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The emergency return vehicle, a Soyuz I think, could either float next to the dock or be tethered nearby.

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    $\begingroup$ tethering is actually a pretty bad idea; because of how orbital dynamics work, a tethered vehicle is going to wind up spinning around the center of mass of the shared system (which will be the ISS CM) and contact ISS, which they're only supposed to do at docking ports $\endgroup$ Commented Sep 8, 2024 at 19:52
  • $\begingroup$ What do you mean by “emergency return vehicle”? Are you referring to an additional capsule with no assigned crew, to be used as a lifeboat/return vehicle in emergencies? $\endgroup$ Commented Sep 8, 2024 at 21:16
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    $\begingroup$ It would have to be quite a space pileup for that to be necessary. Doesn't the ISS have eight docking ports? $\endgroup$ Commented Sep 9, 2024 at 10:18
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    $\begingroup$ @Cadence The US side of the ISS has two docking adapters for crewed spacecraft, See space.stackexchange.com/a/2326/6944 (you can skip down to Docking Ports). Russian ISS and US ISS crewed vehicle docking systems are incompatible. $\endgroup$ Commented Sep 9, 2024 at 12:11
  • $\begingroup$ There isn't one specific capsule that is THE emergency return vehicle. All the capsules docked at the station are emergency return vehicles for their specific crews, and also their normal non-emergency return vehicles when it's time for them to leave. The recent starliner trouble means they had to basically make up a way to get those two home in case of an evacuation, which -- as I understand it -- is essentially laying in the floorboard of a Dragon. $\endgroup$ Commented Sep 9, 2024 at 20:23

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No, because any time a crew return vehicle is undocked, the crew assigned to that vehicle has to board it.

This is to guard against a failed re-docking stranding crew members on the station.

See Why are three cosmonauts required to move a Soyuz? for more detail.

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  • $\begingroup$ Can vehicles intended to dock be handled by the standard end effectors on the arm, or are these only provided on modules intended to be berthed? $\endgroup$ Commented Sep 9, 2024 at 7:52
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    $\begingroup$ @MarkMorganLloyd only venicles meant to be grappled have the grapple fixture. Thus far no vehicles have had grapple fixtures and also been able to dock, unless Shuttle had fixtures for rescue contingencies that I'm not aware of $\endgroup$ Commented Sep 9, 2024 at 11:05
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    $\begingroup$ @ErinAnne Hence anything undocked would either have to have autonomous (re-)docking capability, or crew with sufficient provisions on board. Boeing: "Give us six months to plan and test that." SpaceX: "We can organise that overnight, and if it fails we'll have collected useful data from the attempt." $\endgroup$ Commented Sep 9, 2024 at 11:33
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    $\begingroup$ @MarkMorganLloyd Doesn't really matter. If one of your lifeboat capsules was out on the end of an arm or floating autonomously, the evacuation procedure in case of an emergency suddenly goes from a few minutes of "everyone go to your assigned hatch and get in" to a complicated process of launching one lifeboat, then maneuvering the second capsule to that dock and securing it, then finally boarding and leaving. It's not going to be a few minutes, you're talking about like half an hour if not more. It makes that capsule just not a lifeboat anymore, which defeats the whole purpose. $\endgroup$ Commented Sep 9, 2024 at 20:14
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    $\begingroup$ Unless your crew return vehicle happens to be a Boeing Starliner with multiple failures making it unsafe for return. But I would think this is definitely the exception to the rule. arstechnica.com/space/2024/09/… $\endgroup$ Commented Sep 11, 2024 at 21:42
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Even ignoring the safety aspects of not having your emergency escape vehicle readily available, there's another major problem: it won't work.

"Float next to" isn't a thing in space. In order to have two objects passively stay in the same relative positions, they need to be in different parts of the exact same orbit, to sub-millimeter precision. Errors accumulate rapidly in space; if you get the velocity wrong by even a single millimeter per second, the vessel will have drifted by five and a half meters over the course of a single trip around Earth.

Tethering is an even worse idea: you've got the same drift issue, but now, when the vessel reaches the end of its tether, it's probably going to bounce or swing back and collide with the ISS.

In order to keep two spacecraft close to each other, you need active stationkeeping, and that means burning maneuvering fuel. And stationkeeping in close proximity is tricky, because you need to keep the exhaust plumes from one vehicle from striking the other one.

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    $\begingroup$ Most people simply do not understand enough about orbital mechanics to even know how it works. It really is nothing like an empty pool of water. The station is orbiting at literal breakneck speeds. I've heard of orbits being described as "Falling so fast gravity can't catch it". To keep something moving in sync at very high speeds is very tricky. Much harder than flying planes in formation. $\endgroup$ Commented Sep 10, 2024 at 2:58
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    $\begingroup$ @Nelson The fact that the ISS is moving fast relative to the ground (or ECEF or whatever) isn't really relevant to the difficulty. The ISS is moving even faster relative to the Sun or Galactic core. Just because you changed the frame of reference doesn't make the problem harder. The challenge is in the details outlined in this answer. How do you keep two objects in space close to each other with extreme precision while not having the two affect each other (via tethers or exhaust or whatever)? Especially since you must constantly thrust to correct course and maintain separation. $\endgroup$ Commented Sep 10, 2024 at 20:37
  • $\begingroup$ @zephyr The issue isn't really the frame of reference, but evidence of complex forces being exerted on the orbital objects to require said precision. Is it complicated to keep two parked cars side by side? No, because there is constant, static forces applied to it to keep it in place, like gravity, fraction, etc. People think orbits are like parking two cars in a parking lot, which it definitely is not. $\endgroup$ Commented Sep 11, 2024 at 3:10
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    $\begingroup$ @Nelson, the complex forces make the outcome of a mistake counterintuitive (you get loops and wobbles rather than straight-line motion), but the fundamental issue is that there is no friction: errors are free to accumulate indefinitely. $\endgroup$ Commented Sep 11, 2024 at 3:24
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    $\begingroup$ Although this answer is correct in what it says, this isn't really a deal-breaker for what the question is about. What you would actually do is put the vehicle in an orbit where you're sure that it's drifting away from the station, as slowly as possible. Then you'd do whatever you need to while the capsule is undocked. After this, yes, the vehicles may be kilometres apart, but they will still be in very similar orbits requiring only a small amount of Δv to rendezvous again. That will take time and fuel, but no more than nomal docking after an accurately injected launch. $\endgroup$ Commented Sep 11, 2024 at 16:12
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The emergency vehicles are for emergencies! You never know when one might happen! It's like saying, "I'll just put on my seatbelt right before a crash." The answer to your question is: You can, but it would be a very bad idea to actually do it.

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I don't think the ISS currently has a specific "emergency return vehicle", the current modus-operandi is that the vehicle that brings a crewmember to the station also acts as emergency return vehicle for that crewmember

That said, NASA has worked with spacex to develop contingency procedures for bringing home extra astronauts on a Dragon capsule in an emergency. There have been a couple of situations recently where NASA has assigned extra astronauts to a dragon capsule for emergency evacuation due to problems on other capsules.

Aside from concerns about failure to re-dock or what happens if an emergency happens while a vehicle is undocked, I see two other things to bear in mind.

  • Soyuz uses different docking ports from Dragon/Skyliner. Undocking a soyuz (with or without it's crew) would not free up a port that could be used by a Dragon.
  • Keeping things "floating" in close proximity to the station is risky, they may collide or drift apart.
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