Saturday, February 16, 2013

Some references and thoughts about the meteor explosion

300 kilotonne explosion
> 100 kilotonne explosion

18 km / s (at entry?)

15 m size (diameter?  radius?)

  • volume if radius = 14137 m^3 ~ 14e3 m^3 = 14e9 cm^3
  • volume if diameter = 1.75e9 cm^3

7000 metric tonnes

  • 7e9 g
  • density if size is radius:  7e9 g / 14e9 cm^3 = 0.5 g / cm^3
  • density if size is diameter:  7e9 g / 1.75e9 cm^3 = 4 g/ cm^3
  • density of iron:  7.874 g/cm^3

chondrite rather than iron, iron tends to reach the earth

  • a hodge podge, but basically / mostly an oxide
  • not as dense as iron
  • not as heat conductive?
exploded at 15-20 km altitude
energy release began higher at 50 km altitude

google translation of statement from Russian Academy of Sciences:
This morning in the city of Chelyabinsk registered a decline of the cosmic body, which caused a bright flash of light and a strong shock wave. Reported shattered windows in homes. We estimate the size of the body was a few meters, the weight of the order of ten tons, the energy of a few kilotons. Body fell into the atmosphere at a speed of 15-20 km / s, collapsed at an altitude of 30-50 km, the movement of the fragments at high speed caused powerful glow and a strong shock wave. The main part of the substance of the falling body has evaporated (burned), the fragments that remain stalled and could fall to the ground as meteorites. Usually, the total mass of meteorites is found no more 1-5% of the initial mass. The main energy will be released at an altitude of 5-15 km. The bodies of this size are falling quite often, several times a year, but usually burn at high altitudes (30-50 km). Considered body seems to be very strong, probably iron. The last time a similar phenomenon was observed in Russia in 2002 (Vitim bolide). More accurate estimates can be given after receipt of all the information available.

Why do meteors explode:
Asteroids are just chunks of rock, so what makes them so explosive? In a word: speed.
The kinetic energy, or energy of motion, of a speeding asteroid is enormous. The Russian meteor entered the atmosphere going 40,000 miles per hour (64,374 km per hour), Bill Cooke, lead for the Meteoroid Environments Office at NASA’s Marshall Space Flight Center in Huntsville, Ala. said in a NASA press briefing.
The chunk of asteroid or comet that caused the 1908 Tunguska event is estimated to have entered the atmosphere at about 33,500 mph (53,913 km/h).
The shock wave from an asteroid's interaction with the atmosphere heats up the rock, essentially vaporizing it, Boslough said. The hot vapor then rapidly expands in the atmosphere, with explosive results.
"It's just like TNT going off, only much more energy," Boslough said.

  • Ideas: 
    • leading edge is super heated above boiling point
    • Or:  because pressure on leading edge is high, boiling point of material is much higher
    • Material is not heat conductive so either way there is thermal gradient, thermal stress.  
    • At some point the object cracks / fractures - this causes "instant" vaporization - 
      • either the superheating condition is triggered and the material goes to equilibrium - the vapor phase
      • or the fracture/crack causes the fragments to rotate so that pieces that were previously on the leading edge are no longer facing their individual direction of travel.  With the pressure reduced the material vaporizes "instantly" causing the explosion

Sunday, February 3, 2013

Factorial Design of Experiments

This blog post is about how scientific experiments can be designed such that the system being tested does not have to be measured at every possible combination of variables, or if it is how second order effects between variables can be calculated.  I'll work through an example to illustrate the principle:  in this example the "system" being tested is brewing beer.  Considering that brewing a batch of beer takes at least 2 weeks, involves many hours of work, it could be very worthwhile to find a way to get the same information from fewer experiments.

For some examples of practical applications / examples of factorial design of experiments (and statistical design more generally) here are some papers that Joshua L. Hertz and I wrote when we were in Stephen Semancik's group as post-doc's at NIST:
Combinatorial Characterization of Chemiresistive Films Using Microhotplates
A Combinatorial Study of Thin-Film Process Variables Using Microhotplates


Assume that we are interested in the effect on the color of the beer of three variables:  ferment temperature, type of yeast used, and mash temperature.  We will test each of these variables at 2 different settings:
ferment temperature:  50 F, 45 F
type of yeast:  WP004, WP005
mash temperature:  145 F, 150 F

With 3 variables that have 2 settings each there are 8 possible combinations of experiments that can be run.

These three variables and their 2 settings each can be represented as a cube, where each dimension is a variable, each side has fixed value for one of the variables, and each vertex represents one of 8 possible combinations of the variables: