The Early Space Exploration Building
houses an engineering model of the Soyuz side of the Apollo-Soyuz Test Project docking module. It
is located near the Soyuz model hanging
from the lobby ceiling.
The docking module served several functions.
The Apollo spacecraft used a probe and
drogue docking mechanism that differed substantially; the DM had an
Apollo-type docking mechanism on one end and an "international" docking
mechanism, used by the Soyuz especially for ASTP, on the other. Thus, the
DM physically enabled the two spacecraft to dock.
The Apollo spacecraft used a 100% pure oxygen atmosphere at a pressure of
about 5 psi. The Soyuz used a mixed nitrogen-oxygen atmosphere (80-20) at
about 14.7 psi. Thus, an airlock/decompression chamber was required for
The DM housed communication gear which allowed the Apollo spacecraft to
communicate on Soyuz frequencies.
Finally, the docking module provided additional space: The Apollo
spacecraft was designed to house 3 astronauts and the Soyuz typically
accommodated 2 or 3 cosmonauts. The DM provided additional volume.
It was in the docking
module that the famous handshake
in space took place.
The NTRS has operations handbooks for docking module DM 1 and docking
module DM 2.
DM 2 was the docking module which flew on the mission; since the National Air & Space Museum exhibits the
backup docking module in their ASTP
display, I assume that it is DM 1.