Sentence Correction

dadu wrote:Ron, could you let me know the errors in the non-underlined portion. Even that would be an opportunity to learn.

well -- two of them are things that are not tested on the gmat (there should be "the" in front of "asteroid", and there shouldn't be a comma). the gmat doesn't test articles (a/an/the) and doesn't test punctuation, so those two are not worth worrying about.

the third mistake is something that the gmat could possibly test, though it's not terribly likely: "since X" implies that X is a specific point in time and/or specific event, so X should be stated in the regular past tense.
i.e., the sentence should say something like "ever since observations ... first revealed" (not "have revealed").

what is the source of this awful thing?