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There are various opinions found on web pages, about capitalizing "Earth".
Many seem to agree with this handout from the University of Nevada.
You don't capitalize "earth" after "the".
http://writingcenter.unlv.edu/w...talization.pdf
In this particular case, you can easily just avoid the whole problem, and still have a good English sentence by just dropping the "the."
Earth has been hit before by asteroids and will be again someday.
