A HERO for General Relativity

HERO (Highly Eccentric Relativity Orbiter) is a space-based mission concept aimed to perform several tests of post-Newtonian gravity around the Earth with a preferably drag-free spacecraft moving along a highly elliptical path fixed in its plane undergoing a relatively fast secular precession. We considered two possible scenarios-a fast, 4-h orbit with high perigee height of 1047km and a slow, 21-h path with a low perigee height of 642km. HERO may detect, for the first time, the post-Newtonian orbital effects induced by the mass quadrupole moment J2 of the Earth which, among other things, affects the semimajor axis a via a secular trend of similar or equal to 4-12 cmyr-1, depending on the orbital configuration. Recently, the secular decay of the semimajor axis of the passive satellite LARES was measured with an error as little as 0.7cmyr-1. Also the post-Newtonian spin dipole (Lense-Thirring) and mass monopole (Schwarzschild) effects could be tested to a high accuracy depending on the level of compensation of the non-gravitational perturbations, not treated here. Moreover, the large eccentricity of the orbit would allow one to constrain several long-range modified models of gravity and accurately measure the gravitational red-shift as well. Each of the six Keplerian orbital elements could be individually monitored to extract the GJ2/c2 signature, or they could be suitably combined in order to disentangle the post-Newtonian effect(s) of interest from the competing mismodeled Newtonian secular precessions induced by the zonal harmonic multipoles Jl of the geopotential. In the latter case, the systematic uncertainty due to the current formal errors sigma Jl of a recent global Earth's gravity field model are better than 1% for all the post-Newtonian effects considered, with a peak of similar or equal to 10-7 for the Schwarzschild-like shifts. Instead, the gravitomagnetic spin octupole precessions are too small to be detectable.