ND Tyson tweeted that Scott Kelley aged 1/100th of a second less than his twin, Mark, while spending nearly a year in space. Isn't this a playing-out of a real "twin paradox" experiment? It seems that we have two identical twins, one of which goes on a (kind of) long space flight while the other stays home. The resolution of the twin paradox, if I understand it correctly, is that the twin who must accelerate to return home breaks the symmetry of the picture and therefore is the twin who ages less. For the Kellys, though, Scott is spending the vast majority of the time in an inertial frame, while Mark is spending all of his time in a non-inertial frame -- he is experiencing a constant acceleration, while Scott's worldline is largely geodesic. Shouldn't this asymmetry mean that Mark is actually the "moving" (younger) twin and Scott is the "stationary" (older) twin?