Solar energetic particle monitoring during the cruise phases of JUICE and BepiColombo

The ESA JUICE and the ESA/JAXA BepiColombo missions, currently in cruise to the Jovian and Hermean systems, respectively, provide a unique opportunity to characterise interplanetary space weather conditions during their extended cruise phases, particularly Solar Energetic Particles (SEPs). BepiColombo’s cruise spans eight years (2018–2026), while JUICE is cruising over a similarly long period (2023–2031). During these long trajectories, both missions sample diverse regions of the heliosphere measuring quasi-continuously solar energetic particles and, in the case of BepiColombo, also interplanetary magnetic fields that are modulated by solar activity, shaping the radiation environment experienced by spacecraft and influencing planetary space weather conditions. In particular, BepiColombo offers a rare opportunity to sample SEPs in the inner heliosphere systematically, while JUICE provides unique interplanetary SEP measurements at heliocentric distances greater than 1 AU.

BepiColombo carries a comprehensive suite of plasma and radiation instruments that operate regularly during cruise, including the Solar Intensity X‑ray and Particle Spectrometer (SIXS), the BepiColombo Environmental Radiation Monitor (BERM), the Solar Particle Monitor (SPM), and the Mercury Planetary Orbiter Magnetometer (MPO‑MAG), complemented by dedicated solar‑wind campaigns. JUICE provides continuous energetic particle measurements via the RADiation-hard Electron Monitor (RADEM), with additional instrument operating during health checks, planetary flybys, and later cruise phases.

Here we present an overview of SEP observations recorded by JUICE (RADEM) and BepiColombo (BERM, SIXS, SPM) during their cruise periods, together with interplanetary magnetic field measurements from BepiColombo (MPO-MAG). We discuss how these cruise phase datasets complement observations from heliophysics missions such as Parker Solar Probe and Solar Orbiter, and how both missions can act as upstream solar wind and radiation monitors for other planets, including Venus, Earth, Mars, and Jupiter. In particular, we characterise the propagation of SEPs through the heliosphere. By combining measurements from both missions with observations from other heliophysics missions (e.g., Parker Solar Probe, Solar Orbiter, and near‑Earth spacecraft), we constrain SEP onset times, angular extent, and longitudinal spread. This approach provides insight into SEP transport processes, magnetic connectivity, and the influence of interplanetary structures on particle propagation between the Sun and planetary environments, as well as help us to characterise the environment that both missions will fate in their corresponding orbits.

JUICE and BepiColombo cruise observations demonstrate the significant and often under exploited potential of planetary cruise phases to advance both heliophysics and planetary space weather studies across the inner Solar System.