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In very low Earth orbit there are (at least) two problems; aerodynamic drag, and atomic oxygen (O rather than O2). Drag can be compensated with a low thrust from an ion engine.

Question: But what exactly makes atomic oxygen "bad". How exactly does atomic oxygen cause problems for spacecraft in VLEO? I assume it has something to do with the higher reactivity of single oxygen atoms compared to oxygen molecules, but what are the specific problems?

Does it just slowly eat the whole spacecraft, or are there specific materials or devices that are particularly sensitive?

Does it get inside and munch on insulation and rubber seals like the Andromeda Strain?

People might want to put Earth-viewing telescopes in VLEO to be closer. Since the glass in lenses and reflective aluminum coatings on mirrors are pretty much fully oxidized, is it the thin optical coatings on their surfaces that get "eaten" for example?

uhoh
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1 Answers1

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It literally eats away at surfaces made of certain materials. It's pretty crazy. According to Space Mission Analysis and Design ("SMAD", 3e, by Wertz and Larson), Atomic oxygen (which I'll call ATOX thanks to user1209304's comment), is "the predominant atmospheric constituent" from 200 to 600km, and is a dominant force above 170km (around which it actually attains a maximum in terms of number of atoms per cubic meter). As a spacecraft moves through its orbit, it encounters a flux of oxygen atoms, which react with surfaces.

Kapton (again from SMAD), will degrade at a rate of 2.8 μm for every $10^{24}$ atoms/m$^2$ of atomic oxygen time-integrated flux density encountered, with silver degrading much faster (somewhere around 10-20 μm). For context, the JWST sunshield's layers can be less than 25$\mu$m thick, although of course the JWST is not in LEO.

The ATOX flux is given by:

$$F_O = \rho_N VT$$

Where $\rho_N$ is the density in atoms per cubic meter, $V$ is the spacecraft velocity, and $T$ is the time interval (makes sense - speed * time is meters, $\rho_N$ is a volume measurement, so $F_O$ gives atoms per square meter).

As an example, $F_O$, for a spacecraft in a 7km/s, 200km orbit, over a year long mission gives approximate value of $F_O$ of $2\times 10^{23}$ ($\rho_N$ value from SMAD). This would degrade several nanometers of Kapton and several mircometers of silver over the course of that year long mission.

Since it is reactive, ATOX also causes some unwanted oxides to be created as this degradation is taking place. These oxides are problematic in and of themselves (in that your surface isn't pure anymore), but also because they are "radiatively active" (SMAD) - that is, they have unwanted thermal radiation characteristics, which can interfere with optical sensors.

uhoh
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Michael Stachowsky
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    Here's a beautiful picture of atomic oxygen glow on the shuttle : https://i.pinimg.com/originals/4a/c6/07/4ac60761a7301264f8a0fb2f19114366.jpg – Organic Marble Oct 23 '19 at 13:49
  • That is neat. What exactly is it? ATOX reacting with the shuttle (I hope not...) or ATOX at the shuttle's surface illuminated in some way? – Michael Stachowsky Oct 23 '19 at 13:49
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    It was most likely reacting with traces of propellants deposited on the tiles https://www.nature.com/articles/354048a0 – Organic Marble Oct 23 '19 at 13:51
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    @uhoh: edited. It is thickness of material lost given a flux per square meter – Michael Stachowsky Oct 23 '19 at 17:02
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    @uhoh: I see your edit. Just want to make sure it's clear what is meant: if I have a 1$m^2$ piece of Kapton that encounters a total of $10^{24}$ atoms of oxygen that is more or less evenly distributed across that surface, then the kapton will be 2.8$\mu$m thinner across its entire surface. Hopefully that comes across in the answer? – Michael Stachowsky Oct 23 '19 at 17:06
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    NASA conference publication 3257 LDEF Results for Spacecraft Applications contains several papers on atomic oxygen effects, but I can't seem to find it online atm. Guess I'll toss it in the "to be uploaded" queue. – Organic Marble Oct 24 '19 at 01:43
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    Here's a very in-depth paper on the interactions of atomic oxygen on spacecraft parts. @OrganicMarble – Magic Octopus Urn Oct 24 '19 at 16:55
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    Further expanding this, in general most AO exposure concerns are from ram AO, where the atomic oxygen is encountered at high velocity, which provides the necessary activation energy to initiate the reactions that cause erosion. Thus, for most materials, the threat is highly directional on an orbiting spacecraft. Silver is a special case, though. Even thermalized AO (i.e., where the velocity-associated kinetic energy is nullified) will oxidize silver, and unlike other metals where the oxide serves to passivate, silver oxide sloughs off continuously, exposing new metal. – Tristan Oct 24 '19 at 17:51
  • @Tristan wow, that's so much useful information that readers might be better served with it in an answer. $$\frac{1}{2} 16 \times 938 \times 10^6 \text{ eV c}^2 \times \left(\frac{7670 \text{ m/s}}{c} \right)^2$$ is almost 5 eV! Add $k_B T$ and the distribution could go past 6 eV. I never realized that orbital velocity was high enough to drive chemical reactions, cool! – uhoh Oct 25 '19 at 00:08
  • @MagicOctopusUrn ditto; why not add an answer with critical information from that paper? – uhoh Oct 25 '19 at 00:09
  • @OrganicMarble ditto of the ditto – uhoh Oct 25 '19 at 00:10
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    @MichaelStachowsky I further adjusted the edit. I think this is a standard, conventional way to write it. $10^{24}$ atoms/m$^2$ is understood as a unit of time-integrated flux density, as would 2.8 μm be understood as a unit of thickness loss. To double check on this, I've just asked How does the term "time-integrated flux density" apply to the effluence of a stream of atoms on a surface? in Chemistry SE. – uhoh Oct 25 '19 at 00:22