The question Orbit Guardians - bs, right? mentions a company proposing a smallsat that will capture then give a roughly 200 m/s retrograde "kick" to liquid metal NaK alloy droplets in LEO circa 800 to 900 km, apparently leaked by some old nuclear reactors still in orbit.

Apparently these droplets are believed to be liquid not solid, but I think that would depend on several things, including their chemical composition (a wide range of ratios are soluble) and their emissivity = 1 - albedo.

Pristine metal is presumably shiny with very high albedo $A$, but they could have space dust, dirt or other non-soluble contaminants their surfaces making them darker.

For a sphere (blob of metal, planet, or otherwise) the equilibrium temperature is given as:

$$T_{Eq} = \left(\frac{I_0 (1-A)}{4 \sigma} \right)^{1/4}$$

where $I_0$ is the solar intensity at 1 AU of about 1361 W/m2 and $\sigma$ is the Stefan-Boltzmann constant 5.67E-08 W/m2/K4.

For example:

albedo      T_Eq (K)      T_Eq (C)
 0.1          271           -2
 0.5          234          -39
 0.9          157         -116


  1. How much is known or at least generally believed to be true about those liquid metal droplets orbiting the Earth? (e.g. sizes, composition (stoichiometry), contamination, albedo, temperature...)
  2. Are any actually tracked? Are any specific orbits known? Or would a spacecraft have to go out there and scour the orbits of old nuclear reactors looking for them?
  3. Have any individual droplets ever been detected by any means, or are they strictly deduced to exist?

While individually small, the 480,000,000 West Ford dipoles needles made a "shiny" radar target. These droplets are old and spread out and not so numerous, so it's conceivable that they've never actually been observed.

phase diagram of NaK alloy


Legend: Na - melting point of pure sodium, K - melting point of pure potassium, E - eutectic point, P - peritectic point, blue line - melting point of eutectic mixture, black line - melting point of compound Na2K, red line - melting point of surplus (Z. Anorg. Chem. 74 (1912) 152-156)

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    $\begingroup$ Liquid? They should freeze at about -8C, for the ration NaK used... Being moderately reflective metallic spheres, they should stabilize at a temperature of -80C or so.. Simple empirical prof they are not liquid: NaK has a very low but non-zero vapor pressure in the liquid state. If these droplets were still liquid, they should have evaporated ages ago. And if they ARE liquid, why bother to try to shove them to deorbit. Just whack with a giant flyswatter, breaking them up into zillions of microscopic fragments, that are mostly harmless in themselves and will de-orbit very quickly. $\endgroup$
    – PcMan
    Apr 22 at 17:35
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    $\begingroup$ @PcMan write all that wisdom up as an answer post and let people vote on it $\endgroup$
    – uhoh
    Apr 22 at 18:18
  • $\begingroup$ @TooTea thanks for that! I swear it was my fingers or at most my reptile brain that did that. If it were my conscious mipselling I would have added an h in there somewhere or ended with two n's. $\endgroup$
    – uhoh
    Jun 24 at 13:14

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