08 October 2016

I don't update enough

How did it get to be October?

Commonweal #4 has a draft.  Commonweal #5 is making modest progress.  The first half of The Human Dress (which people of a certain Usenet vintage will recall being mentioned as the Doorstop) is back from the copy-editor.  (The copy editor has been restrained in their imprecations.)

Google Sheets has started putting in spurious @table:number-columns-repeated attributes when exporting to ODS.  This made my week much more entertaining than it might otherwise have been.

I keep seeing a peregrine at the Victoria Park subway station.  The pigeons are consistently distraught.


Kiya Nicoll said...


Hooray! Hooray! Hooray!

Kiya Nicoll said...

(A picture of my late cat reading the Doorstop can be found here.

Graydon said...

+Kiya Nicoll -- I got through writing Commonweal #4 ("Under One Banner"), and realized there were things I wouldn't know until after I'd written Commonweal #5 ("A Mist of Grit and Splinters") because the timelines overlap. (#5 starts before, and ends after, #4. I probably need to get a whiteboard.) There was no way I'd get them _both_ written in time.

Which meant, nothing for 2017. Which means, hurm, I could publish the Doorstop, it's not actively terrible... And nothing for 2017 *would* be actively terrible. So here we are.

Unknown said...

I will be very glad to purchase a copy of The Human Dress. If it were actively terrible I would not have reread the draft you sent me a dozen times over the years.

Anonymous said...

I'm Rob Ellwood, a retired Canadian engineer and ex-RASF-ite. In G+, you asked about a metallic object moving at 40 km/sec appearing at sea level. Here are some back-of-the-envelope estimates.

IIRC, it takes ~1 MJ to heat up & melt a kg of iron, so it would take maybe 5 MJ to vaporize it. The object has 800 MJ of kinetic energy per kg, enough to vaporize itself a hundred times over.

Each kg of the object has the energy of 200 kg of TNT. (Take V in km/sec, square it, and divide by 8.4.) Use a 100 tonne object if you want a 20 KT Hiroshima-class event.

If the object is heading down, you get an immediate energy release and the properties of the object don't matter. The rest of this mess assumes that the object is moving horizontally.

Scale and object strength matter a lot. One kg of powder (regolith) releases all its energy within, say, 10-100 meters. In the first millisecond, the initial blast from hitting the air spreads out the particles, so the second millisecond sees the particles hitting even more air, giving a bigger blast, so the third millisecond.... The blast ends up shaped like a teardrop. For 100 tonnes of powder, the center of the fireball would be (blind guess) maybe 1 km downrange.

With the equivalent of 200 kg of TNT exploding over a line 10-100 meters long, I would guess the lethal radius would be a few meters near the start and 10 meters near the end.

Metal teleported in from the outer solar system will be below the brittle/ductile transition temperature. It will shatter fairly quickly. At a guess, a 1 kg chunk would go through 20-100 meters of air.

Rock is stronger, but eventually shatters. Looking at the ~10,000 ton Chelyabinsk meteor re-entry (https://www.youtube.com/watch?v=4ZxXYscmgRg), there are several flare-ups during the first few seconds. These would be chunks fracturing off, then immediately shattering. You can clearly see where the main body shatters: there's a huge flare-up. On another video, the trail of recondensed rock becomes far wider there, and has irregularities from the larger fragments. Note that it broke up at ~30 km, where the air density is, what, 1.5% of sea level?

*Ductile* metal is even stronger. For a 1 kg compact chunk, you get a 100-1000 meter streak of fire through the atmosphere. (Most of the energy goes into the shock wave, and just heats up the air. Blind guess: assume 10% of the energy goes into vaporizing the metal. Hitting 1 gram of air vaporizes 10 grams of metal. 1 kg with density 10 gives a sphere of r=3 cm. Treat it as a 1 kg hockey puck with r=3 cm. 1 gram of air takes up about 1000 cc's. The hockey puck vaporizes completely after 300 meters.)

The path of any object will be dead straight. Sure, a small, irregular object might experience 10 gees of lateral acceleration, but that will make it tumble. It will then have random accelerations in random directions, which will average out to near-zero. I suspect that, to the human eye, even a sustained 10 gees would look straight instead of curved.

Graydon said...

Thank you, Rob!

While I think there's likely to be a "bag of gravel" analog-to-grapeshot application, that confirms my suspicion that carbon-carbon heat shields aren't going to suffice. :)