# New Horizons spinning about "minimum axis of inertia"? Misunderstood or misspoke?

I recently stumbled upon the question Why did New Horizons have to be spin-balanced to grams-level precision? (With quarters!) and the accepted answer written by an authoritative source (the guy who created & performed the test).

I am puzzled by this statement:

However most spacecraft (New Horizons and Ladee included) spin about their minimum axis of inertia

I take "minimum axis of inertia" to mean the principal axis with the lowest moment of inertia. However, looking at the configuration of the New Horizons spacecraft, I intuitively (if moments of inertia can be intuitive) expect the spin axis (Y) to be the maximum axis of inertia:

1. The New Horizons Spacecraft, Fountain et al. https://www.boulder.swri.edu/pkb/ssr/ssr-fountain.pdf

This is confirmed by Guerra et al.[2] which states:

The nominal spin rate of New Horizons is ω0 = 5 rpm, and the location of the centre of mass is the origin of the reference frame [...]. In this reference frame the principal moments of inertia are A = IXX = 161.38 kg/m2, B = IYY = 402.12 kg/m2, C = IZZ = 316 kg/m2 [19].

Where reference 19 is a private communication with "G. H. Fountain" (author of previously sourced "The New Horizons Spacecraft").

Is the comment made in the accepted answer wrong? Am I misunderstanding what minimum axis of inertia means?

1. Estimating the thermally induced acceleration of the New Horizons spacecraft, Guerra et al. (2017) (archived)
• Minimum/maximum perhaps related to the en.wikipedia.org/wiki/Poinsot%27s_ellipsoid representation ? And you sure don't want the tennis racket theorem coming in to play so don't rotate around z... Nov 2, 2021 at 19:27
• @BrendanLuke15 Two other possible explanations, both wild guesses really: i) the diagram could be incorrect (though it looks plausible to me) or ii) there are different mission phases with the rotation about different axes. You could add a link this question from jon harrison's answer to see what he has to contribute. Nov 2, 2021 at 21:20
• @Puffin generally a good idea, but it seems they are a "one and done" kind of user as all of their reputation is from that one single answer. Nov 2, 2021 at 21:55
• I'm a co-author of Guerra et al. and IIRC, we had to contact G. H. Fountain because we couldn't find New Horizons' moments of inertia anywhere. I believe the most probable explanation is that he was thinking about the semi-axis of the ellipsoid, as referred by @Jon Custer, and misspoke Nov 4, 2021 at 9:27
• @Ng Ph, I don't have a statistic but would say false. It's not a good idea because vibrations in antennas and other flexible parts make the S/C to loose kinetic energy of rotation and the axis of minimum moment of inertia becomes unstable. This is well known since Explorer-1. Rigid antennas help, but I would say it's always a liability. So, unless you can't avoid it it's better to use the axis of the largest moment of inertia. Nov 6, 2021 at 23:00