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Following the recent Soyuz failure news, I found out that the currently docked Soyuz expires in the foreseeable future, leaving the crew with no means to return back to Earth.

What are the reasons behind Soyuz having a limited lifespan in orbit? Does it mean it's not safe to fly it after the expiration date, or is it not possible at all?

Steve Linton
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Denis Kulagin
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1 Answers1

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Scott Manley answers this question here.

Basically, it is due to the "shelf life" of the hydrogen peroxide propellant used by the attitude control thrusters of the Soyuz descent module. When hydrogen peroxide is passed over a heated catalyst, it decomposes to water and oxygen, and releases lots of energy, so the propellant goes from liquid to steam/gas, providing the thrust. But hydrogen peroxide will also slowly break down spontaneously, so the stored propellant will gradually lose its potency.

The 200 day value is a threshold of safety: decomposition hasn't progressed too far to affect usability of the peroxide propellant to perform a controlled re-entry, landing within a targeted area.

He also goes on to describe the consequences of an uncontrolled (ballistic) re-entry - not a disaster, but definitely undesirable because of G forces imposed on the crew and unpredictability of the actual landing point.

Anthony X
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    Confirmed in this reference: http://space.nss.org/media/Space-Manufacturing-conference-11-144-Soyuz-Spacecraft.pdf – Organic Marble Oct 13 '18 at 21:07
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    As he said, they have a few options for how to solve this. I wonder if they will end up going for the ballistic re-entry. – Tim Oct 13 '18 at 23:21
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    @Tim If Soyuz can be flown/docked automated, an option might be to fly the next one up uncrewed to take over as lifeboat, and return the current one empty. Wouldn't have to wait for full safety checkout, just need enough confidence that it is likely to succeed. Could make use of the opportunity for resupply. Ballistic re-entry then becomes a fallback if that launch attempt goes wrong. – Anthony X Oct 13 '18 at 23:40
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    @AnthonyX as Manley says it would also act as a demonstration that Soyuz isn’t inherently flawed, which might speed up any reconfirmation process. On the other hand it might fail and then we have no human rated craft until April. I suppose the current crew could extend their stay somewhat with additional supplies from falcon 9 & dragon 1. It would be a shame if the ISS was left unmanned. – Tim Oct 14 '18 at 00:00
  • @AnthonyX Does it serve any purpose to have the capsule return empty? If I understand correctly it is going to be scrapped anyway, so it might as well stay for as long as it could theoretically be used as plan C og D for a safe return. – kasperd Oct 14 '18 at 13:29
  • @Tim What would such a launch demonstrate that hasn't been demonstrated by the large number of earlier Soyuz launches? – kasperd Oct 14 '18 at 13:30
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    @kasperd that there isn’t a fault in their more recently manufactured capsules (I’m thinking about the hole drilled in the one they’ve got, this recent failure) – Tim Oct 14 '18 at 13:31
  • @kasperd I assumed there would be limited space to dock or berth an extra Soyuz. – Anthony X Oct 14 '18 at 14:01
  • @AnthonyX Surely if they send an extra to dock the current one would not undock until the replacement vehicle has been confirmed to have docked safely. But if they run out of docking space that would be a reason for a capsule to return empty. (I don't know how many Soyuz can dock simultaneously, but it must be at least two) – kasperd Oct 14 '18 at 14:11
  • I've just asked Does the Soyuz spacecraft chemically stabilize the hydrogen peroxide (H2O2) to remain ISS-dockable for 200+ days? – uhoh Oct 14 '18 at 15:21