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If anyone could work out how much mass (not weight :) ) have humanity added to the Earth, seeing that the LHC (atlas, I believe, weighs 7000 tonnes), and the Three Gorges dam in China slowed the Earth's rotation by 0.06 micrometers. How much does the collective building of humanity added to the mass of the Earth?

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    $\begingroup$ A number of answers have pointed out that we have negligible impact on the totall mass of the earth. To address the second part of your question: the Three Gorges Dam would have slowed the earth's rotation by moving that mass around, in this case raising it in height (presumably from a quarry to above a river valley). By conservation of angular momentum, raising mass higher will slow the rotation - like a skater in a spin who stretches their arms out to the sides. $\endgroup$ – Semidiurnal Simon Dec 12 '17 at 11:45
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All mass used in construction on Earth comes from material on Earth. Since mass is conserved, there is no change. If we start harvesting material from the moon or asteroids for use in construction on Earth, then there would be a net increase in mass. But currently, the net mass of Earth has decreased due to man, since we have launched many satellites into space which came from material on Earth.

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    $\begingroup$ But the mass has increased by several hundred kg of moon rocks, and a few grams from things like the Stardust comet sample return mission. $\endgroup$ – jamesqf Dec 5 '17 at 20:22
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    $\begingroup$ @jamesqf Good point -- apparently 382 kg of lunar rocks have been returned. More than I expected, but still tiny compared to the mass we've removed from Earth. $\endgroup$ – Pont Dec 5 '17 at 20:48
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    $\begingroup$ Even smaller, there has also been a small mass loss due to accelerated nuclear reactions - ie. in fission reactors and nuclear bombs (both types). Yes most of that mass would have been lost eventually but in millions/billions of years rather than seconds / days. $\endgroup$ – winwaed Dec 6 '17 at 15:18
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    $\begingroup$ And, the contribution of meteorites is a net positive. Not sure how big of a contribution that is. $\endgroup$ – farrenthorpe Dec 6 '17 at 22:13
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    $\begingroup$ I would consider the oxygen used in chemical reactions to have originated on Earth. No net change there. $\endgroup$ – farrenthorpe Dec 12 '17 at 21:47
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Don't confuse redistribution of mass with gain/loss of mass. There's a net loss of hydrogen and helium from the uppermost tenuous atmosphere. This far outweighs the incoming meteorites and miscellaneous space dust - by some 50,000 to 90,000 tonnes per year. Both of these components eclipse, by orders of magnitude, the loss of mass from radioactive decay, satellites that drift into space, recovered moon rocks, and assorted other human activities. The exception to losing mass may be those rare years when large bodies collide with Earth, in which case there will be anomalous net gains in mass.

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