How does the earth maintain its average mass even though it looses mass to space?

100,000 tonnes of extraterrestrial material fall to earth every year. That's what helps keep the balance in check.

It will be interesting in a century or so from now as Earth begins to develop some sort of solar system based economy on how much mass we actually gain or lose. We may build a lot of things lo leave orbit, but It's projected that we could be importing raw materials mined from asteroids by the tens of thousands of tons each year. It'd be an academic question though, we're never going to lose or gain enough material to ever noticeably affect the planet's gravity!

The Earth also adds mass from meteors burning up in the atmosphere and meteorites impacting the Earth - 30,0000 to 120,000 metric TONS per year.

The Sun swallows a half dozen or more comets per year. only very rarely does the comet survive long enough to make it out of the Sun's photophere again, but when that happens, the comet gets evaporated pretty quicly by the solar wind, so that all that's left behind is trail of meteoroids.

Even though Earth both looses and gains mass, the amount is so small that even if the amount gained does not equal the amount lost, Earth will not change it's mass by even 1% in billions of years.

The Earth does not "maintain its average mass." If it gains or loses mass there is nothing to reverse the gain or loss except the luck of the draw. Its mass is not in equilibrium of any kind.

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Matter does not just drift into space, it's held in place by gravity. Any minute amounts that did drift away would be more than compensated for by meteorites and meteors.

Uniontera number 2, hope it helps. (about Mass)

Existence is the time expressed by light itself. http://www.youtube.com/watch?v=MjmOw1Pnc4s