[liquidise]

In N. Stephenson's Seveneves, a major plot point was that sufficient falling debris can superheat the atmosphere. Granted, we've orbited a trivial fraction of the Moon's mass, but have we orbited enough material to cause any warming to the atmosphere upon re-entry? Are we many orders of magnitude away?

I'm frankly not even sure how to napkin math such a question.

[didgeoridoo]

About 3,000 metric tons in LEO[0]. Don’t worry about mass above LEO, as there’s far less up there and it’s not decaying any time soon.

LEO velocity is about 8km/s.

Plugging that in to mv^2 gets you about 200 trillion joules of kinetic energy.

Assuming the chemical (burning) energy contained in space debris is negligible, that 200 trillion joules is the maximum energy you’d transfer to the atmosphere if all this debris suddenly deorbited.

The sun, in contrast, transfers about 430 quintillion joules per hour to the earth[1].

So the kinetic energy of all LEO mass is about a millionth of the energy the Earth receives from the sun every hour.

Prob not a heating concern.

(Please check my math. This was a wonderful nerd snipe but I did it on my phone while defrosting chicken nuggets).

0: https://www.sciencedirect.com/science/article/pii/S009457652....

1: https://thatsbyers.com/blog/how-much-energy-does-the-sun-rea....

[liquidise]

What a wonderful response to return to. Appreciate the work you put into this.

[coumbaya]

I would think, instinctively, that anything we put in orbit and then it falls down would be roughtly equal to roughtly 2x the energy we spent to put that mass in orbit in the first place, but maybe I'm missing something obvious.

[mrguyorama]

Why would it be 2x? In terms of gravitational potential energy, you get out exactly what you put in, minus things like friction and air resistance.

Also the vast majority of the energy in an orbiting body is not in gravitational potential energy (not that you said that) but in the kinetic energy of the object moving at something like 4.75 miles per second.

The end result is that the energy an object decelerating back into atmo releases is about the same magnitude as the energy of the rocket that got it up there in the first place.

[coumbaya]

Well not 2x as the deceleration is 2x, but in total like you said, you put energy rocketing it up and sideway fast, it uses that same amount of energy going down.

[uhhhhhhh]

Most of the energy used to put it into space was to push the fuel and delivery vehicle mass not the payload, that is not present on return.

[K0balt]

It will come down with exactly the energy we put into it minus the energy it already lost in orbital degradation.

But the energy we put in was with an efficiency below 25 percent (I would guess) so the energy it will release will be equal to a fraction of the energy we put in plus the energy released from oxidizing whatever burns on the way down. But still probably less than it already had put into it.

TLDR inconsequential effects. Not enough mass to matter. 5200 tons of space rock falls on earth every year.

[bckr]

Oh come on man, I'm on page 21.

[TomK32]

Stop reading it after Part 2, I had the same feeling about Part 3 as this critic for the The Guardian did: "Once we arrive in the novel's snail-paced last third, there are lots and lots of lavish descriptions of imaginary machines: city-sized orbiting habitats, giant pendulums reaching down into the Earth's atmosphere, 'sky trains'. After scores of pages of this, my eyelids were succumbing to a powerful gravitational force."