Gonggong & Xiangliu

[Art Blos]

For the first time I came across a situation where I do not know which version of the orbit to choose. The moon of (225088) Gonggong, Xiangliu, can rotate both in prograde and retrograde orbit. I suggest reading the article and making a choice. Arguments are welcome.

[Gurren Lagann]

Gonggong-Xiangliu is a tight binary - the a/r_H of the binary is 0.007 at max, less than the 0.05 defined as the border between tight and wide binaries, and TNO moon orbits strongly prefer prograde orbits in tight binaries. So I think that Xiangliu has a prograde orbit, based on math and Grundy et al. (2019) (paper seen here: http://www2.lowell.edu/~grundy/abstracts/2019.TNB_orbits.html).

[pedro_jg]

I would go with prograde. According to this study by Grundy et al. (2019), most fully determined orbits for TNO binaries are prograde, at least for tight binaries (semi-major axis of satellite smaller than 5% of the total Hill sphere radius). A quick calculation show that Gonggong's Hill radius is large enough (3.35 million km at perihelion) to contain Xiangliu's orbit and still be classed as a tight binary, so there's a statistical argument to consider one solution more likely than the other.

Edit: I see that Gurren Lagann already posted a similar comment to mine. But nevermind.

[Art Blos]

Thanks to all.