Preparation of a celestial field for precise VLBI observations of the JUICE spacecraft: a test case of the Venus flyby

Abstract

The Jupiter Icy Moons Explorer mission, launched in 2023, is on its way to the Jovian system, to arrive in 2031. The Planetary Radio Interferometry and Doppler Experiment performs radio astronomical observations of the spacecraft already in its interplanetary cruise phase. Since the sky density of potential compact radio sources suitable as phase-reference calibrators in very long baseline interferometry observations is low, densifying the celestial reference frame along the Ecliptic is an essential task in support of JUICE. In preparation for the JUICE gravity assist manoeuvre at Venus in August 2025, to support the accurate reconstruction of the spacecraft state vector, we selected in-beam calibrator source candidates along the JUICE trajectory. Additionally, we attempted to link known nearby bright compact reference sources from the Radio Fundamental Catalog to the International Celestial Reference Frame. We performed nodding-style phase-referencing VLBI observations with the European VLBI Network at 8.4GHz, targeting nine in-beam calibrator candidate and three RFC sources in the celestial vicinity of JUICE's trajectory during its Venus flyby. We linked their positions to nearby ICRF sources by means of relative astrometry. Four of the ICC targets and all RFC sources were detected and imaged with the EVN, and their coordinates determined with typically sub-mas precision. Although none of the targeted ICC sources proved suitable as VLBI phase-reference calibrators because of being too faint or resolved, their newly-determined positions and flux densities could in principle be used to verify the detection of the spacecraft at the time of the actual flyby around Venus. During the detailed analysis of the ICC targets, we found that the structure of one of them, J0843+1826 is resolved on mas scale, and the other one, J0843+1814 is a young radio galaxy, likely a member of a cluster.