Tuesday, November 8, 2011

What If That Asteroid Were to Hit Earth?

At 6.28 p.m. Easter time today, an asteroid ¼ of a mile long will pass between the Earth and the Moon (see graphic) and roughly 201,700 miles from our planet. By any reckoning of astronomical distance standards this is what's called a "grazing pass" or if you prefer more colorful parlance, a "near miss". Again, two hundred thousand miles doesn't sound like a "near miss" but by the astronomical standards of astronomical units (1 AU - 93 million miles) and light years, it certainly is.

We understand from the data that no major effects or influences will occur, including gravitational ones - which should mean no tides or any earthquake activity triggered by a differential gravitational pull on Earth's crust. However, it is instuctive to consider the possible effects if this moderate asteroid actually struck the Earth at 46,400 kilometers per hour (12, 900 meters per second), and with a putative or effective mass of 1.2 x 10^11 kg. (After ablation, evaporation of some mass on entry through the atmosphere.)

First, the effective kinetic energy (½ mv^2) would be:

½ (1.2 x 10^11 kg) (12, 900 m/s)^2 = 9.9 x 10^18 J

This is roughly the equivalent of 400 single megaton H-bombs going off, and from my computations, would carve out a crater nearly 7 miles wide and 3,000' deep. In other words, if such a monster hit Manhattan, that would pretty well be the end of it. If it struck Barbados nearly a third of the island would be cratered. A water or ocean strike, meanwhile, would trigger a 90' high tsunami though some low-balled values of 70' have been circulated. (But bear in mind, in the scheme of "rogue waves", 70' is considered more or less high normal and we're talking about an object 200' longer than the Nimitz aircraft carrier striking the ocean at more than 12,000 meters per second!)

It is known that a scale exists, called the "Torino scale" to measure and reference the potential destructive scale of asteroids, much like the Richter scale does for earthquakes. 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.

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.

Some bloggers have opined and worried about "How can any scientists actually know that this thing will miss us?"

The answer is that they 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. See, by way of illustration,


Once the object's orbital elements are known, then they can be processed into the perturbation equations and the knowledge obtained of whether there will be any direct interaction or not.

Given I have used celestial mechanics over many decades myself, first in an undergrad astronomy course to compute the position of Jupiter in the year 2010 (from the year 1970) and later to use it to find the perturbations on Halley's comet in 1986, I have no fear...ZERO...that any mistakes will be made.

However, I do believe this close pass endorses once again the saner position of not giving up on manned space flight. As both the late Arthur C. Clarke and Isaac Asimov have observed, it makes little rational sense - given we do have the technology or the potential to develop it - to keep all our eggs on this one little orb in the hope that we will never ever face a mass-sterilizing impact and being wiped out.

The dinos had no choice, as they were merely dumb beasts with brains the size of walnuts.

We have no similar excuses, and saying "we don't have the money" merits a swift kick in the posterior, and nothing more!

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