The Russian Soyuz vehicle has made about 100 manned orbital flights since 1967. Like the U.S. Apollo spacecraft and the future Orion capsule, it consists of a crew cabin and a rear-mounted equipment module containing its propulsion system. But unlike those craft, it also includes a forward-mounted habitation cabin. When returning to Earth, all these spacecraft perform a de-orbit rocket burn, cutting about 125 meters per second off their 8000 m/s orbital velocity. They then jettison their attached modules and turn their heat shields into the wind. The atmosphere slows the vehicle—it loses most of its speed in a matter of minutes as it decelerates to three to four times the normal acceleration due to gravity (which is 9.8 m/s²). When the craft is low and moving slowly enough, it deploys parachutes for a soft surface touchdown that is cushioned by a brief braking rocket burst.

But this time, something seems to have gone wrong with the mechanical separation of the Soyuz from its aft equipment module. Separation is supposed to occur 32 minutes after the de-orbit burn, as the craft is descending toward the sensible atmosphere, a point 100 km up where aerodynamic effects start to matter. Something in the pyrobolts or cable cutters malfunctioned, and the crew felt the cabin shuddering as it was jerked around by a partly loose massive equipment module.

SOURCE: NASA documents

SOYUZ SCHEMATIC: The details of a Soyuz.

The still-linked modules entered the sensible atmosphere but at the wrong orientation. Air drag would have turned the ship like a shuttlecock, directing its lightly shielded nose (with the precious parachutes) into the onrushing plasma. Several minutes of this thermal stress would have been enough to breach the hull and possibly destroy the vehicle.

But at some point, the aft section did tear free, probably because of the mounting aerodynamic torques and heating. This may have been no mere lucky break. Russia suffered a series of near disasters early in its program, including Yuri Gagarin's first mission in 1961 and a chillingly similar Soyuz nosedive in January 1969. So Russian engineers apparently modified module-to-module connectors to tear loose, if needed, under such entry stresses, even if the separation system fails entirely. If that modification was indeed made-which is unknown outside of Russian space circles-this may have been the first time this fail-safe system was flight-tested.