The post-Newtonian gravitomagnetic spin-octupole moment of an oblate rotating body and its effects on an orbiting test particle; are they measurable in the Solar System?

Abstract

We analytically work out the orbital effects induced by the gravitomagnetic spin-octupole moment of an extended spheroidal rotating body endowed with angular momentum $\boldsymbol{S}$ and quadrupole mass moment $J_2$. Our results, proportional to $S J_2 c^{-2}$, hold for an arbitrary orientation of the body's symmetry axis $\boldsymbol{\hat{S}}$ and a generic orbital configuration of the test particle. Such effects may be measurable, in principle, with a dedicated spacecraft-based mission to Jupiter. For a moderately eccentric and fast path, the gravitomagnetic precessions of the node and the pericenter of a dedicated orbiter could be as large as $400~\textrm{milliarcseconds~per~year}$ or even $1,600-4,000~\textrm{milliarcseconds~per~year}$ depending on the orientation of its orbital plane in space. Numerical simulations of the Earth-probe range-rate signal confirm such expectations since its magnitude reaches the $\simeq 0.03-0.3~\textrm{millimeter~per~second}$ level after just 1 day. The precision of the current two-way Ka-band Doppler measurements of the spacecraft Juno, presently orbiting Jupiter, amounts to $\simeq 0.003~\textrm{millimeter~per~second}$ after $1,000$ seconds. Also other general relativistic effects could be measurable, including also those proportional to $GMJ_2c^{-2}$, never put to the test so far. Most of the competing Newtonian signals due to the classical multipoles of the planet's gravity field have quite different temporal signatures with respect to the post-Newtonian ones, making, thus, potentially easier disentangling them.