been later found to exhibit some cometary activity. We are recognizing these years that the boundaries between different classes of minor bodies are often quite fuzzy.

The above considerations should urge us to look at pebbles with more respect, mainly those which came from the space. But, as we will see below, asteroids are much more interesting than purely being samples of primitive material of our Solar System.

Plot of proper eccentricty versus proper semi-major axis for the main belt asteroid population. Proper elements computed by A. Milani and Z. Knezevic, publicly available at the web site http://hamilton.dm.unipi.it/astdys/. The plot clearly shows the presence of the most important mean-motion resonances with Jupiter (empty strips in semi-major axis), as well as some evident clusters of objects, the so-called dynamical families

Spectroscopic observations show that asteroids exhibit a variety of prop- erties, and belong to a number of different taxonomic classes. Different classes are thought to correspond to differences in mineralogical composi- tion of the surfaces. The relative abundance of objects belonging to different taxonomic classes varies as a function of heliocentric distance, and this is an important evidence of the existence of a general gradient in composi- tion of the original proto-planetary disk. This general trend, however, is not as sharp as one might expect, and is partly hidden by a con- siderable amount of mixing. One of the most common taxonomic classes, which is more abundant in the outer belt, is believed to correspond to bodies having the same composition of the oldest and most primitive meteorites, the carbonaceous chondrites, which date back to the epoch of early forma- tion of our Solar System. However, some apparent paradoxes are evident as one looks at the available data.

One of the major problems is the great difference between the properties of the big objects (1) Ceres and (4) Vesta. (1) Ceres is now classified as a dwarf planet, has a diameter of about 1,000 km, and exhibits a reflectance spectrum that is diagnostic of a primitive, and possibly hydrated, composition. As oppo- site, (4) Vesta, which has a diameter of about 500 km, exhibit evidence of a basaltic crust, which is interpreted as evidence of full melting and dif- ferentiation occurred in ancient times. The internal heating that could produce the melting is thought to have been provided by the presence in the interior of short-lived radiogenic nuclei like Al₂₆.

The problem here is that, if it is admissible to accept the idea that Vesta had a thermal history in some way similar to that of the terrestrial planets, it is much more difficult to explain why and how Ceres did not experience the same kind of evolution. Being twice as large as Vesta, Ceres is expected to have accreted even earlier, and if the composition of the solid material in the asteroid belt at that epoch had been the same, it should have incorporated an amount of Al₂₆ more than sufficient to fully melt it. Apparently, this did not happen, and Ceres’s composition is that of a primitive body that never experienced strong heating episodes. Today, the difference in heliocentric distance between Vesta and Ceres is about 0.4 AU (Vesta being closer to the Sun). The general gradient in composition of the asteroid population as a function of heliocentric distance does not suggest that the original com- position of Vesta and Ceres could be so different. How to explain this apparent paradox? We hope to obtain some critical body of evidence from the results of the Dawn space probe, which is on its way to reach Vesta, and later Ceres, in the next years.

Gaia will be the most powerful tool ever developed for remote sensing, in terms of astrometric accuracy. The measurement of the tiny de°ections caused by mutual close encounters involving the largest main belt asteroids, is expected to produce reliable mass measurements for about 100 objects. Coupled with size measurements, directly obtained by Gaia signals, average densities will be also obtained for the same asteroids, which will belong to a variety of taxonomic classes. This will be a first, fundamental step to get essential information about the overall composition and plausible internal structure of a large variety of main belt asteroids. Obtaining reliable clues concerning the internal structure is currently the Holy Grail of asteroid science. Not only this is an essential information to understand these elusive bodies, but it is also a necessary pre-requisite to develop credible systems of defense and mitigation of the damages from impacts of interplanetary objects with the Earth.

The topic of catastrophic impacts with celestial bodies is in some respect a dangerous one from the point of view of being able to provide correct scientific information to non-specialists. The danger is to feed non-rationale and unmotivated beliefs, and to provide information that can be misunderstood and used to support a general demand of catastrophism that has nothing to do with science.

The particular characteristic of the asteroid hazard is that of being related to events that are extremely rare, but that may produce extraordinarily heavy consequences when they occur. Since the concept of risk is related to both the probability of the occurrence of a certain phenomenon, as well as to the likely consequences of such event, the asteroid hazard can reach some level of risk that is comparable with that of other natural events that are much more common, including disasters such as floods and earthquakes.

One key-point when interpreting Figure  is the uncertainty in our knowledge of the size distribution of NEAs, due to the fact that the population is fully known only down to a certain completeness limit in magnitude (size). At smaller sizes, an extrapolation is needed, and different models are possible. A general consensus is that the current NEA population should include about 1,000 objects larger than 1 km. This size is conventionally considered to mark the threshold between global and local catastrophes in terms of corresponding effects on the biosphere. At smaller sizes the impact energy decreases sensibly, but the number of possible impactors increases much, as shown by the predictions of different models plotted in Figure.

In any case, it is certain that the Earth has been hit many times by wandering asteroids during its history. Impact craters tend to disappear over long timescales due to erosion and tectonic phenomena, yet many craters are still clearly visible on the Earth surface, as shown in some Figures in what follows. It is believed that at least in some cases the impacts may have been highly energetic, with profound consequences on the terrestrial biosphere.

To make a well known example, the complete eradication of the dinosaurs 65 million years ago is believed by many scientists to have been a consequence of the impact of an asteroid was about 6-miles in size (10-km). The explosion was probably the equivalent of about 200 million megatons of dynamite (or hundreds of nuclear bombs). However, not just the blast, but the prolonged effects on the atmosphere and climate, all contributed to the mass extinctions which followed. The hypothesis that an asteroid impact led to the extinction of the dinosaurs is not universally accepted, a competing theory being that of an episode of paroxysmic volcanic activity in the Deccan region in India. However, the point is that the impact with a 10-km asteroid certainly delivers an amount of energy that is more than sufficient to have catastrophic consequences on the biosphere, and events of this kind have certainly occurred during the Earth’s history, independent on the exact event which caused the dinosaurs’ extinction.

Some scientists even believe that an asteroid impact may have split apart the giant super-continent, Pangea, 250 mya, whereas others have proposed that 65 mya a 25-mile-wide (40-kilometer-wide) asteroid slammed into what today is India, causing it to crack and shatter, propelling part of the subcontinent into Africa and leaving the Seychelles islands behind.

In much modern times, in 1908, a much smaller object (asteroid or comet) that was perhaps 20 – 50 meters across, exploded in the atmosphere above Tunguska, Siberia, at a height of 8.5 kilometers, with a force equal to an atomic bomb (10 megatons of TNT), and destroyed over 60 million trees. If the object had struck 5 hours earlier, it would have exploded above St. Petersburg, and possibly killing hundreds of thousands of innocent people.

In March of 1989, the 300 meter (1,000-foot) diameter Apollo asteroid 4581 Asclepius (1989 FC) missed the Earth by just six hours. If it had struck our planet, it would have released as much destructive power as 1000 nuclear bombs. Finally, many small objects have been found to pass very close to the Earth, at distances much closer than the moon.

We can conclude that not only purely scientific reasons, and the beauty of some physical and dynamical effects acting on the asteroids, but also some more pragmatic considerations should leave little doubt that the study of asteroids is of vital importance if we are to avoid in the future another mass extinction. But this is of course another story…

[ref: Cosmology Magazine]