"Jetpacker" is an Achievement/Trophy in RAGE. This Achievement/Trophy can be earned by killing an Authority Enforcer during jetpack descent.
Shooting the "flying" Enforcers seen in the Authority Prison yard while they are descending is the earliest that the player can get this Achievement/Trophy. One shot, even regular Pistol Rounds, will cause them to detonate, so the kill is easy. Can also be done when returning to the Dead City during the Lost Research Data mission.