For Your Consideration - Thom Barker

For Your Consideration - Thom Barker

What are the odds? Close enough to zero to be zero

Ever thought about what the probability of getting a visit from the meteorite fairy is?

Let’s just say it is, well, astronomical.

Nevertheless, Ruth Hamilton, a resident of Golden B.C., awoke on the night of Oct. 4 to a loud crashing sound and the feel of debris from her roof falling on her face.

After shaking off the shock, turning on the light and getting her bearings, she discovered a black rock the size of her fist on her pillow. (See story Page A10).

Ruth is exceptionally lucky the meteorite ultimately came to rest on the pillow next to her and not the one she was sleeping on, but the odds of it happening in the first place are so slim as to be almost non-existent.

I’ve written before about how meaningless really huge numbers are to humans, but since we’ve got one, let’s put it out there. The odds of this happening are one in 100 billion according to researcher Peter Brown of Western University, where Ruth’s rock is currently being studied.

That number itself is way bigger than we can ever really appreciate, but the calculation of it is fairly straightforward.

Consider the surface area of the Earth is approximately 510 billion square metres. This is disputed, of course. The formula for calculating the surface area of a sphere is pretty simple, A=4Πr2. The Earth is not a perfect sphere, however, and it’s not smooth, so finding an exact surface area is virtually impossible, getting into fractal theory and such, well beyond my first-year Calculus for Geologists capacity.

Let’s just say, 510 billion square metres give or take a few billion is close enough for the purposes of understanding just how unlikely it is to get a meteorite through your roof.

The other main factor in the calculation is the estimated 500 meteorites that reach the ground each year. Less than 10 are recovered, which makes sense since the majority of the surface is covered in water (approximately 71 per cent) and only a small percentage is actually inhabited by humans.

The percentage that is actually inhabited by humans isn’t really a factor, but it makes for a fascinating debate on sites such as Quora and Curious Meercat.

While perhaps as much as 50 per cent of the planet’s landmass has now been modified by humans in some way, (particularly agriculture) we still only actually occupy a very small percentage. Urban areas, where we are largely concentrated only take up approximately three per cent of the land.

Also not really a factor is the 7.9 billion of us now alive.

The two numbers that matter in my calculation are 510 billion square metres and 500 meteorites that make it to the surface annually. OK, I get one in 1 billion. Of course, that may not take into account other factors such as orbits and whether all of those rocks are big enough to penetrate a house’s roof etc.

Brown’s explanation is this: “There has been one bed hit by a meteorite somewhere on Earth roughly every decade over the last half-century (using the Meteoritical Bulletin which lists falls and cases where Beds are hit). There are roughly 10 billion beds on Earth – hence 100 billion to one.

I’m not sure I buy his calculation any more than mine, but he is the research chair in meteor physics for the Western Institute for Earth and Space Exploration and I’m a newspaper editor with a geology degree and first-year physics and calculus for non-physics majors under my belt.

I would submit, though, that one in one billion and one in 100 billion are both close enough to approaching zero that Ruth Hamilton’s close encounter is indeed one of the most extraordinary events in meteoritical history.