Asteroid impact effects May 12, 2006
Posted by dr. gonzo in Asteroids, Space.trackback
If you saw my last post, Asteroids and comets, and actually read it, you may have noticed that the two small objects I listed were also the two objects, out of the four, which were most likely to impact Earth, albeit that still represents a small chance.
I was interested in what types of effects the impact of a 7 meter wide object (such as 2006 JY26) or a 40 m wide object (such as 2003 SG344) would have on planet Earth. To that end the Department of Planetary Sciences Lunar and Planetary Lab at the University of Arizona has developed an interesting web based impact effects calculator.
The calculator has the user input a number of different parameters ranging from size of the object, type of rock the object is made of, and impact surface composition. Other variables, that are harder to discern, such as impact velocity and impact angle are best inputted as the averages given on the application page.
So, for impact velocity: “Typical impact velocities are 17 km/s for asteroids and 51 km/s for comets.”
And impact angle: “The most probable angle of impact is 45 degrees.6.7 years”
A few calculations
Let’s take 2006 JY26.
A 7 m wide asteroid, with a density of 1,500 kilograms per cubic meter (porous rock), an impact velocity of 17 kilometers per second, an impact angle of 45 degrees impacting on a surface of sedimentary rock*, would:
-begin to break up at 238,000 feet
-burst into a cloud of fragments at 140,000 feet
-have an airburst energy of 1.14 x 1013 Joules = 0.27 x 10-2 MegaTons
-not form a crater, although large fragments may strike the surface.
The average interval between impacts of this size somewhere on Earth is 3.0 years. No major global changes arise and at a distance of 6.21 miles (10 km) from impact no air blast would be noticed.
Defintion: airblast-
The energy due to the impact causes a distortion in the air. This distortion travels in the form of a wave. If the energy of the impact is very high, the wave may initially be a shock wave, travelling at a velocity greater than the speed of sound in air. The wave eventually decays into a sound wave travelling at 300 m/s (671 mph).
For the same object: change of density to 3,000 kg/m3 (dense rock). The object:
-begins to break up at 177,000 ft
-bursts into a cloud of fragments at 113,000 ft
-has an airburst energy of 2.75 x 1013 Joules = 0.66 x 10-2 MegaTons
-does not form a crater, although large fragments may strike the surface.
The average interval between impacts of this size somewhere on Earth is 5.1 years. There are no global changes and the air blast would not be noticed from a distance of 6.21 miles (10 km) away.
For the same object: change of density to 8,000 kg/m3 (iron). The object:
-begins to break up at 43,700 ft
-bursts into a cloud of fragments at 29,500 ft
-has an airburst energy of 1.66 x 1014 Joules = 0.40 x 10-1 MegaTons
-has large fragments that strike the surface and may create a crater strewn field
The average interval between impacts of this size somewhere on Earth is 10.9 years. There are no major global changes and the air blast would not be noticed from a distance of 6.21 miles (10 km) away.
What happens if the object is larger?
Let’s try 2003 SG344.
A 40 m wide asteroid with a density of 1,500 kg/m3 (porous rock), an impact velocity of 17 km/s, impact angle of 45 degrees impacting on a surface of sedimentary rock. The object:
-begins to break up at 238,000 ft
-bursts into a cloud of fragments at 55,700 ft
-has an airburst energy of 5.88 x 1015 Joules = 1.41 x 100 MegaTons
-does not form a crater, although large fragments may strike the surface
The average interval between impacts of this size somewhere on Earth is 168.1 years. There are no major global changes and the air blast would not be noticed at a distance of 6.21 miles (10 km).
For the same object: change in density to 3,000 kg/m3 (dense rock). The object:
-begins to break up at 177,000 ft
-bursts into a cloud of fragments at 35,500 ft
-has an airburst energy of 1.20 x 1016 Joules = 2.87 x 100 MegaTons
-does not form a crater, although large fragments may strike the surface
The average interval between impacts of this size somewhere on Earth is 286.7 years. There are no major global changes and the air blast would not be noticed from a distance of 6.21 miles (10 km).
For the same object: change in density to 8,000 kg/m3 (iron). (Again, all effects are measured from a distance of 10 km)The object:
-begins to break up at 46,100 ft
-reaches the ground in broken condition
-has an impact energy of 1.33 x 1016 Joules = 3.17 MegaTons
-fragments strike the ground in an ellipse of dimension 0.301 km by 0.213 km
-forms a crater:
Crater shape is normal in spite of atmospheric crushing; fragments are not significantly dispersed.
The floor of the crater is underlain by a lens of broken rock debris (breccia) with a maximum thickness of 116 m = 382 ft.
At this impact velocity ( Richter scale: 4.9, Mercalli Scale Intensity at a distance of 10 km: IV, V.
-has most of its ejecta blocked by Earth’s atmosphere
-has an air blast which will arrive in 30.3 seconds with a maximum wind velocity of 35.1 m/s = 78.6 mph and a sound intensity of 84 dB (Loud as heavy traffic). Damage resulting will include shattered glass.
The average interval between impacts of this size somewhere on Earth is 610.2 years. There are no major global changes.
*Note: All calculations were at impact velocity 17 km/s, impact angle 45 degrees and surface composition of impact site was always sedimentary rock.
In Summation
A few things to be garnered from this. Low density, small object impacts are relatively common. They result in negligible damage on the ground. Higher density objects, especially iron ones, can cause significant damage on the ground and create a fairly large crater if the object is large enough. Luckily, these collisions are much rarer.
Imagine a 3,800 foot wide, 800 foot deep crater suddenly becoming an obvious landmark in your hometown, wild stuff.
From the Gonzo 
Interesting stats. Truly wild stuff. Those larger iron meteorites sound wicked. Thanks for the read.
what would be the effects and energy if ceres the largest asteroid hit earth at 95,000 mph.