Combining a stock buyback plan is smart at least because perhaps they can balance the two and counteract the dividend effect by buying up shares.
In reality Apple could probably just become a private company by buying back most/all the public shares, but I doubt they want to do that. There are obvious PR benefits to being the "most valuable" or "most profitable" public company in the world.
Also, as far as space travel and exploration, that makes no sense for Apple. Apple is not a company that invests huge in R&D projects that won't pay off for decades. It would ruin their incredible focus on what they are doing now and for the next 5 or so years. If you get too far out ahead, you build things the world isn't ready for yet or you stop executing on the here and now products and services.
Apple's still got a ton of room for potential growth on phones, pc's, and tv's. There are also a whole slew of "post pc" computing interfaces and devices that haven't even been dreamed up yet. Space travel would be a distraction.