Comments on: Reconstructing the Chelyabinsk meteor’s path, with Google Earth, YouTube, and math

By: Good Stuff

Good Stuff — Tue, 19 Feb 2013 02:38:00 +0000

Why did NASA not see the Chelyabinsk meteorite?

1) because it came out of the daytime sky. These are nearly impossible to find ahead of time because telescopes can only spot asteroids during the night

2) The Russia meteor is estimated to have been less than 20 meters in diameter, which is considered a tiny asteroid

3) The small asteroid hit the atmosphere moving at a blistering 40,000 miles per hour. That’s more than twice as fast as asteroid 2012 DA12 is moving. The space rock lasted about 30 seconds in the atmosphere before breaking apart 12 to 15 miles above Earth’s surface.

following this story using the Facebook platform – http://www.facebook.com/photo.php?fbid=10151748294638677&set=a.453208868676.239413.805278676&type=1

By: dculberson

dculberson — Mon, 18 Feb 2013 15:42:00 +0000

Only instead of an earth shattering kaboom, it was more of a glass shattering kaboom.

@Christian Buchner: 30 seconds is a much longer time line than milliseconds or nanoseconds – and it makes a huge difference in how the energy is absorbed.

By: Christian Buchner

Christian Buchner — Mon, 18 Feb 2013 15:16:00 +0000

One could argue that releasing 500 kilotons of TNT equivalent over a time span of just 30 seconds amounts to one big explosion.

By: Zak McKracken

Zak McKracken — Mon, 18 Feb 2013 13:15:00 +0000

TL:DR:
There’s no explosion. The energy is being released into the atmosphere over the whole time of the meteor’s descent, not in one big KABOOM.

By: Zak McKracken

Zak McKracken — Mon, 18 Feb 2013 13:01:00 +0000

The analogy is somewhat flawed here, because it invites the idea that there was an explosion of the power of 30 Hiroshima bombs. Which is entirely untrue, and actually there wasn’t an explosion at all. Most of the kinetic energy that the object had was used to heat it up, melt and evaporate parts of it, heat up and ionize the atmosphere along its path (making lots of light in the process) and push it around rather violently.
Said pushing around results, if done at supersonic speeds, in a conical shock wave. If done at hypersonic speeds, that shock wave will be mightily strong and travel a long way, and that is what caused the damage on the ground. This is the same as observed here:
http://www.youtube.com/watch?v=g0Ed_fx9IFc
…except a lot harder. In some videos you can hear the shock wave two minutes after the breakup, meaning ~ 40 km away from the event. This is where most of the energy went.

What looks like an explosion is the object falling apart during the part of the trajectory when the heat is at its peak. The thermal stresses are cracking it up and the individual parts then create a lot more drag combined (since they have more surface area), which creates even more light and a stronger shockwave. It also decelerates the objects even quicker which explains why the trail fades rather quickly after that and there’s no huge crater or any particularly large impact visible.

We are quite lucky to have this atmosphere thing to cushion the fall of such objects.

By: Jasper Johns

Jasper Johns — Mon, 18 Feb 2013 01:44:00 +0000

There’s a great, funny, novel about an asteroid heading toward earth and how people deal with it. It’s called THE MYOSHI EFFECT.

By: Brian Fort

Brian Fort — Mon, 18 Feb 2013 01:30:00 +0000

They have since updated it to 500 kilotons.

By: bzishi

bzishi — Sun, 17 Feb 2013 20:33:00 +0000

That’s right: KE = 1/2 mv^2. At 15 – 18 km/s, 10,000 tonnes mass, and 4.184 GJ/(ton of TNT), this gives an energy of 269 – 387 kilotons of TNT. The uncertainty in the velocity will dominate the equation so the mass doesn’t need to be as precise.

Edit: Whoops. I see that NASA used the imperial ton instead of the metric tonne. That was silly. The correct values would be 244 – 352 kilotons of TNT.

By: theophrastvs

theophrastvs — Sun, 17 Feb 2013 20:07:00 +0000

This is the sort of stuff that’s really needed – good on-going work!

now, if some kind physics type would work out how you get 30 Hiroshima bombs worth of energy (uhm ~2000TJ) out of a 10000 ton rock, admittedly traveling at impressive speeds, i’d really love to see that. (let’see, one half em vee squared…)