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During re-entry, SpaceX's Starship flight IFT-4 video stream showed an initial reddish glow (link to cut video stream, not official video). However, later on, the color of the glow turns purple. At this point, there doesn't seem to be any burning, sparks, or any other sign of chemical interaction with Starship's skin. From my understanding, the blackbody spectrum doesn't get to purple; rather, it should go from red, yellow, then white (and then, maybe blue-ish). What is there to explain the purple color of the glow?

Starship initial red glow

Purple glow later on

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    $\begingroup$ I'm not sure, but the obvious answer for why the color does not follow the black body spectrum is that the radiation is not black body radiation. For example, it might be excitations of atoms, e.g. when excited hydrogen drops down to its normal energy state, it emits purple-ish light. $\endgroup$ Commented Jun 9 at 1:03

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Upon reentry, space vehicles have incredible amounts of kinetic energy which must be dissipated, typically through heat. Contrary to popular belief, this is typically not just from friction (e.g. atmospheric drag), but instead from a shock wave formed by this immense kinetic energy compressing the atmosphere in its path. Along with atmospheric drag, this compression heats the air in the path, causing reentry heating, and most notably, a plasma sheath to form around the vehicle. This plasma sheath consists of ionized air, which tends to glow bluish-purple (also see: Why do electric sparks appear blue/purple?; this also applies to lightning and ionized air in general).

My cursory research has not shown me much on why the color of the plasma shifts, but I believe it is due to a combination of differences between camera settings between the two camera views shown on this Starship flight, as well as the differing composition of the atmosphere with respect to altitude. Lower in the atmosphere (81km in the image showing purple plasma), there is likely more water vapor, and thus more ionizable hydrogen, which would lead to a purpleish hue due to hydrogen's emission spectrum, whereas higher in the atmosphere, the color would likely be primarily due to emissions from oxygen (whether monatomic, diatomic (typical oxygen gas), or triatomic (ozone)). However, this is just an estimated guess and may not be the actual cause. I invite anyone with more knowledge to add to this answer or correct me!

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    $\begingroup$ Scott Manley made a comment about it (timestamp 11:11) but he didn't go into much detail, saying only "you can see how the glow has changed colors as it's got deeper and deeper and the plasma is getting denser and denser". Whether that was due to different molecular combinations at lower altitudes or just the higher density changing the color effect in some way wasn't specified. He's pretty knowledgeable on topics like this so I couldn't tell if he knew the reason but just didn't go into that level of detail. $\endgroup$ Commented Jun 9 at 8:09
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    $\begingroup$ Isn't nitrogen's spectrum pretty much purple? $\endgroup$ Commented 2 days ago
  • $\begingroup$ So can someone clarify and provide a more complete answer? CC: @DarthPseudonym $\endgroup$ Commented yesterday
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What you're seeing there is ionized nitrogen, the same as you'd see from an electric discharge:

Electric discharge Image by Iantresman at English Wikipedia. - Transferred from en.wikipedia to Commons., CC BY 2.5, Link

The compression from reentry heats the air, so you get thermal emission, yes; but the shock is also ionizing the air in front of Starship, which rips some electrons off the atoms but leaves others in an excited state, kicked into a higher orbital without being removed entirely. The electrons then drop down to their base state and emit light. Nitrogen emits primarily red and blue light, which we see as purple, and since the atmosphere is dominated by nitrogen, that's what ionized air looks like.

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