theory (this specific aerodynamic instability occurs below hypersonic speeds). For a nitrogen atmosphere (Earth or Titan), the maximum allowed half-angle is approximately 60°. For a carbon dioxide atmosphere (Mars or Venus), the maximum-allowed half-angle is approximately 70°. After shock wave detachment, an entry vehicle must carry significantly more shocklayer gas around the leading edge stagnation point (the subsonic cap). Consequently, the aerodynamic center moves upstream thus causing aerodynamic instability. It is incorrect to reapply an aeroshell design intended for Titan entry (Huygens probe in a nitrogen atmosphere) for Mars entry (Beagle 2 in a carbon dioxide atmosphere). Prior to being abandoned, the Soviet Mars lander program achieved one successful landing (Mars 3), on the second of three entry attempts (the others were Mars 2 and Mars 6). The Soviet Mars landers were based upon a 60° half-angle aeroshell design. A 45° half-angle sphere-cone is typically used for atmospheric probes (surface landing not intended) even though TPS mass is not minimized. The rationale for a 45° half-angle is to have either aerodynamic stability from entry-to-impact (the heat shield is not jettisoned) or a short-and-sharp heat pulse followed by prompt heat shield jettison. A 45° sphere-cone design was used with the DS/2 Mars impactor and Pioneer Venus probes. Notable atmospheric entry accidents Not all atmospheric reentries have been completely successful: Voskhod 2 – The service module failed to detach for some time, but the crew survived. Soyuz 5 – The service module failed to detach, but the crew survived. Apollo 15 - One of the three ringsail parachutes failed during the ocean landing, likely damaged as the spacecraft vented excess control fuel. The spacecraft was designed to land safely with only two parachutes, and the crew were uninjured. Mars Polar Lander – Failed during EDL. The failure was believed to be the consequence of a software error. The precise cause is unknown for lack of real-time telemetry. Space Shuttle Columbia STS-1 – a combination of launch damage, protruding gap filler, and tile installation error resulted in serious damage to the orbiter, only some of which the crew was aware. Had the crew known the true extent of the damage before attempting reentry, they would have flown the shuttle to a safe altitude and then bailed out. Nevertheless, reentry was successful, and the orbiter proceeded to a normal landing. Space Shuttle Atlantis STS-27 – Insulation from the starboard solid rocket booster nose cap