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CNN's August 19, 2024 SpaceX is about to send four people on a wild — and risky — mission into the radiation belts. Here’s what to know includes:

Almost immediately after reaching space, the Polaris Dawn crew will begin a “pre-breathe” process to prepare for the spacewalk. It’s akin to what scuba divers do to avoid decompression sickness, otherwise known as “the bends.” The crewmates must purge nitrogen from their blood so that when the Dragon capsule is depressurized and exposed to the vacuum of space, the gas doesn’t form bubbles in their bloodstream — a potentially lethal condition.

“We don’t have an airlock on this mission,” Gillis told CNN, referring to the areas on board the International Space Station (ISS) that serve as special decompression chambers for astronauts heading out for a spacewalk. Polaris Dawn will instead take “a really novel and different approach” to the pre-breathing process that involves “slowly decreasing cabin pressure and raising oxygen concentration.”

Unlike any pre-breathe attempted on the International Space Station, the process will take roughly 45 hours — nearly two days, said Gillis, who works as a lead space operations engineer at SpaceX and trained the Inspiration4 crew for their mission.

Answers to How long were the Apollo astronauts allowed to breathe 100% oxygen at 1 atmosphere continuously? indicate that oxygen toxicity is a real thing and becomes a problem as the duration increases, and suggest to me at least that you'd like to keep exposure to high concentrations of oxygen even at sub-atmospheric pressures relatively short, so I'd like to ask:

Question: Why will the Polaris Dawn "pre-breathe" be so long (45 hours) and gradual? What are the proposed advantages over a short one?

The technique seems to have some scheduling challenges if applied to say space station maintenance. You can't go back out again a day later if you forgot something for example, and everyone in the spacecraft has to do it together it seems, so where is the advantage?

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  • $\begingroup$ companion question (related but different) What (if any) pre-breathes were "attempted" on the ISS, and why? $\endgroup$
    – uhoh
    Commented Aug 23 at 0:56
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    $\begingroup$ It's not really novel. Shuttle had an EVA prebreathe protocol where they took the cabin to 10.2 psi for 24 hours prior to the EVA and often stayed there. For example, on STS-088 they did the 10.2 depress 19 hours into the mission and stayed there until 7 days, 20 hours into the mission. See your other question for prebreathe rationale. $\endgroup$
    – Organic Marble
    Commented Aug 23 at 1:31
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    $\begingroup$ It’s not clear to me what pressure they’re going to be at, or not that there’s danger in longer exposures to pure O2 at low enough pressures (e.g. 0.2-0.3 bar). $\endgroup$
    – fyrepenguin
    Commented Aug 23 at 1:44
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    $\begingroup$ The technique is being used on a small capsule which does not have an airlock, so the answer to why they are using this method is likely due to the specific type of scenario. The method might not have any advantages that would apply to a larger vehicle and/or one which has an airlock. But it would be interesting to know if there is. $\endgroup$
    – Steve Pemberton
    Commented Aug 23 at 13:31
  • $\begingroup$ related: How will the Polaris Dawn cabin pressure and oxygen partial pressure dovetail with that of their EVA suits? (100% oxygen?) $\endgroup$
    – uhoh
    Commented Sep 11 at 6:55

2 Answers 2

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It may be due to the difficulties in navigating the atmospheric composition and pressure from mostly high pressure nitrogen with 21% oxygen to low pressure oxygen with no nitrogen whilst avoiding too much or too little oxygen (hyperoxia and anoxia) at the wrong pressure or nitrogen narcosis (lots of nitrogen at low pressure). The problems would be exacerbated by not having an air lock meaning the not inconsiderable volume of the whole capsule needs to be adjusted. And they are probably being hyper safety conscious as well.

https://cdn.mos.cms.futurecdn.net/Gu9APTXv293FqfjwbW2zWf-1200-80.jpg

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  • $\begingroup$ I think also possibly exacerbated by the small volume of the cabin, making managing the composition more difficult than it would be in a larger volume. So considered safer as you mentioned to take it slower. $\endgroup$
    – Steve Pemberton
    Commented Aug 23 at 13:35
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Partial Pressure of O2 is What Matters

[...] indicate that oxygen toxicity is a real thing and becomes a problem as the duration increases, and suggest to me at least that you'd like to keep exposure to high concentrations of oxygen even at sub-atmospheric pressures relatively short [...]

The emphasized part is not quite right.

The partial pressure of oxygen in normal air is 0.21 bar or 160 mmHG. This means that, as long as you do it slowly, you can set atmospheric pressure nearly anywhere you want below 1 bar and it'll be ok, as long as you adjust the gas mixture to keep partial pressure of O2 near 160 mmHG.

The medical conditions that Uwe mentions in his answer to your linked question occur at much higher partial pressures of Oxygen. The first occurs in a 100% oxygen atmosphere at 0.5 bar after 24 hours - meaning nearly 400 mmHG O2.

And that leads directly to my guess for why the soak is so long; they're probably adjusting atmosphere, waiting for measured pressure and O2 to stabilize, and then adjusting again.

They've got a lot of nitrogen to remove, but it's also important to remove it at the correct rate, and mixing rate and sensor lag might be a concern.

Fire is the Risk

The actual health risk of a low total pressure / high O2 environment is the risk of fire, but that's a whole separate question.

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