We might say that the main character of The Martian Chronicles is the human race. Each story focuses on a different character, but the book as a whole is about all of us. If we look at it that way, we could say that the book is the story of a Voyage and Return.
In "Rocket Summer," we meet people getting ready for a trip. Except they're not exactly the people you'd expect to be heading off to Mars, because Bradbury only specifically talks about "housewives" and "children."
The first few expeditions to Mars are exciting, even if they're not always what you'd call successful. We hear about them in "Ylla,""The Earth Men," "The Third Expedition," and "—And the Moon Be Still As Bright." Members of the Second Expedition are proud of their accomplishment, and the Third Expedition finds a place that seems like Heaven (complete with dead loved ones).
We might even put "—And The Moon Be Still As Bright" in this category. Although it's pretty dark (it involves a lot of murder), it shows us that the Fourth Expedition is still all excited and hopeful about their missions.
But Mars isn't the easiest place for Earth people to settle, as we see in "The Green Morning" (Mars needs oxygen), "The Musicians" (where Firemen are burning away the very thing that makes Mars special—that is, all the dead Martians), and "The Fire Balloons" (where it turns out that Martians don't need religion, after all). It's almost as though coming to Mars hasn't solved all our problems.
What's really nightmarish about Mars is... other humans. We see this in "Usher II" (where a book-lover murders censors); in "The Martian" (where people demand so much from a Martian that it kills him); and in "The Off Season" (where we see how shallow and unpleasant people can be). So even though we're on Mars, we can't escape from our usual problems.
"The Watchers," "The Silent Towns,"and "The Long Years" are all about the thrilling escape and return back to Earth—because, if we're just going to take our problems with us, we might as well go home. Even if, as "There Will Come Soft Rains" shows us, home seems to do just fine without us.
So, here's the question: when people return to Mars in "The Million-Year Picnic," are they simply starting the cycle all over again? Or is their trip to Mars actually part of a new beginning?