The structure of a complex, asymmetric coronal mass ejection revealed by 17 spacecraft across the inner heliosphere

Coronal mass ejections (CMEs) are enormous bursts of plasma and magnetic field from the Sun that propagate explosively through the Solar System. CMEs are the primary drivers of the most severe space-weather events, posing major radiation hazards for human exploration beyond Earth's magnetosphere, including missions to the Moon and Mars. During Europa Clipper's cruise between Earth and Mars, its Plasma Instrument for Magnetic Sounding (PIMS) recorded unusual solar-wind conditions later identified as the wake of a complex CME. Data from an unprecedented network of 17 spacecraft across the inner Solar System revealed a hidden Earth-directed component that propagated asymmetrically and could not have been predicted without off-Sun-Earth-line observations. This component also affected Europa Clipper and Mars, demonstrating how a missed forecast of this kind could endanger a crewed mission. These findings highlight the need for multipoint observations, including planetary missions in cruise, to improve space-weather forecasting, mission planning, and operations.