What usually happens to parts of a rocket after lift-off?

SpaceX is conducting tests to return the upper stages of its rockets back to Earth Rockets are often built in sections, called stages. Normally, such stages fall back into Earth's atmosphere and burn up or plunge into the ocean after boosting the rocket's payload to a precise spot in orbit. In this case, as our earlier post explains, the rocket's job was to fire its upper stage hard enough to send both landers out of Earth orbit and onto a path towards the Moon, and it succeeded. But the spent s