To a decent approximation, the astronauts took off from the Earth
and landed back on the Earth -- so their total displacement was zero!
With that simple approximation, 

            average velocity  =   0  m/s


  One might argue that the astronauts left Florida and returned to
the Pacific Ocean, making a displacement of about 11,000 km.
In that case, one could calculate

                                     11,000,000 m
            average velocity  =   --------------------------
                                  (8 days) * (86,400 s/day)


                              =    approx  16 m/s