Classical and relativistic orbital motions around a mass-varying body

TL;DR

This paper analyzes how isotropic mass loss from a body affects the orbital motion of a test particle, revealing secular changes in orbital elements and differences between osculating and true orbits, with implications for solar system dynamics.

Contribution

It provides a perturbative analysis of orbital effects due to mass variation, including relativistic considerations, and compares different approaches to understanding these effects.

Findings

Osculating semimajor axis and eccentricity decrease secularly.

True orbit expands despite osculating elements decreasing.

Relativistic effects are negligible for current solar system evolution.

Abstract

I calculate the classical effects induced by an isotropic mass loss of a body on the orbital motion of a test particle around it; the present analysis is also valid for a variation of the Newtonian constant of gravitation. I perturbatively obtain negative secular rates for the osculating semimajor axis, the eccentricity and the mean anomaly, while the argument of pericenter does not undergo secular precession; the node and the inclination i remain unchanged. The anomalistic period is different from the Keplerian one, being larger than it. The true orbit, instead, expands, as shown by a numerical integration of the equations of motion in Cartesian coordinates; in fact, this is in agreement with the seemingly counter-intuitive decreasing of the semimajor axis and the eccentricity because they refer to the osculating Keplerian ellipses which approximate the trajectory at each instant. A comparison with the results obtained with different approaches by other researchers is made. General relativity induces positive secular rates of the semimajor axis and the eccentricity completely negligible in the present and future evolution of the solar system.