| We already have a lot of mass of random junk falling into the atmosphere from space all the time. A result from a quick google search: > Estimates for the mass of material that falls on Earth each year range from 37,000-78,000 tons. Most of this mass would come from dust-sized particles. http://curious.astro.cornell.edu/about-us/75-our-solar-syste... I guess this means the Earth gets slightly bigger and gravity becomes slightly stronger over time, but it's a much smaller effect than, say, the ocean's tides creating drag that slows the rotation of the Earth. Presumably, the materials we mine and bring back aren't going to have the same composition as micrometeorites, since space gravel exactly isn't economically valuable. As long as we aren't bringing back super toxic or environmentally damaging stuff that's rare on Earth, I'm not too worried. Also, I expect all but the most valuable elements (e.g. gold, platinum) will be re-used in space to build infrastructure. It's interesting that a robust space launch economy could compensate for the thousands of tons of annual space gravel, and cause the Earth to maintain its mass at a constant value. |