To confirm the identity of a 1966 Centaur booster returning near earth in 2020, NASA took some spectroscopy observations. They didn't match those of the same metal (301 stainless steel) on earth, apparently due to the booster's 54 years in "harsh" space. But they did more closely match observations of a similar 1971 booster that stayed near Earth.

How/why did the steel's spectral signature change? Something about radiation rather than chemical reactions? Heating/cooling cycles? Outgassing of trace carbon and nitrogen?

Did it really change so much that it could be confused with something like a metal-bearing asteroid? High-chromium steel, of whatever type and in whatever condition, is quite different from the usual nickel-iron stuff we see out there.

(Addendum: does this make stainless steel a poor choice for missions longer than a decade or so?)

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    $\begingroup$ What's taking so long getting that space electron microscope (1, 2) up there so we can find out? Can we all chip in 10% of our Space SE reputation points, convert to bitcoin and crowdsource a Thrown-together $200 million mission to asteroid 2020 SO; check out or nudge to longer-lasting mini-moon orbit $\endgroup$
    – uhoh
    Dec 2, 2020 at 23:19
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    $\begingroup$ Should we make 2020-SO a tag yet? $\endgroup$ Dec 2, 2020 at 23:38
  • $\begingroup$ This might turn out to be an active object of discussion here, but that might not turn out to be the best name for the tag if it's really a rocket body (which it seems that it is). I think you could just go ahead and post that as a new question in meta. Just fyi nothing yet on the Reddy group's news page but this is the infrared telescope that they used. $\endgroup$
    – uhoh
    Dec 2, 2020 at 23:54
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    $\begingroup$ @uhoh No need to send an electron microscope. Just send a \$20 webcam (well, perhaps \$40 given the current webcam market) and then use advanced image analysis techniques (the stock Gallery app on your phone plus a couple of Mk 1 Eyeballs) to tell if it has the shape of a Centaur and a US flag and/or a NASA logo on it. $\endgroup$
    – TooTea
    Dec 3, 2020 at 13:48
  • $\begingroup$ @TooTea ya okay, but I wanna know what it's been through and how the surface has been modified, not what it is. I wanna know that even if it's an alien spacecraft, it doesn't matter to me in the least if it's got NASA logos or something unrecognizable! $\endgroup$
    – uhoh
    Dec 3, 2020 at 13:52

2 Answers 2


The spectral data came from the surface of the material only a few atoms thick which is exposed to hard vacuum. The solar wind has ions of many materials. It is mostly hydrogen, which as an ion is just a proton. Or hydrogen atoms. Either one can react. The solar wind is not very energetic but it contains small amounts of other elements like oxygen and nitrogen, as well as alpha particles which in this case are just helium ions.

Hydrogen is so small that it can slip in among the iron and chromium and nickel and carbon of stainless steel and cause embrittlement that weakens it, among other effects (which is why hydrogen pipelines are not common). Things that change the properties of a material will change its spectrum, and may also show a long term effect from the solar wind that is not the same as surface reactions or collections.

Given enough time the Solar Wind should tarnish or in some way chemically effect the surface of steel. Also, the extreme heating and cooling cycles might also change the steel structure with atomic migration.

  • $\begingroup$ I think that this should be the accepted answer; it contains all the essential points without all the distracting elements in my answer. +1 for a concise yet complete answer! $\endgroup$
    – uhoh
    Dec 3, 2020 at 23:49
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    $\begingroup$ Thanks. I should add something about hydrogen and steel. $\endgroup$ Dec 4, 2020 at 3:27

Light interacts with fresh metal surfaces in only the first few atomic layers. What makes metals "metals" is the very high electron density, and we can think of that electron "plasma" as having such a high plasma frequency that the light barely penetrates a tiny fraction of a wavelength before being re-radiated backwards by all those electrons vibrating along with the incident electric field.

See Wikipedia's Skin effect. In the plot below we can see that even at a radio frequency of 1 MHz an electromagnetic wave's fields will have dropped by 1/e in only 10 microns when incident on a smooth, polished surface of 304 stainless steel (we can assume 301 to be similar). It drops as $1/ \sqrt{f}$ so for red light of 600 nm or 5E+14 Hz that line would reach about 1 Angstrom. We can't do that because we have to take microscopic plasma density effects and other goodies into consideration, but it works out about right. If we wanted a more accurate answer we'd have to look up the complex index of refraction $n + ik$ and then calculate the attenuation coefficient.

But I digress

because the surface of a rocket ship is not an atomically smooth polished surface even before launch. Even for stainless steel will be some contaminants adsorbed on the surface and some of the impurities will be oxidized, those "nooks and crannies" of realistic surfaces will have wavelength-dependent scattering effects.

Put it in deep space for 50 years and the effects of micrometeorites will modify the surface and that of ultraviolet light and the continuous onslaught of charged particles and neutrals from the Sun will have implanted and modified the top several tens of nanometers of the surface so much as to make the electronic structure and optical response very different than stainless that's remained on Earth.

The operative concept here

is "the top several tens of nanometers" which is pretty much all that the solar wind will effect.

This has no effect on 304 stainless steel's structural properties.

If it were a front-surface telescope mirror or even a dish for a sub-millimeter radio telescope, it would matter optically, but not structurally.

Wikimedia's File Skin_depth_by_Zureks.png

Wikimedia's File Skin_depth_by_Zureks.png

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    $\begingroup$ So the skin has absorbed enough solar wind particles to Look Filthy to a spectrometer's sensitive eye, at least compared to pristine steel; exactly how filthy and in what way, probably we didn't know until 2020-SO itself was measured. $\endgroup$ Dec 3, 2020 at 1:43
  • $\begingroup$ @CamilleGoudeseune you didn't include any quoted material from your link, but it says only that "While not immediately a perfect match..." I don't know if "looks filthy to a spectrometer's sensitive eye" is a proper description of the difference or not, but I certainly think we should have scrambled that mission to find out exactly that! :-) $\endgroup$
    – uhoh
    Dec 3, 2020 at 2:01
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    $\begingroup$ "Keep the contaminants, ya filthy Centaur." $\endgroup$ Dec 3, 2020 at 14:32
  • $\begingroup$ If the effects are in optical wavelengths, could we see the effects? $\endgroup$ Dec 3, 2020 at 16:32
  • $\begingroup$ Yes, because 50 nm deep is a sizeable fraction of an optical wavelength. Sea waves with wavelength, say, 20 m, reflect quite differently from a smooth concrete wall than from one fronted by 2 m of boulders. $\endgroup$ Dec 3, 2020 at 21:24

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