Russian asteroid explosion and past impactors paint a potentially grim future for Earth

March 8, 2013 by Dan Majaess, Universe Today
More than a dozen known impactors created 50 km sized craters (and larger) on Earth. One such example is the Manicouagan crater in Quebec, Canada. The crater is 215 million years old, and exhibits an 85 km diameter (image credit: NASA).

The recent meteor explosion over Chelyabinsk brought to the forefront a topic that has worried astronomers for years, namely that an impactor from space could cause widespread human fatalities. Indeed, the thousand+ injured recently in Russia was a wake-up call. Should humanity be worried about impactors? "Hell yes!" replied astronomer Neil deGrasse Tyson to CNN's F. Zakharia .

The geological and biological records attest to the fact that some impactors have played a major role in altering the evolution of , particularly when the underlying terrestrial material at the impact site contains large amounts of carbonates and sulphates. The dating of certain large (50 km and greater) found on Earth have matched events such as the extinction of the Dinosaurs (Hildebrand 1993, however see also G. Keller's alternative hypothesis). Ironically, one could argue that humanity owes its emergence in part to the impactor that killed the Dinosaurs.

Only rather recently did scientists begin to widely acknowledge that sizable impactors from space strike Earth.

"It was extremely important in that first intellectual step to recognize that, yes, indeed, very large objects do fall out of the sky and make holes in the ground," said Eugene Shoemaker. Shoemaker was a co-discoverer of Shoemaker-Levy 9, which was a fragmented comet that hit Jupiter in 1994 (see video below).

Hildebrand 1993 likewise noted that, "the hypothesis that catastrophic impacts cause mass extinctions has been unpopular with many geologists … some geologists still regard the existence of ~140 known impact craters on the Earth as unproven despite compelling evidence to the contrary."

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Beyond the asteroid that struck Mexico 65 million years ago and helped end the reign of the dinosaurs, there are numerous lesser-known terrestrial impactors that also appear destructive given their size. For example, at least three sizable impactors struck Earth ~35 million years ago, one of which left a 90 km crater in Siberia (Popigai). At least two large impactors occurred near the Jurassic-Cretaceous boundary (Morokweng and Mjolnir), and the latter may have been the catalyst for a tsunami that dwarfed the recent event in Japan (see also the simulation for the tsunami generated by the Chicxulub impactor below).

Glimsdal et al. 2007 note, "it is clear that both the geological consequences and the tsunami of an impact of a large asteroid are orders off magnitude larger than those of even the largest earthquakes recorded."

However, in the CNN interview Neil deGrasse Tyson remarked that we'll presumably identify the larger impactors ahead of time, giving humanity the opportunity to enact a plan to (hopefully) deal with the matter. Yet he added that often we're unable to identify smaller objects in advance, and that is problematic. The meteor that exploded over the Urals a few weeks ago is an example.

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In recent human history the Tunguska event, and the asteroid that recently exploded over Chelyabinsk, are reminders of the havoc that even smaller-sized objects can cause. The Tunguska event is presumed to be a meteor that exploded in 1908 over a remote forested area in Siberia, and was sufficiently powerful to topple millions of trees (see image below). Had the event occurred over a city it may have caused numerous fatalities.

Mark Boslough, a scientist who studied Tunguska noted, "That such a small object can do this kind of destruction suggests that smaller asteroids are something to consider … such collisions are not as improbable as we believed. We should be making more efforts at detecting the smaller ones than we have till now."

Neil deGrasse Tyson hinted that humanity was rather lucky that the recent Russian fireball exploded about 20 miles up in the atmosphere, as its energy content was about 30 times larger than the Hiroshima explosion. It should be noted that the potential negative outcome from smaller impactors increases in concert with an increasing human population.

In 1908 the Tunguska impactor toppled millions of trees in a rather remote part of Siberia (image credit: Kulik). Had the object exploded over a city, the effects may have been catastrophic.

So how often do large bodies strike Earth, and is the next catastrophic impactor eminent? Do such events happen on a periodic basis? Scientists have been debating those questions and no consensus has emerged. Certain researchers advocate that large impactors (leaving craters greater than 35 km) strike Earth with a period of approximately 26-35 million years.

The putative periodicity (i.e., the Shiva hypothesis) is often linked to the Sun's vertical oscillations through the plane of the Milky Way as it revolves around the Galaxy, although that scenario is likewise debated (as is many of the assertions put forth in this article). The Sun's motion through the denser part of the Galactic plane is believed to trigger a comet shower from the Oort Cloud. The Oort Cloud is theorized to be a halo of loosely-bound comets that encompasses the periphery of the Solar System. Essentially, there exists a main belt of asteroids between Mars and Jupiter, a belt of comets and icy bodies located beyond Neptune called the Kuiper belt, and then the Oort Cloud. A lower-mass companion to the Sun was likewise considered as a perturbing source of Oort Cloud comets ("The Nemesis Affair" by D. Raup).

A halo of comets designated the Oort Cloud is theorized to encircle the periphery of the Solar System, and reputedly acts as a reservoir for objects that may become terrestrial impactors (image credit: NASA/JPL).

The aforementioned theory pertains principally to periodic comets showers, however, what mechanism can explain how asteroids exit their otherwise benign orbits in the belt and enter the inner solar system as Earth-crossers? One potential (stochastic) scenario is that asteroids are ejected from the belt via interactions with the planets through orbital resonances. Evidence for that scenario is present in the image below, which shows that regions in the belt coincident with certain resonances are nearly depleted of asteroids. A similar trend is seen in the distribution of icy bodies in the Kuiper belt, where Neptune (rather than say Mars or Jupiter) may be the principal scattering body. Note that even asteroids/comets not initially near a resonance can migrate into one by various means (e.g., the Yarkovsky effect).

Indeed, if an asteroid in the belt were to breakup (e.g., collision) near a resonance, it would send numerous projectiles streaming into the inner solar system. That may help partly explain the potential presence of asteroid showers (e.g., the Boltysh and Chicxulub craters both date to near 65 million years ago). In 2007, a team argued that the asteroid which helped end the reign of the Dinosaurs ago entered an Earth-crossing orbit via resonances. Furthermore, they noted that asteroid 298 Baptistina is a fragment of that Dinosaur exterminator, and it can be viewed in the present orbiting ~2 AU from the Sun. The team's specific assertions are being debated, however perhaps more importantly: the underlying transport mechanism that delivers asteroids from the belt into Earth-crossing orbits appears well-supported by the evidence.

A histogram featuring the number of asteroids as a function of their average distance from the Sun. Regions depleted of asteroids are often coincident with orbital resonances, the latter being a mechanism by which objects in the belt can be scattered into enter Earth-crossing orbits (image credit: Alan Chamberlain, JPL/Caltech).

Thus it appears that the terrestrial impact record may be tied to periodic and random phenomena, and comet/asteroid showers can stem from both. However, reconstructing that terrestrial impact record is rather difficult as Earth is geologically active (by comparison to the present Moon where craters from the past are typically well preserved). Thus smaller and older impactors are undersampled. The impact record is also incomplete since a sizable fraction of impactors strike the ocean. Nevertheless, an estimated frequency curve for terrestrial impacts as deduced by Rampino and Haggerty 1996 is reproduced below. Note that there is considerable uncertainty in such determinations, and the y-axis in the figure highlights the "Typical Impact Interval".

Estimated frequency of impactors as a function of diameter, energy yield, and typical impact interval. Results assume an impact speed of 20 km/s and density of 3 g/cm^3 (image credit: Fig. 2 from Rampino and Haggerty 1996, NASA ADS/Springer).

In sum, as noted by Eugene Shoemaker, large objects do indeed fall out of the sky and cause damage. It is unclear when in the near or distant future humanity will be forced to rise to the challenge and counter an incoming larger impactor, or again deal with the consequences of a smaller impactor that went undetected and caused human injuries (the estimated probabilities aren't reassuring given their uncertainty and what's in jeopardy). Humanity's technological progress and scientific research must continue unabated (and even accelerated), thereby affording us the tools to better tackle the described situation when it arises.

Is discussion of this topic fear mongering and alarmist in nature? The answer should be obvious given the fireball explosion that happened recently over the Ural mountains, the Tunguska event, and past impactors. Given the stakes excessive vigilance is warranted.

Fareed Zakharia's discussion with Neil deGrasse Tyson is below.

The interested reader desiring additional information will find the following pertinent: the Earth Impact Database, Hildebrand 1993, Rampino and Haggerty 1996, Stothers et al. 2006, Glimsdal et al. 2007, Bottke et al. 2007, Jetsu 2011, G. Keller's discussion concerning the end of the Dinosaurs, "T. rex and the Crater of Doom" by W. Alvarez, "The Nemesis Affair" by D. Raup, "Collision Earth! The Threat from Outer Space" by P. Grego. **Note that there is a diverse spectrum of opinions on nearly all the topics discussed here, and our understanding is constantly evolving. There is much research to be done.

Explore further: Crashing comets not likely the cause of Earth's mass extinctions: new research

More information: Dan Majaess is a Canadian astronomer based in Halifax, Nova Scotia. He researches the cosmic distance scale, variable stars, star clusters, and terrestrial mass extinctions linked to asteroid/comet impacts.

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3.3 / 5 (8) Mar 08, 2013
We are already awake and have many pretty 'N-E-A-T' projects to track these killers. yet we have these unfortunate chelyabinsk incident (especially all that time we were tracking a different asteroid!)

(Just reminds me of the film 'Armageddon' and Mr. Trumans explanations when asked why NASA didnt discover the global killer sooner..)
2.1 / 5 (25) Mar 08, 2013
But CO2 is more dangerous?
3.8 / 5 (16) Mar 08, 2013
What the hell is wrong with you CO2 deniers? The existence of asteroids and comets doesn't change anything about climate science.

CO2 effects on climate can be reduced by burning less fossil fuel. Do the math on what it costs to use other energy sources (and please include the lowering of cost that would come from widespread adoption and massive subsidies that are currently enjoyed by the fossil fuel interests). Do the math on what it would cost to correct for the effects of CO2 if we just burn every atom of carbon we can find. Compare the costs. It is cheaper to switch to other fuels.

The only people who might be inconvenienced are those who currently own the fossil fuel infrastructure. And I guarantee you, those are exactly the people who will come out on top when the switch is made. They are just using the gullibility of people like you to delay the change so they are best positioned to own everything again afterward.
1.2 / 5 (19) Mar 08, 2013
"Should humanity be worried about impactors? "Hell yes!" replied astronomer Neil deGrasse Tyson"

should cocaine be legalized in the US? "Hell yes!" replied
The Columbian growers.
2.3 / 5 (11) Mar 08, 2013
A grim future for humanity. But, I believe we are more likely to be wiped out by viruses.
3.9 / 5 (7) Mar 08, 2013
A grim future for humanity. But, I believe we are more likely to be wiped out by viruses.

...or by consequenses of our own stupidity: defoliation, population growth, pollution, overfishing, regional nuclear wars...
3.3 / 5 (7) Mar 08, 2013
Given that a doubling of CO2 is projected by biologists to cause 30 percent of all existing species to go extinct, the CO2 problem is as damaging as a large asteroid.

"But CO2 is more dangerous?" - RyggTard

Preventing one does not preclude preventing the other.

But if RyggTard is to be consistent then he will support steps to prevent both catastrophe's.
3.4 / 5 (8) Mar 08, 2013
I wasn't aware that Tyson was growing asteroids.

"Hell yes!" replied The Columbian growers." - Conservative Whiner

Your whining doesn't appear to be very effective.
2.9 / 5 (8) Mar 08, 2013
RyggTard is mentally diseased. You did know that didn't you?

I mention it regularly.

"What the hell is wrong with you CO2 deniers? The existence of asteroids and comets doesn't change anything about climate science." - DNA
2.9 / 5 (8) Mar 09, 2013
There is a 100% probability Earth will be hit again!
The only remaining question is, when?

We should get our skates on now, to defend ourselves!
1 / 5 (6) Mar 09, 2013
Geoengineer the asteroid...that will heat it up...
1.7 / 5 (6) Mar 10, 2013
We discussed this already.

The Yarkovski effect.
I made the suggestion to cast a permanent shadow on the approaching asteroid. (Not paint it like the professor wants).

Companies that want to save the earth will have their shadows in the shape of their logo cast upon asteroid.

It is the least I can do for life on earth. Your welcome.
1 / 5 (1) Mar 10, 2013
@Tausch - that's actually an idea worth pursuing - a big rotation-stabilized thin-membrane sheet would be really light, and meteorite hole in it wouldn't much matter (and it wouldn't even have to be a mirror, just fairly opaque). Keeping it 'hovering' at a distance would take very little fuel (compared to a closer and much more massive gravity tractor, for example). And one could selectively shade area, for example to let sun hit the early 'morning' side if that would help deflect the asteroid.

It has its limits - it can't be scaled beyond the amount of light that hits the asteroid - but actually fewer limits than painting the asteroid.
1 / 5 (4) Mar 10, 2013
Yes. Q-star is on the same wavelength as you or I. Notify Q-star. This is his calling. Besides he was looking forward to contacting NASA - why not with the suggestion made? The suggestion was in direct reply to his comments made.

See thread link above.
NASA's home base is the U.S. I live in Europe. Q-star lives in the U.S. I'm sure this project will be everything he expects it to be. I know, if realized, it will be.
2 / 5 (4) Mar 11, 2013
@Tausch - I do not think it is likely a shadow can be used for this - the reason being the sun has a diameter of 1,392,684 km, - just see what happen when Venus is passing in between the sun and earth... There is no shadow to speak of despite Venus having a diameter of about 12 000. A mirror would likely have a better chance.
1 / 5 (4) Mar 11, 2013
Yes. Either way. Illumination or shadow. The Yarkovski effect is all you want to invoke with the least of resources and effort.

Proof of concept is no obstacle. To save the earth is fun and exciting. At least the dress rehearsal is.
5 / 5 (1) Mar 11, 2013
@ka - It is a matter of distance. For example, as person casts a shadow on the earth in spite of a person being smaller than Venus.

The light from a mirror also spreads by the same angle as a shadow shrinks, and for the same reason.

However this is not a problem:
Consider an asteroid in an orbit similar to earth's. The distance to the sun is ~100 times the sun's diameter, so diameter of the fully-dark part of a shadow shrinks by ~1/100 times the distance that the shadow is projected.

If we 'hover' a 1 km shield in an orbit 10 km closer to the sun than 1 km asteroid, then the dark central shadow loses only 10 km / 100 = 0.1 km diameter and is still 0.9 km in diameter (and the partial shadow on the last bit of asteroid is still almost fully dark).
1 / 5 (3) Mar 11, 2013
NASA is funding the paint job.
There is no reason not to fund a shadow.
Considering the earth is at stake.

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