# Did the Space Shuttle crew have to worry about monatomic oxygen?

Comments below this answer tell us that the Space Shuttle always remained in Earth's atmosphere. When it visited the Hubble Space Telescope or the ISS or Mir it was still in the thermosphere and simultaneously the ionosphere.

Above the turbopause the oxygen component of Earth's atmosphere changes from diatomic (O2) to monatomic (O), while nitrogen remains mostly diatomic. The most noticeable effect of this is the "knee" at about 100 km altitude where the scale height doubles, i.e. the exponential rate that the density decreases with height drops to about half, i.e. the scale hight increases significantly, because O is lighter than O2.

Believe it or not, monatomic oxygen is the primary constituent of the atmosphere between about 175 and 450 km!.

Monatomic oxygen is a drag.

Of course the crew of any spacecraft in LEO will need to understand the impact of atmospheric drag since it has a continuous and significant effect on orbit altitude and phasing, and spacecraft attitude and tilt of solar panels can have a significant impact on that.

Question: But what (if anything) were crew taught about monatomic oxygen and its interaction with the Space Shuttle besides the increased drag? Did they have to worry about spacecraft erosion due to the chemical activity? On long missions it "eats" spacecraft parts, especially things like plastics, organics, things that can oxidize and will suffer from it. Were there any materials on the outside of the shuttle, or of space suits, or of any other equipment or items used that were vulnerable to monatomic oxygen such that the crew needed to be aware of it?

note: the three thin lines are simple scale-height plots with $$h_{scale}$$ of 6.5, 7, and 7.5 km, bottom to top, just for reference.

Volume fraction of the main constituents of the Earth's atmosphere as a function of height, based on the MSIS-E-90 atmospheric model.

Source

• – uhoh
Oct 6, 2020 at 15:38
• Can't recall much ever being said about this for any purposes beyond informational... Oct 6, 2020 at 18:46
• @BowlOfRed I know what you mean and yes, this is intentionally narrow for several reasons; 1) maintain parity with the ionosphere question in order to establish relative importance of the two 2) address the history and evolving nature of spaceflight knowledge, 3) the wide variety of Shuttle missions and exposure of objects and astronauts to space compared to ISS's more static nature 4) it's a lot of work to review all large array of existing posts were monotomic oxygen's effects are addressed in various ways in order to avoid a duplicate. It should be done but I can't this week.
– uhoh
Oct 6, 2020 at 21:35
• @Digger Thanks for the feedback! I understand you didn't pilot the Hubble itself (humor), but there's always a chance you might have run across something related to What were the relative orientations of the Hubble Space Telescope's three star cameras, six rate gyros and four reaction wheels optimized for exactly? (needs a better answer)...
– uhoh
Oct 7, 2020 at 15:12
• Sorry, can't provide definitive answers to either...I just helped get us there and back... Oct 7, 2020 at 15:29

Obviously, @Digger can provide a first-hand crew experience answer, but in general no.

Atomic oxygen (commonly called just AO) is a designer's problem that manifests primarily in the restriction of materials that are used on the exterior of the vehicle. This is a much bigger deal on ISS than it was on Shuttle. In general the erosion rate for most spacecraft polymers is measured on the order of tens of microns per year at ISS orbit altitude. Interestingly, silver erodes at a rate one to two orders of magnitude higher than, e.g., Kapton. This is why you never see silver or silver-plated materials directly exposed to the space environment in LEO.

Put another way -- for AO to become a crew concern would imply a serious, fundamental engineering failure in the design of the spacecraft they're in.

• "... primarily in the restriction of materials..." leaves the door open for the possibility of other effects specific to AO secondary to its chemical reactivity. Is that there because there are other effects that you know of that you haven't mentioned yet, or is it there because you just can't rule anything else out?
– uhoh
Oct 7, 2020 at 17:14
• @uhoh Not so much that as there may be cases where AO-incompatible materials need to be used outdoors. For a lot of materials, they may be vulnerable to ram AO (i.e., AO that you are blasting into head-on, so you are getting the kinetic energy of the impact in addition to its baseline reactivity), but not vulnerable to thermalized AO. In those cases, as long as they are not directly exposed to the ram direction, they may be fine. Oct 8, 2020 at 0:16