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Assuming that in the not too distant future space travel developed enough, that large numbers of ships and people were leaving the earth's surface on a regular basis. Would this ever reach a point were the amount of mass leaving the earth became dangerous?

Would the resulting decrease in mass lead to changes in the earth's orbit that in turn lead to catastrophic climate change?

Would the amount of metal leaving the earth cause changes to the gravitational filed?

Would policies need to be put in place to regulate the amount of material resources that can be shipped off planned every year?

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    $\begingroup$ Compared to earth, we are small. Really really really [...] really really small. $\endgroup$ – PatJ Nov 28 '16 at 22:27
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    $\begingroup$ Once interplanetary travel is pretty easy, I'm sure we'll be mining unimportant asteroids for materials. (And I still like the question!) $\endgroup$ – BrettFromLA Nov 28 '16 at 23:47
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    $\begingroup$ The only concern would be depletion of rare stuff that is hard to find in places other than Earth (e.g. perhaps if space ships needed rare earth metals or gold or platinum in large amounts an exhausted Earth's supply of them). $\endgroup$ – ohwilleke Nov 29 '16 at 1:35
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    $\begingroup$ I am appalled at the lack of Douglas Adams references in this thread. Will rectify when I find the appropriate one. :-) $\endgroup$ – clacke Nov 29 '16 at 12:56
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    $\begingroup$ ... Like the fact that the fabulously beautiful planet, Bethsellamin, is now so worried about the cumulative erosion caused by ten million visiting tourists a year, that any net imbalance between the amount you eat and the amount you excrete whilst on the planet, is surgically removed from your bodyweight when you leave. So every time you go to the lavatory there, it’s vitally important to get a receipt. -- The Hitch-Hiker's Guide to the Galaxy (Fit the Second), Douglas Adams clivebanks.co.uk/THHGTTG/THHGTTGradio2.htm $\endgroup$ – clacke Nov 29 '16 at 13:31
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Earth is big.[citation needed]

The mass of Earth is about $6 * 10^{24}$ kilograms. Not all of that mass is usable to make spaceships, of course, but let's just hand-wave that and look purely in terms of how much stuff we have.

The Star Trek spaceship USS Enterprise is 3,205,000 metric tons; that's $3 * 10^9$ kilograms.

In one year, there are about $3.154 * 10^7$ seconds.

If we were to launch one USS Enterprise from Earth every second, it would take 590,805 years to reduce the mass of Earth by one percent.

In half a million years of interstellar spaceflight and expansion, we'd surely find other sources of building material as well. From a pure mass point of view, Earth will be fine.

Now, like I said, not all of Earth's material can necessarily be converted into spaceships. We may need to find other sources of some materials faster than others, but that's what the vast expanse of space is for. As we expand into new star systems, we can start dismantling other barren worlds, many of which are even bigger than Earth.

EDIT:

@Dercsar made a good point in the comments, that the reduced mass of Earth would shorten the length of the days. Fortunately angular momentum is easy to work with. Assuming the radius of Earth stays the same (which seems logical enough, since we would keep our houses and such where they are), the only two variables are mass and time. It works out that to reduce the length of a day by one second, we need to reduce the mass by about $7 * 10^{19}$ kilograms. That's $2.3 * 10^{10}$ USS Enterprises, which would take just over $729$ years at one spaceship per second. The good news is that it would in actuality take much longer than that (as one spaceship per second is simply not feasible), and when we finally do launch that much mass, we won't need to add a leap second nearly as often!

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    $\begingroup$ Also, lifting stuff to orbit is incredibly expensive, a few thousand per kilo with current technology. Even at \$10 per kg, it's \$30 billion per USS Enterprise. $\endgroup$ – ventsyv Nov 28 '16 at 22:59
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    $\begingroup$ For clarity that's the Star Trek Enterprise - the actual USS Enterprise is much smaller (but I'm sure everyone would agree that it's still very large), and 'only' weighed ~100,000T. $\endgroup$ – Matt Bowyer Nov 28 '16 at 23:36
  • $\begingroup$ It's also worth considering the energy requirements of putting things into space - even with 100% efficiency, putting the 'little' (real) USS Enterprise 400km from Earth (and not even into orbit) will require 400PJ of energy - about the same as humanity's entire energy consumption for 20-30 minutes. In reality, the actual consumption will be much higher - so I suspect that if you somehow get to the stage that your plan is causing issues then energy consumption will be a problem long before mass consumption is. $\endgroup$ – Matt Bowyer Nov 28 '16 at 23:50
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    $\begingroup$ "Earth is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to Earth." $\endgroup$ – Quuxplusone Nov 29 '16 at 9:33
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    $\begingroup$ However, you should calculate how many mass must be lost to lengthen days by one second -- because that could be an issue. $\endgroup$ – Dercsár Nov 29 '16 at 12:44
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Long and short of it is No.

We are not going to start exporting dirt off of the Earth any time soon, and even lifting water into orbit is far too expensive to keep up in the long term.

In fact, there aren't enough readily accessible resources on Earth to get mankind truly established as an interplanetary species. We'd have to start mining asteroids, and at that point we'll have access water in the form of comets, etc. (or we'll just make it out of hydrogen and oxygen).

On top of all this, comets and meteor regularly crash on Earth, adding mass to the planet. (not sure what the loss to gain ratio would be, but it's really, really not a problem either way)

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  • $\begingroup$ In fact, there aren't enough readily accessible resources on Earth to get mankind truly established as an interplanetary species. - may you show those facts, some of them in your answer? $\endgroup$ – MolbOrg Nov 29 '16 at 11:36
  • $\begingroup$ @MolbOrg I think that is a better issue for a space.SE question. $\endgroup$ – Mindwin Nov 29 '16 at 14:44
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    $\begingroup$ According to this Wikipedia page, there are several mechanisms of mass loss and gain. The largest gain is in-falling matter. The largest loss is atmospheric gas escape. The approximate net appears to be about 50,000 tons lost each year. $\endgroup$ – James Nov 29 '16 at 15:34
  • $\begingroup$ @Mindwin it is about correctness of the answer. Also space.SE do not have "get mankind truly established as an interplanetary species." in a sense WB may have. $\endgroup$ – MolbOrg Nov 29 '16 at 17:54
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This is completely the wrong problem to think about. Once humanity becomes a properly interplanetary civilisation, the mass of the earth will increase, for the simple reason that most manufacturing will be done in space.

The ships you talk about moving off to distant planets, these will all be built in orbit, out of metal mined from asteroids. But things built in space will also end up back on Earth, for the simple reason that not having to contend with gravity will greatly simplify a lot of manufacturing processes. There's the possibility that we could set up hydroponic zero-gravity farms in near-solar orbit, the raw nutrients and water for these plants can be harvested from space rocks, and the finished crops can be blasted off back to Earth. Transporting the vehicles that brought these crops over back up to the space farms will be more expensive than having some automated asteroid-based space factory build a new one, so the spent space transport drones will just need to stay on Earth.

As others have said, Earth is big, so it will be a long time any of this makes any difference to anything, but I don't think it's unreasonable to expect that within a century or so Earth will be a massive net importer of goods manufactured in space.

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