
This mission-critical porthole was used for docking with the ISS and for reentry burn orientation, bringing space tourist Greg Olsen back to Earth.
The porthole is made of multilayered high temperature glass with a heat resistant composite and titanium alloy frame, which is engraved and stamped with registration, identification and serial numbers. It mates with the external periscope Vzor (see photos below).
When I showed it to astronaut Ed Lu, he immediately recognized it, since he used and photographed the equivalent on TMA-2. The other end of the periscope can tilt almost 90°. When it is pointing forward, it is used for docking, with a mechanical alignment reticle much like was used on Apollo with the window-mounted COAS.
When the periscope is rotated to point away from the craft, it provides a mechanical means of orienting the craft for a reentry burn. To get the alignment right, the earth should be moving in a straight line from top to bottom in the central window. The eight peripheral windows are connected to prisms, creating a wide-angle lens array. The flight director needs to make sure that the perimeter of the Earth runs through the center of all of the eight windows.
So, even if all flight electronics have failed, the mission critical operations of docking and reentry can be accomplished with unpowered optics.









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