22. When the next trip to Mars is undertaken, they will have to determine if it is made of Ben-Gay.

Rewrite:

22. When the next trip to Mars is undertaken, the astronauts will have to determine if it is made of Ben-Gay.

Why?

"They" seems to refer back to those who have undertaken the expedition to Mars (astronauts?), but these people are not explicitly mentioned; therefore, one should explicitly name who has undertaken the expedition in the subject of the main clause.