Complex sentence: Rocket stages (or satellites) with enough propellant may make a direct, controlled de-orbit, or if this would require too much propellant, a satellite may be brought to an orbit where atmospheric drag would cause it to eventually de-orbit.
Split into simpler sentences: Rocket stages (or satellites) with enough propellant may power themselves into a decaying orbit. If a direct, controlled de-orbit would require too much propellant, a satellite may be brought to an orbit where atmospheric drag would cause it to eventually de-orbit.