I love this question because, at first sight, I interpreted the wrong meaning.
So before I propose the solution, would like to talk of the pitfalls/mistakes that one may do(As I did)::
Pitfalls:
1: Assuming the first phrase to be a modifier for "amateur astronaut": In the first go, it seems as if "just inside the ... of Jupiter" modifies the astronomer. Technically many are conditioned in such a manner. The moment we see a phrase, we presume it to be a modifier.
2: Wrong meaning: Some may confuse an astronomer for an astronaut and interpret that astronomer is orbiting Jupiter and while he/she does so notices the comet.
SolutionKindly note the following pointers::
1: "Although" is conjunction, hence the first phrase should in fact be a clause. This means "ALTHOUGH" should be followed by SUBJECT + VERB. This can also be taken as a rule.
I repeat, pls note:: For sentences containing "ALTHOUGH" immediately look for "sub+verb" after "although"2: "ALTHOUGH" showcases a contrast and it generally does so by using a comma " , ". So if a sentence with "ALTHOUGH" has only one comma, it's highly probable that the two statements (before and after the comma)
in the sentence have to be contrasting statements. Also, the two statements have to be clauses each with a sub + verb.
Only one choice adds subject to the dependent clause. Hence, answer - B