Meteorite just misses skydiver



Alternate headline:

Skydiver Captures Video of All-Time Slowest Meteor


of all things a meteorite would be the only thing flying down to earth like Superman at high speed.

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Unfortunately, Bad Astronomy (above) is fairly sure it’s a pebble that fell out of his parachute, not a meteorite.

I had previously read Bad Astronomy’s earlier post on the subject, and was excited that it could be true.

But the reasoning displayed in the second article seemed sound: essentially, apply Bayesian inference (though he didn’t say it exactly like that). To wit: Yes, the object in the video may more closely resemble a meteorite than a pebble. But after you factor in the zillion-to-one prior probability of a meteorite passing that close by, vs. the fairly common occurrence of a pebble falling out of a parachute, you’re going to need extraordinary evidence to say that it’s probably a meteorite.



Phil Plait, Bad Astronmer:
“my conclusion is that it is much more likely to be real than not.”


pesco - the link you give is his first post on the subject; the link ericjs gave is a followup.

plus, this:

Plait posted two articles, and the second one (previously linked) is basically a retraction / rethinking.

I am not normally one who would dare contradict the math of a NASA scientist, but if quoted accurately Bill Cooke is effectively saying that a 1kg meteorite passes within 30’ of someone on the surface of the Earth every 12 hours, which is clearly not the case. The odds are far more astronomical than 500 billion to 1. It’s something closer to 500 bazillion to 1.

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Ah, thank you!!! Apologies!


It’s true. I am that person. And they’re really annoying.


I’m not going to get in to the physics of it, but as a metaphysicist I have to object to reasoning to a conclusion using Occam’s Razor. Don’t get me wrong, the Razor is a useful tool, but only so far. If we reached conclusions based only on how likely an event was, we might conclude that there are no such things as meteorites, only debris falling from badly-packed parachutes.


No sane person can ever acknowledge any unlikely thing to have ever happened.

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Who is using Occam’s Razor?

Bayesian inference, however, is a very mathematically-sound method of reasoning, as I mentioned in my post above.

The prior chance of an event occurring has to be taken into account.

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It depends on how you parse that sentence. I think he’s saying that the odds of a 1KG meteorite passing within 30 feet of anyone in a ten minute window is 1 in 500 billion, not some specific person.

By those odds, it would take about 7 million years for there to be greater than 50% chance of it happening.

Math: The odds of happening at least once in 400 billion ten-minute segments is (1 - ((499999999999/500000000000)^4e11)) = 0.55. 400 billion x ten minutes is 7 million years.

OK, supposing that’s a pebble. How did it achieve enough momentum when it was released from the parachute to hurtle past the camera lens (also in free fall) at such a huge speed differential? A few seconds earlier it would have been traveling at the same speed. It would have to be made of some honking dense material!

And while we’re at it, I’d like to see a calculation of the odds that a pebble trapped randomly inside of a packed parachute would fly out and zip down in front of the parachutist’s camera lens in near-exact mimicry of a meteorite…

I would be interested to know how we could ever conclude that!

We have meteorites lying on the ground. We can count them and group them by size. From this we know how often something big enough to break a window falls out of the sky. Basically about 100 tons of crap falls out of the sky per day. But it’s a big sky, so that’s how they are still rare events in any region of sky.

How does such an application of probability make those rocks disappear from museums all over the world?

Because the camera lens was not in free-fall. The camera lens was attached to a person who had just moments before opened his parachute.

And, in fact, just before opening his parachute, the person was not even free-falling, but was actually wearing a wingsuit. (The parachute will have made the vast majority of the speed change here, though.)

Edit: Also, note that you cannot tell the speed of the rock without knowing either its size or its distance from the camera. If it’s a big meteorite, it must be further away and be falling much faster. If it’s a small pebble, it must be closer and must be falling slower. This stuff is covered in the Bad Astronomy article, and the Facebook post. In the latter, it is calculated that, if it’s a near-by pebble, then it is falling just 10 m/s faster than the camera.


Lies, damn lies, and Bayesian inference.

It seems a lot of people think that dense or heavy materials fall faster than lighter things. Please lookup Galileo Galilei, who proved otherwise.

  1. There is a limit to the speed an object can reach when falling from a certain distance
  2. Small items fall faster than large items, because they have less air resistance.

Mass is not a factor in the equation of accelation due to gravity. v=gt, or velocity is gravitational constant times time, where g=9.81.

So, all objects fall at the same speed, unless they are hindered by air resistance. Take a piece of paper and drop it. It will drop slowly. Put the piece of paper on a book, then drop the book. It will fall very fast, just as fast as the book.
note: unless the object is very very heavy in comparison to the earth. So e.g. an asteroid the size of the moon would actually attract the earth toward itself thus changing this equation.