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I was clicking through YouTubes and came across the NASA Video ISS Expedition 53 Landing in Kazakhstan (also here). I don't recall seeing such prominent and continuous smoke from returning Soyuz capsules before. Is this in fact smoke? What exactly is the source (smoldering heat shield, steam?), and is this phenomenon standard/typical?

Expedition 53 Soyuz capsule return

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It's a purge of remnant hydrogen peroxide from the "secondary control system".1 "On the “ОТСТРЕЛ ЛОБОВОЙ ТЕПЛОЗАЩИТЫ” (Bottom Shield Jettison) all УРМД thruster valves are opened and all the peroxide residuals are drained out."3

A secondary guidance, navigation and control system in the Descent Module enables the crew to maneuver the vehicle after the Instrumentation/Propulsion Module has been jettisoned. The Soyuz commander can pilot the module using a rotational hand controller that manages the firing of eight hydrogen peroxide thrusters on the vehicle's exterior. This system is deactivated 15 minutes before landing, when the parachutes are deployed. 2

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  3. Source: Soyuz Crew Operations Manual, paragraph 3.18
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  • $\begingroup$ Wow, not what I expected! So it is steam or condensed water droplets in the cold atmosphere , and not smoke? $\endgroup$
    – uhoh
    Commented Apr 8, 2018 at 0:20
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    $\begingroup$ I didn't know! Unfamiliar with hydrogen peroxide. But it appears that yes, it decomposes into water and oxygen. chemistry.stackexchange.com/questions/43233/… $\endgroup$
    – Organic Marble
    Commented Apr 8, 2018 at 0:59
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    $\begingroup$ I see what you mean. I think that hydrogen peroxide thrusters are based on expansion of water into steam heated by the decomposition of H2O2 into water and oxygen, plus the liberated oxygen. Unless they spill it on something that oxidizes and smokes (making white smoke) it seems like condensed water vapor is most likely. $\endgroup$
    – uhoh
    Commented Apr 8, 2018 at 1:03
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    $\begingroup$ Also, the first source says that they had "never seen the smoke before" (words to that effect) implying that it's rare, but I wasn't sure how to confirm that. $\endgroup$
    – Organic Marble
    Commented Apr 8, 2018 at 1:06
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    $\begingroup$ Aww. I was hoping that it was wooden heatshield. $\endgroup$
    – spender
    Commented Apr 8, 2018 at 3:04

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