Kilogram
As regards the kilogram, relativity’s effect upon the constancy of matter’s mass is simply an interesting scientific phenomenon that has zero effect on the definition of the kilogram and its practical realizations. Objects are "weightless" for astronauts in microgravity. However, the objects still have their mass and inertia. Astronaut must use ten times as much force to accelerate a ten-kilogram object at the same rate as a one-kilogram object.