An aeroshell is a rigid heat-shielded shell that protects a vehicle from pressure and heat created by drag during atmospheric entry (see blunt body theory), slows it down during entry, and may protect it from debris during spaceflight. 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.
Aeroshells are a key component of interplanetary space missions. They were used in the Apollo program to the moon in the 1960s and 1970s, the 1975 Viking program to Mars, the 1997 Mars Pathfinder mission, the 2003 Mars Exploration Rover missions, the 2007 Mars Phoenix mission, and are scheduled for use in the Mars Science Laboratory mission in late 2011.
NASA's Planetary Entry Parachute Program (PEPP) aeroshell, tested in 1966, was created to test parachutes for the Voyager Mars landing program. To simulate the thin Martian atmosphere, the parachute needed to be used at an altitude more than 160,000 feet above the earth. A balloon launched from Roswell, New Mexico was used to initially lift the aeroshell. The balloon then drifted west to the White Sands Missile Range, where the vehicle was dropped and the engines beneath the vehicle boosted it to the required altitude, where the parachute was deployed.
The Voyager program was later canceled, replaced by the much smaller Viking program several years later. NASA reused the Voyager name for the Voyager 1 and Voyager 2 probes to the outer planets, which had nothing to do with the Mars Voyager program.
There is one PEPP Aeroshell left, at White Sands Missile Range in New Mexico.