The ISS? It's above the atmosphere so there is no air resistance to slow it down. The velocity is constant after the initial boost into orbit. Actually that's

The ISS? It's above the atmosphere so there is no air resistance to slow it down. The velocity is constant after the initial boost into orbit. Actually that's not quite correct, I believe it is not quite out of the atmosphere completely and as such there is a (very) small amount of air resistance and they have to fire engines to boost back into the appropriate orbit every once in a while.