I
chose the #2 purpose: To suggest a drawback
to space flight.
I think the author's purpose in this selection
is to suggest a drawback in space flight.
(Notice strong intro
sentence.) First they give examples
of how people can get sick in space, then
they tell how muscles can deteriorate under
the influence of microgravity, if astronauts
don't exercise. Also, they tell about
how most of your bones lose calcium and become
weaker. Finally, in the last paragraph
they tell about on a long space trip there
might still be some unkonwn dangers. (Note
also the transition words to help the reader
follow the writer's logical flow of text-based
details.)