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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?

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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.

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