Posted by: bkivey | 15 February 2013

KE=1/2mv^2

Asteroid 2012 DA14 is a 150 foot rock that’s in the news today because it’s going to pass about 17,000 miles from Earth, and is noteworthy as the closest known approach by an object of that size. The experts assure us that there is zero chance we’ll get dinged, and even if we did, from the planet’s perspective it wouldn’t be a big deal.

If a rock that size did hit Earth, it would be a very big deal for any people in its path. Just for fun, I looked up the Impact Earth! page. This is a calculator that simulates the effects of objects of various sizes and densities striking the planet. You can also adjust parameters for angle and speed of impact, as well as distance from the impact and whether the object strikes land or water. According to the information we have on the asteroid, the relevant parameters for calculating impact effects are:

  • Diameter: 50 meters
  • Density: 2600 kg/m^3
  • Relative velocity: 12.6 km/s
  • Angle of impact: 45 degrees

Impact effects are dependent on the distance from the impact, so I used Portland, OR as Ground Zero, and I live about 8 miles from the city center. Running the numbers, we find that the asteroid would start to break up at 177,000 feet and explode at about 30,000 feet, creating an airburst with an energy of about 2.6 megatons. If that happened over Portland, the shock wave would arrive in just under 50 seconds, generating a wind of just over 40 mph and a sound pressure of 78 dB. Peak overpressure would be a little over 1 psi. That’s a sizable blast. To see what that means in practical terms, I went to the HYDESim page. This calculator simulates atmospheric overpressure (blast effect) from large explosions. The simulation shows overpressure values as concentric rings from Ground Zero, and doesn’t take terrain into effect. There is a range of 700′ hills between me and Portland, but if something explodes at 30,000 feet, the hills probably aren’t going to mitigate the effects too much. In reality, the blast energy would be oriented along the line of entry, producing an elliptical pattern rather than concentric circles.

To correlate the blast pressures with physical effects, I found this handy table (scroll down a bit) from Riegal’s Handbook of Industrial Chemistry. If 2012 DA14 exploded over Portland, we’d have something like the following effects:

  • Downtown Portland would be completely destroyed.
  • Catastrophic damage would occur from the east side of the West Hills to the outer neighborhoods in east Portland.
  • Aircraft and structures at the airport, and in Beaverton where I live, would be seriously damaged. Glass would be blown out, and people knocked down.
  • At distances up to ten miles from Ground Zero, houses would be rendered uninhabitable.

That’s a lot of energy for a relatively small rock. KE=1/2mv^2 indeed.

Word Watch

Preantepenultimate: This neologism by James Taranto apparently means the ‘fourth from last’.

 

 

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