Why can astronauts not use parachutes to help slow down their spacecraft as it approaches the moon how do they slow down their spacecraft?

A parachute would not work in the vacuum of space as there is no air for it to catch and create drag. Instead, spacecraft approaching the moon slow down through a combination of a retrograde rocket burn and gravity assist maneuvers to enter lunar orbit. Mission planners carefully calculate the spacecraft's trajectory and use precise engine firings to control its speed and trajectory.