Probing the gravity field of Jupiter and Saturn with Doppler tracking of the Juno and Cassini spacecraft

Current deep space tracking systems enable accurate reconstructions of the orbit and of the forces acting on a spacecraft. The estimation of the gravitational forces exerted by the bodies the spacecraft approaches are especially interesting, since the gravity field is a direct, although degenerate, expression of the internal structure. Microwave tracking of planetary probes is thus one of the few available sources of information on the interior of planets and satellites.

 NASA’s Deep Space Network will play once more a crucial role in planetary geodesy between 2016 and 2017, when the Juno and the Cassini spacecraft will probe the gravity fields of the two largest planets of the solar system. Both Juno and Cassini will be inserted into highly eccentric orbits with low pericenter altitudes, respectively around Jupiter and Saturn. The proximity to the planets offers an unprecedented sensitivity to the high degree harmonics of their gravity fields. The DSN, together with the onboard radio systems, will allow a precise measurement of the spacecraft range rate, the primary observable quantity in the determination of the gravity field.

Juno will benefit from the use of an advanced tracking system based upon a full two-way radio link in Ka-band. Onboard the spacecraft a Ka-band digital transponder guarantees a very high frequency stability in the coherent retransmission of the received radio signal. During the 2002 solar conjunction of the spacecraft, a similar radio system onboard Cassini, together with a triple-link plasma calibration technique, enabled the most accurate test of General Relativity ever carried out. A failure of the Cassini analog Ka-band transponder forces the use of the less accurate X-band tracking system.

The main objectives of the gravity measurements at the two gas giants are the determination of the size of a heavy elements core, and of the amount of mass involved in the atmospheric circulation. Given the simultaneity and similarity of the two experiments it is possible to make the best use of the experimental and modeling efforts, while providing an immediate comparison of the results for the two planets. We will compare the experimental setup of the two experiments and outline the expected accuracies in the estimation of the gravity field obtained with numerical simulations.