Thursday, October 6, 2016

Asteroids: The Celestial 'Swords of Damocles'


Nearly forty years ago, my article on Earth -impacting asteroids ('Target Earth?') was published in the Barbados NATION newspaper, not long after I'd completed my Peace Corps service. For many citizens of the island nation it was the first time they'd ever been awakened to a potential threat from space.(Ten years later a more direct wake up call would arrive with the impact of a large (~ .5m wide) meteoroid in what became known as the "Mount Tenantry event" - which I helped investigate.

As I noted in my article the most worrisome class of asteroids were precisely the "celestial swards of Damocles" or Earth-crossers, otherwise known as Apollo objects.  How big a threat are the Earth-crossing asteroids?
First,  it is instructive to see how they can emerge as a threat when their orbits are perturbed as shown in the sketch above, e.g. for the asteroid known as Icarus.  What makes any large asteroid dangerous, is its mass in combination with its relative velocity or speed of approach. Thus, large Apollo objects (and other asteroids) would typically have relative velocities from 11,000 to 30,000 meters per second, or about 6.8 to 19 miles per second.

For an asteroid with a 1 kilometer (0.62 mile) diameter, at a density three times that of water, this would generate an equivalent energy on impact of one million Hiroshima sized atomic bombs. Since the Hiroshima explosion was equivalent to 13,000 tons of TNT, this means a larger asteroid would generate an explosive force of 13 thousand megatons. This already is roughly equal to the total equivalent of all U.S. nuclear warheads.

For a ten kilometer asteroid (roughly 6 miles across) the explosive equivalent would translate into 13 million megatons. This would dwarf all man-made nuclear stocks and warheads, and is rightfully called ‘planet –killer’. An asteroid about this size is believed responsible for the extinction of the dinosaurs nearly 65 million years ago. For present day inhabitants of Earth, the consequences would hardly be less startling or grave. A Gaspra (12.5 x 7 x 7.5 miles) impact would obliterate all life on Earth - never to arise again. On the Torino scale, which registers the magnitude of asteroid devastation we are talking about a Torino scale 9 - the maximum.

                                                 The asteroid Gaspra - A planet killer

The crater and blast effects alone would eliminate most of the population on the continent it struck. An ocean strike, creating tidal waves upwards of 2 miles in height, would be even worse. The debris, for its part, would block out most solar radiation for years and probably usher in massive extinction of plants-  including sea plankton – that account for ninety percent of our oxygen.

How probable is such a Torino 9 asteroid impact?

The calculation of the probability is based on an estimated 15 Apollo Objects acquiring an Earth-crossing orbit every million years. From this it has been reckoned that the odds of a given Apollo Object eventually hitting Earth are 5 in one billion. This works out to one impact, on the average, every 200 million years. But wait. There are about one thousand such objects in all so that the probability that any one will strike Earth is now: (1000) x (5/1000000000) = 5/1000000. This figure: 5 in one million means that time wise, 5 asteroids would be expected to strike Earth every million years, or one every two hundred thousand years.

The problem with statistics like this is that they can lull a person into a false sense of security. It is true that the impact rate is low, relative to the scale of recorded history, but it is comparatively high in the context of geological time (thousands of millions of years). To fix ideas, consider that the most recent crater formed by a true Apollo object (as opposed to a large meteoroid) is at Lake Bosumtwi basin in Ghana, Africa. This is about 1.3 million years old. On this basis, the Earth is certainly long overdue for another hit of staggering proportions. Indeed, from the mean frequency of collisions we have worked out, the Earth should have been struck at least six times since the last impact by an Apollo object.

I should point out that in all of these statistics no account has been taken of the colliding comet factor. However, estimates by the Ames Research Center suggest that- with comets included - there are at least 2,000 objects that can cross the Earth's path. (Recall the captured 1994 impacts on Jupiter were from a large comet). Since my calculations were based on 1,000 objects - e.g. asteroids only, the probabilities would increase by a factor two. The time interval between major collisions would be halved.

Of course, statistical behavior does not follow rigid rules. There is no statistical law that pre-ordains an asteroid impact on Earth with the precise regularity of a 200 thousand year interval. By analogy, there is absolutely no reason why I shouldn't get a run of heads if I flip a coin ten times. Maybe I will get six heads in a row in one such run. The point, and it is an important one, is that the string cannot continue indefinitely. Assuming the coin is "true" the probability of heads must average 1 out of every 2 tosses. Looking at 100 tosses, for example, the total heads and tails will probably be very close to 50 each. The run of heads at some stage would have been compensated for by a similar run of tails to balance out the "law of averages".

Somewhat similar conditions apply to asteroid strikes on Earth. The run we are currently experiencing: no major hits over 6 successive periods of two hundred thousand years each, can’t go on indefinitely. As certainly as heads become tails for coin tosses, "no impacts" must become registered impacts for Apollo objects approaching Earth. The next big hit could come at any moment from an undetected asteroid. Or, it may come in the year 2036 (April 13, to be exact)  when the near Earth asteroid  99942 Apophis that some forecasts have now assigned a 2.7 in 100 chance of striking Earth. Fortunately, even if such an impact occurred, the destruction would be localized, given the object's 325m diameter.

This highlights the reality that there are millions of smaller, less visible or less detectable objects, that can also wreak havoc.  Few may recall, but as recently as February, 2013,  our Earth faced not one but TWO cosmic shots over our planetary bow.

The first shot - now known as the Chelyabinsk object- occurred with the explosion of an 11 ton (~2.2. x 10 4 kg) meteoroid over the Russian Urals creating an airburst over Chelyabinsk, Russia  The blast broke windows in 6 cities according to an MSNBC report the morning of Feb. 15, and injured 1,000 people, while 3,000 buildings sustained damage mostly from the blast shock wave.

Some 3D simulations and analyses at Sandia National Laboratories in 2014 showed the following about the object, actually since designated a small asteroid:

The entry at 19 km/s meant that it originated from the asteroid belt between Mars and Jupiter - not from a ballistically launched missile whose velocity would only be about 11.2 km/s or a short period comet with a mean speed of 35 km/s.

-  The altitude of the blast indicated the object was small and weak. The diameter of 20 m (66 feet) was estimated base on the observed velocity factored together with the assumed density of the material.

- The asteroid first felt the presence of Earth's atmosphere while it was thousands of miles above the Pacific Ocean and for a dozen minutes the 10,000 ton rock fell swiftly and unobserved passing at shallow angle through the atmosphere where the molecular mean free path was much greater than the 20 m diameter.

- When it crossed over the border into Russia at 3:20:20 UT and was 100 km in altitude 99.99997 % of the atmosphere still lay beneath it.

- For the better part of 10 seconds the asteroid hurtled through the air as a rigid body moving at a shallow angle, 17 degrees relative to the horizon and descending 1 km for every 3 km of flight.

- At about 45 km altitude the entry dynamics began to change. The dynamic pressure then built up from 0.7 Mpa (millions of Pascals, where 1 Pa = 1 atm equivalent), Within a couple more seconds, below 40 km, pressure on the now fracturing asteroid increased past 1 MPa, breaking it into a number of smaller fragments.

- As the pressure then grew exponentially the process cascaded and formed ever smaller fragments that rapidly increased the surface to volume ratio. As the fragments ablated the hot gas between them built up finally resulting in a chain reaction and a massive explosion converting the asteroid's kinetic energy into heat and pressure (yielding the shock wave that shattered windows).

- Only one significant piece-fragment remained post-explosion. This continued to fall like a ballistic missile in 'dark flight' at terminal velocity until it punched through the ice of frozen Lake Chebarkul.  This 1.5 m diameter boulder thereby became the largest Chelyabinsk object found.

As noted by one of the simulation authors, reported in Physics Today, September, 2014 (p. 35):

"In Chelyabinsk, the energy deposition that led to the explosion took place in stages and was spread out over a long distance because of the shallow entry angle. Energy was deposited at linear densities greater than 1 kiloton per kilometer and rose to a peak of 80 kt/km; most of the energy deposition occurred at altitudes from about 38 km down to 23 km. It took four seconds for this to happen during which the asteroid left a 50 km wake of hot expanding gas and ablation products."

The incident recalled for many the Tunguska Siberian event in 1908 which knocked down nearly 80 million trees and decimated nearly 820 square miles, about two-thirds the size of Rhode Island.  And this was from an object perhaps ten times the size of the 11 ton meteoroid that just hit the Urals this morning.

Meanwhile, on the same date, a much larger asteroid, the one identified as DA 14 2012, narrowly missed. See its path below:
In this oblique view, the path of near-Earth asteroid 
2012 DA14 is seen passing close to Earth on Feb. 15, 2013

 It packed a mass of 130,000 metric tons (~ 2.9 x 108  kg) and potential for an explosive release equivalent of a 20 megaton nuclear bomb.  Incredibly, the object was first discovered back in February, 2012  by a DENTIST using a high powered telescope. Why had no national scientific agencies made the discovery? What would have been the case if,  instead of a minor midget asteroid making this close pass, it was a planet killer 6 km across or larger? (As opposed to only 150'  or 45 m  DA 14 2012) Would we be ready to do anything other than piss, moan and pray?

Projects Space Watch and Spaceguard remain on funding life support, though a bit more largesse has come through in recent years. But mainly, asteroid tracking is the work of unsung, unpaid amateurs..Space Watch , had been based at the University of Arizona, and featured generous grant allocations in its early years when it  discovered over 1,300 approaching asteroids by 2002..  Later, as the austerity mindedness infected all science research areas, that capability  wound down a lot. Especially since the GOP takeover of the House in 2010, so. amateurs took over more of the load.  Not that the latter aren't observationally capable, but we also need professionals to spot these things and then (using the well known equations of celestial mechanics)  obtain the orbital elements as fast as possible. -.

Spaceguard extends asteroid detection to focus more on NEOs (near Earth objects) as opposed to singling out near Earth asteroids (NEAs).  Again budget limitations have loomed and imposed  truncated observation time frames and selectivity. Although there have been many discoveries of near-Earth asteroids  the Spaceguard Project has been criticized for not having an in-depth master plan if our humble planet is directly in the crosshairs of a giant asteroid. It has also been criticized of having false alarms that could lead to dangerous implications.

NASA published an article in April of 2001 to clarify many questions that have come up with this project. The report distinguished the difference between NEOs and NEAs, why the organization selected NEAs with a diameter of at least one kilometer in diameter, why the number of 90 percent was chosen and many other questions that have perplexed the general public.

Astronomer Paul Chodas, who works in NASA's Near-Earth Object program in Pasadena, Calif., decribed the Russian object as:

"....a tiny asteroid.  It would be very faint and difficult to detect — not impossible, but difficult."

Meanwhile, the actual asteroid DA 14 2012 that made a heart stopping close pass, was three times the size of the Russian object, and coming in at nearly 40,000 mph. Its blast -energy release was more like a 20 megaton blast.

Neither Space watch or Spaceguard is adequately funded, which means that either an NEO or NEA can escape through the observational 'net' and could wreak havoc, especially if the size and mass approaches that of the object that created the Barringer crater in Arizona, e.g.

The crater was formed by the impact of an iron-nickel object  50 m (165') across, coming in at 28,600 mph, generating an impact energy of 10 megatons. This crater, in other words, would have reduced a city the size of Boulder, CO to rubble. The impact occurred roughly 50,000 years ago. By comparison, the meteoroid that exploded over Russia was about 49 m in diameter, and Russian physicists estimate its explosive release at about 20 Hiroshima bombs of 10 kilotons each, or 200 kilotons - thus, a fifth of a megaton. Small change, in other words.

.Most disturbing , there is no formal, well-funded program or plan - of which anyone is aware  - for taking out a planet killer - say 6-10 km across, if one does target Earth. There have been past ruminations about using nuclear weapons to try to split the object into smaller pieces, but that was put aside when the ''architects" decided a number of smaller (1 km ) strikes might not be so good either. But still, a planet killer of 6 km arriving intact is not something to dismiss lightly and our space gurus may well have to opt on the side of losing half of the earthly populace to losing ALL of it, say from a nuclear winter onset as a result of some 10 28 kg of dust and debris in our atmosphere, cutting off sunlight

The worst nightmare is the Earth-sterilizing asteroid or "planet killer", with explosive equivalent of > 100,000 megatons. This will be from a rock greater than 10km across.  Some of the Torino scale levels and gradations (registered by mass and velocity) are as follows:

i) Regionally devastating impact, e.g. June 30, 1908 Tunguska impact. Devastation range approx. 10,000 sq. kilometers, killing crops, humans, animals.

Size of object: 20 m (~ 66') to 100m (~330') diameter .

Explosive release: 1 Megaton to 100 megatons TNT equivalent. Collision probability between ~ 1 in 100 yrs. and 1 in 1000 yrs

ii) Mass extinction impact: e.g. KT-boundary impact of 65 million years ago . Devastation range ~ 10 million sq. km., killing all extant dinosaurs and hundreds of other species.

Size of object: 100m (~330') diameter to 1 km (3330') dia.

Explosive release: 100 Megatons to 100,000 megatons TNT equivalent.

Collision probability: between ~ 1 in 1000 yrs. and 1 in 100,000 yrs.

iii) Earth Sterilizing Impact: Example......not yet. ("Black swan" territory?

Would annihilate every last species on the planet, and sterilize it for thousands of years to come. Devastation-affected area: > 50 x 10^6 sq. km.

Size of object: >> 1 km (3330') dia. (Likely source: any of one hundred Apollo asteroids whose orbits intersect with Earth's)

Explosive release: >> 100,000 megatons TNT equivalent.

Collision probability: Unknown but at least one asteroid specialist (Dr. Basil Booth) has predicted an Apollo asteroid collision some time in the next 250,000 yrs.

As one sees by surveying the preceding, this asteroid falls about midway into the "Mass extinction impact" range. In other words, if it struck the Earth, we'd be looking at a serious disaster indeed, whether water or land strike.

When my NATION article was published many readers in the aftermath asked: "How can any scientists actually know that one of these things could hit  us?" 

My answer - then and now  - is that these specialists (planetary astronomers)  make use of the highly precise branch of astronomy known as celestial mechanics. Especially that branch that has the objective of obtaining the perturbations which an astronomical object is likely to experience under the combined influence of the gravitational forces of other (e.g. larger, more massive ) objects acting upon it

While it is true that the probability of a monster rocks smacking us appears remote it ought not be too readily dismissed. This is because one never knows what sort of gravitational perturbations can affect existing orbits. We already know, for example, that an estimated 1,000 asteroids are perturbed from the asteroid belt each year to become Earth-crossers. A select subset of these are among the planet killing or mass extinction types. The smaller ones? Yes, they are nuisances, but we can deal with them if we have some long range planning wherewithal combined with alert observers.

And what are our illustrious politicos doing in the meantime?  Following the Chelyabinsk event in 2013, Rep. Lamar Smith, R-Texas, then chairman of the House Science, Space and Technology Committee called for a hearings "in the coming weeks". In his words:

"These events are a stark reminder of the need to invest in space science,"

Yes indeed, but we need a lot more than hearings. We need lots more technically-minded people with eyes on the skies.  Investing in their time, especially for professionals with the capability of computing orbital dynamics, takes money. Their services don't come free nor should we expect that.

Astronaut Rusty Schweickart, meanwhile, advised that NASA launch a $250 million-a-year program to survey asteroids and work up a deflection plan. The latter would be most effective for the lesser objects but may also help with the larger ones, of at least mass extinction scale. Ideally Russia and the U.S.  would combine their resources and work together for the planet's benefit - as opposed to more opportunistic war -making (to ramp up defense budgets), including endless stupid wars on terror.

Up tp now nothing has been done about this.  Schweickart noted at the time that "NASA now has $20 million" for searching out these potential cosmic  terrors, and added ... "It's peanuts."

Sadly, it may well take a monster meteoroid or small asteroid (like DA 14 2012) crashing into a populated area before NASA and other agencies, and especially our tightwad politicos,  wake up. That would then be our cosmic 9/11 and prompt us to get off our asses before the really BIG one arrives. Aka, the planet killer!

Incredibly, humans have mocked the fate of those dinosaurs extinguished in the wake of the Chicxulub   impactor event 65 million years ago, but we ourselves have basically done nothing to enhance our own survival chances - should an Icarus-scale asteroid impact. Maybe it's time we seriously begin paying some attention to those celestial "swords of Damocles", large and small.

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