A test of general relativity with ESA’s JUICE mission

The ESA’s JUICE mission is planned for launch in April 2023. After a 8.3-year interplanetary cruise it will reach the Jovian system in mid 2031. The mission is devoted to the study of Jupiter and three of the Galilean moons (Europa, Callisto and Ganymede). The 3GM (Geodesy and Geophysics of Jupiter and the Galilean Moons) radio-science package onboard the spacecraft comprises a Ka-band Transponder (KaT) that will enable a highly stable two-way coherent Ka/Ka link to study the gravity fields of the three Jovian satellites and their internal structure, and an Ultra Stable Oscillator (USO) to probe the atmosphere of Jupiter and the ionospheres of the three moons. The KaT can be used together with the onboard Deep Space Transponder (DST), enabling a full multi-frequency radio link system capable to suppress the dispersive noise, due to solar plasma and ionospheric effects, at nearly all solar elongation angles. The cruise phase of the mission provides three opportunities to perform classical tests of general relativity by measuring the relativistic time-delay and the frequency shift experienced by radio signals during superior solar conjunctions. A similar experiment, performed by Cassini in 2002, provided the most accurate measurement of the post-Newtonian parameter γ so far, with an accuracy of 2.3×10-5. The MORE experiment onboard the ESA/JAXA BepiColombo mission to Mercury is expected to improve the result obtained by Cassini up to a factor of 10 while orbiting the planet. Less significant improvements are expected in the cruise phase. The advanced radio-tracking system of JUICE and the favorable dynamical environment experienced in the outer solar system make a repetition of the Cassini and BepiColombo cruise experiments quite significant. In this work, we show the results of numerical simulations of JUICE superior conjunction experiments. We have found that 3GM can estimate γ with an accuracy at the level of 4.8×10-7, which represents a constraint on the value of γ 50 times tighter than the one obtained by Cassini and about four times better than the expected result from BepiColombo at the end of the mission.