Question: How could a hydrazine and N2O4 cloud (or it's reactants) show up in weather radar?


This is what an ‘anomaly’ looks like on weather radar.

Tweet (with GIF):

Dragon's static fire anomaly was big enough to show up on radar! 🚀💥📡☁️

#SpaceX #CrewDragon #FLwx #LowCC (Non-meteorological radar signature)

Scott Manley's video cued at 00:40:


Hydrazine has a lot of spectral structure in the microwave regions (see paper), as does dinitrogen tetroxide (sorry, only paper I have is paper). This means that they’re pretty good at absorbing and reemitting, hence reflecting, microwave radar energy. Reflections are what the radar displays

Off hand, hydrazine has such a thicket of strong lines that it’s probably a much better reflector (per gm/cm2) than water. It’s probably not even necessary for droplets to form. That means a smaller amount of total mass can give a noticeable signal.

  • $\begingroup$ I'm guessing that of the major constituents of air (N2, O2, Ar) there shouldn't be any strong interaction, but how does Hydrazine compare to the triatomics H2O and CO2, especially since water is so polar? Just curious. I can't see the paper so I don't know what it says. $\endgroup$
    – uhoh
    Apr 21 '19 at 17:31
  • 2
    $\begingroup$ In general, more complex molecules have more rotational and vibrational modes, many (most) of which can interact with microwaves of the correct frequency. More modes -> more cross-section, as the all work independently. The diatomic molecules don’t have nearly as many, and they tend to be at higher frequencies. $\endgroup$ Apr 21 '19 at 17:35

Weather radar is not specifically measuring clouds or water in the atmosphere. What is measured is the amount of microwaves in the 5 GHz range reflected by anything in the atmosphere. This can be water droplets, but as well be insects, birds or any other kind of droplets, like propellant that didn't stay where is was supposed to be.

  • $\begingroup$ by coincidence $2N_2H_4 + N_2O_4 \rightarrow 3N_2 + \mathbf{4H_2O} $ but unreacted propellants can form droplets as well. $\endgroup$
    – uhoh
    Apr 21 '19 at 11:57

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