Aeroshell

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.

First, the aeroshell decelerates the spacecraft as it penetrates the planet's atmosphere and must necessarily dissipate the kinetic energy of the very high orbital speed.

They are also used for all landing missions to Mars, Venus, Titan and (in the most extreme case) the Galileo probe to Jupiter.

[3][4] The size and geometry of an aeroshell is driven by the requirements of the EDL phase of its mission, as these parameters heavily influence its performance.

The backshell typically contains a parachute, pyrotechnic devices along with their electronics and batteries, an inertial measurement unit, and other hardware needed for the specific mission's entry, descent, and landing sequence.

Other rockets may be equipped to provide horizontal force to the back shell, helping to orient it to a more vertical position during the main retrorocket burn.

A spacecraft must have a maximum value of deceleration low enough to keep the weakest points of its vehicle intact but high enough to penetrate the atmosphere without rebounding.

[8] It must also be able to withstand high temperature caused by the immense friction resulting from entering the atmosphere at hypersonic speed.

These factors combine to affect the re-entry corridor, the area in which a spacecraft must travel in order to avoid burning up or rebounding out of an atmosphere.

Future missions, however are making use of atmospheric rebound, allowing re-entry capsules to travel further during their decent, and land in more convenient locations.

This is because a higher mass/drag area means the spacecraft does not have sufficient drag to slow down early in its decent, relying on the thicker atmosphere found at lower altitudes for the majority of its deceleration.

[1] Furthermore, higher mass/drag ratios mean less mass can be allocated to the spacecraft's payload which will have secondary impacts on funding and mission's science goals.

This is why a swept aeroshell forebody as opposed to a blunt one is required; the previous shape ensures this factor's existence but also reduces drag area.

Maintaining a non-zero L/D ratio allows for a higher parachute deployment altitude and reduced loads during deceleration.

[14] Upon slowing to Mach 3.8, the 6-meter (20 ft) tube-shaped Supersonic Inflatable Aerodynamic Decelerator (SIAD-R configuration) deployed.

[18] However, it began tearing apart after deployment,[19] and the vehicle impacted the Pacific Ocean at 21:35 UTC (11:35 local) travelling 32 to 48 kilometers per hour (20 to 30 mph).

[17][20] Despite the parachute incident, the mission was declared a success; the primary goal was proving the flight worthiness of the test vehicle, while SIAD and SSDS were secondary experiments.