An aeroshell is a rigid heat-shielded shell that helps decelerate and protects a spacecraft vehicle from pressure, heat, and possible debris created by drag during atmospheric entry (see blunt body theory). Its main components consist of a heat shield (the forebody) and a back shell. The heat shield absorbs heat caused by air compression in front of the spacecraft during its atmospheric entry. The back shell carries the load being delivered, along with important components such as a parachute, rocket engines, and monitoring electronics like an inertial measurement unit that monitors the orientation of the shell during parachute-slowed descent. Its purpose is used during the EDL, or Entry, Descent, and Landing, process of a spacecraft's mission. First, the aeroshell decelerates the spacecraft as it penetrates the planet's atmosphere. The heat shield absorbs the resulting friction. During descent, the parachute is deployed and the heat shield is detached. Rockets located at the back shell are initiated to assist in the decrease of the spacecraft's descent. Airbags are also inflated to cushion the impact. The spacecraft bounces on the planet's surface directly after the first impact. The spacecraft's lander petals are deployed after the airbags are deflated and retracted. Communication throughout this entire process is relayed back and forth from mission control and the actual spacecraft through low-gain antennas that are attached to the back shell and on itself. Throughout the entry, descent, and landing stages, tones are sent back to earth in order to communicate the success or failure of each of these critical steps. Aeroshells are a key component of space probes that must land intact on the surface of any object with an atmosphere. They have been used on all missions returning payloads to the Earth (if one counts the Space Shuttle thermal protection system as an aeroshell). They are also used for all landing missions to Mars, Venus, Titan and (in the most extreme case) the Galileo probe to Jupiter.