>>3
Define 'depleted'. The amount of matter leaving the planet is insignificant in comparison to the junk raining down on us.
"The Earth gains mass each day, as a result of incoming debris from space. This occurs in the forms of "falling stars", or meteors, on a dark night. The actual amount of added material depends on each study, though it is estimated that 10 to the 8th power kilograms of in-falling matter accumulates every day."
- Samantha Dong -- 2002
Lol, dong.
Space exploration is costly and likely ineffective - much better to focus on not being idiots and using what resources we have to better effect.