NASA's next 60 years will probably be very different than its first six decades. When the agency opened for business in 1958, private spaceflight was just a sci-fi dream. But companies such as Elon Musk's SpaceX and Jeff Bezos' Blue Origin are working to make that dream a reality and open the space frontier to huge numbers of people for the first time. What role will NASA play in the private sector's liftoff? Space.com recently talked to three commercial-spaceflight experts to get some ideas. First, people should understand that about 75 percent of the worldwide space enterprise is already commercial,...