The momentum of the astronaut is conserved. So as you see from eqn. 1.37 that the center of mass velocity of the astronaut is constant.
How can that be? If you're out there in free space, you can certainly move around, I've seen pictures of the spave shuttle and things like that. So can't you change youre velocity? Well you can change the velocity of your hand, or your foot, but sinve momentum is conserved, you can't change that, hence your center of mass velocity will remain the same. If it starts out as 2 m/s, it'll stay that way. If it starts out as zero, it'll stay that way too.