Testing theories of gravitation with the Interstellar Probe Radio Experiment

The Interstellar Probe (and its proposed European contribution, named Stella) is looking to slingshot out of the solar system faster than any previous spacecraft, covering more than 300 AU over its 50-year lifetime. Its spin-stabilization and curtailed need for Earth-attitude reorientation maneuvers enable accurate measurements of non-gravitational accelerations. In light of the performances of the available radiometric techniques and instrumentations, able to operate at such large distances, this constitutes an excellent laboratory to test theories of gravitation at unprecedented scales without any dedicated onboard payload. In this work we investigate the possibility of estimating the mass of the graviton via the Compton wavelength λ, the Nordtvedt parameter η and the change rate for the solar gravitational parameter (dμ/dt) by processing 10 years of radio tracking data. We also model the path delay experienced by the ranging signal due to media and propose a suitable link budget to carry out the experiment, enabling range data accuracies at the level of a few meters. Preliminary experiments suggest a lower bound λ>10 14 km, comparable to the most optimistic bounds obtained from planetary ephemerides (Bernus et al, 2019) and gravitational wave detection methods (Abbott et al, 2021). We estimate 10 -5>σ(η)>10 -6, surpassing the accuracy of the Mercury Orbiter Radioscience Experiment (MORE) onboard BepiColombo performed during cruise (De Marchi & Cascioli, 2020). Finally, 10 -15>σ(dμ/dt)/µ>10 -16 per year, improving the value obtained by processing positional observations of planets and spacecraft (Pitjeva et al, 2021). This experiment complements the science goals of the mission to provide a more comprehensive picture of heliosphere dynamics, whilst investigating violations of the Weak Equivalence Principle postulated by general relativity. References: Abbott, R. et al. (LIGO-Virgo collaboration) (2021). Tests of General Relativity with GWTC-3. arXiv preprint arXiv:2112.06861. Bernus, L., Minazzoli, O., Fienga, A., Gastineau, M., Laskar, J., & Deram, P. (2019). Constraining the mass of the graviton with the planetary ephemeris INPOP. Physical Review Letters, 123(16), 161103. De Marchi, F., & Cascioli, G. (2020). Testing general relativity in the solar system: present and future perspectives. Classical and Quantum Gravity, 37(9), 095007. Pitjeva, E. V., Pitjev, N. P., Pavlov, D. A., & Turygin, C. C. (2021). Estimates of the change rate of solar mass and gravitational constant based on the dynamics of the Solar System. Astronomy & Astrophysics, 647, A141.