Complex sentence: Since no spacecraft can yet aerobrake safely on its own, this requires constant attention from both human controllers and the Deep Space Network, particularly near the end of the process when the drag passes are only about 2 hours apart (for Mars).
Split into simpler sentences: Since no spacecraft can yet aerobrake safely on its own, this requires constant attention from both human controllers and the Deep Space Network. This is particularly true near the end of the process, when the drag passes are relatively close together (only about 2 hours apart for Mars).