
Forensic Ballistics: How Apollo 12 Helped Solve the Skydiver Meteorite Mystery - ColinWright
http://www.planetary.org/blogs/guest-blogs/2014/0419-forensic-ballistics.html
======
tc_
The article's discussion of golf balls reads completely opposite to standard
well-accepted theory. The author writes:

> _[The "drag catastrophe"] is when an object is falling so fast that the
> boundary layer of gas separates off the object and the drag force suddenly
> drops by a factor of almost 10. The reason why golf balls have dimples is to
> cause this drag catastrophe to happen at slightly slower speeds, so the ball
> will travel a lot farther._

This is very confused. Golf balls have dimples to _prevent_ flow separation.
The dimples are turbulators meant to induce turbulent flow around the golf
ball before the laminar flow would otherwise give way to flow separation. Far
from decreasing the drag force, flow separation increases it substantially.

[Also, the term "drag catastrophe" appears to have no relevant hits on Google
other than this one article.]

[Edit 1]: The author is well qualified and unlikely to be confused himself; so
I don't doubt his conclusion. Reading charitably, turbulent flow might be
called a form of separated flow, and this must be what the author means. His
coefficient of drag graph supports this interpretation as his "drag
catastrophe" would be happening when you would expect a transition flow (from
separated laminar to turbulent). Pedagogically he should have more clearly
distinguished it from the typical laminar separated flow.

~~~
certainly_not
I'd never heard of "drag catastrophe" before either, but I think you're right
that he's referring to the onset of turbulent flow that reduces drag on a
bluff body.

That's the sudden drop at the right in all of these graphs:
www.google.ro/search?q=drag+reynolds+number (same as his own Figure 9,
really).

There are a lot of surprising nonlinearities in those graphs. I wish there was
a more detailed article/paper somewhere.

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mixmax
Submissions like this is the reason I frequent HN! A thorough well written and
well researched article that point by point dissects an interesting event and
comes to a surprising conclusion.

If you come across other articles of this calibre please submit them!

~~~
debt
They took a few seemingly unrelated things and uncovered just how intimately
intwined they actually are. Although abstract, isn't that essentially what a
good hacker does on the daily? I think so.

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thedrbrian
Sounds a bit like the fighter pilot who shot himself down

[http://www.check-six.com/Crash_Sites/Tiger138260](http://www.check-
six.com/Crash_Sites/Tiger138260)

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Osmium
Sad conclusion, but that's what science is all about. Fantastic analysis.

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jpswade
tl;dr: Occam’s Razor demands the simplest explanation and so it probably was a
stowaway piece of gravel

~~~
chillingeffect
I came here to post that it was a great article - except for the needless
invocation and anthropomorphization of Occam's Razor.

I. It is not clear that a rock falling out is _simpler_ than a meteorite
falling out. How exactly is a meteor less simple? Sure, it's rarer and less
likely, but it is not simpler.

II. A simple explanation of a how an iPhone works is "magic." But we know
that's not the case. Our invocation of Occam is tautological - we only call on
it when we think it's right, not when we're making a decision. We note that
the article did not begin with: "there must be a simple explanation, because
Occam's Razor always works." Instead, it invoked Occam after the fact.

Conclusion: Occam's Razor is a fetishized social construction and carries no
magical properties and is of no use in predicted phenomena using a scientific
model. It is simply a feel-good mantra like prayer and rosary beads.

~~~
jerf
"Simpler" is how it was formulated at the time... nowadays we'd probably
formulate it in terms of going with the possibility that least amount of
"luck" in its explanation. Something that requires four events with 99%
probability is still more likely than an explanation that requires one at
0.004%, even if it is not "simpler". (Those numbers are just made up, not my
guess at the odds in question... in reality, the gulf is probably even
larger.)

I don't think the odds on filming a meteorite are _so_ bad that it will
absolutely never happen, low probability events do happen in the real world
(people have been directly hit by meteorites, after all [1], and that's much
lower probability than a meteorite passing through a camera's field of vision,
which has a _much_ larger volume per second), but for a given claimed
meteorite filmed, the mundane explanations are more likely.

[1]:
[http://en.wikipedia.org/wiki/Sylacauga_%28meteorite%29](http://en.wikipedia.org/wiki/Sylacauga_%28meteorite%29)

~~~
hammock
Your reformulation reduces Occam's Razor to tautology- "the most probable
explanation is the most probable." You might want to rethink that.

[http://en.wikipedia.org/wiki/Occam%27s_razor#Formulations_be...](http://en.wikipedia.org/wiki/Occam%27s_razor#Formulations_before_Ockham)

~~~
ghkbrew
Isn't that sort of the point? It's true, because it's a disguised tautology.

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mcguire
" _It turns out that when the rock size is set to about 3 centimeters in the
simulation, it passes the skydiver at 12 seconds. This rock size just happens
to have the same terminal velocity as the solution we found by matching the
velocity seen in the video._ "

Hm. Poking around[1], drogue/pilot chutes seem to be around 30 inches or 75
centimeters in diameter. I wonder how you'd get a 3cm piece of "gravel" mixed
up in that?

[1] [http://www.chutingstar.com/skydive/chernis-collapsible-
main-...](http://www.chutingstar.com/skydive/chernis-collapsible-main-pilot-
chute)

~~~
sublimino
The pilot chute can be hastily scrunched up and shoved into the bottom of the
container ("backpack") without any adverse effects to parachute deployment, so
it's plausible that the rock was stowed by accident.

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darkhorn
Is this a second rock?
[http://i.imgur.com/j88wa4u.png](http://i.imgur.com/j88wa4u.png)

Sorurce [http://youtu.be/jfEdEIwhj6s](http://youtu.be/jfEdEIwhj6s)

~~~
thix0tr0pic
It's been said to be a second skydiver

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markbnj
Is someone going to resolve the golf ball drag debate conclusively?

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Gracana
Very interesting but, a little disappointing. Like the author, I was hoping it
really was a meteorite.

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Theodores
It was all going so well...

 _“It can’t be anything else. The shape is typical of meteorites – a fresh
fracture surface on one side, while the other side is rounded,” said geologist
Hans Amundsen._

Every hoax just needs one credible expert to put their name to it. This story
may not have been a deliberate hoax but it ultimately was a hoax. It is good
to see something debunked with some good, old-fashioned science!

~~~
sophacles
I'm pretty sure a hoax requires a deliberate deception.

From the definition:

 _Noun: something intended to deceive or defraud_

Otherwise it's just a mistake, misunderstanding, or other form of "just being
incorrect". If they said "we know it's not a meteor, let's tell everyone it is
anyway" \- then it's a hoax.

Let's not dilute the word.

