HERMES: Hybrid Exosphere Reconnaissance and MErcury Scout mission concept

Mercury is a key target for investigating terrestrial planet formation and differentiation, volatile retention, and atmospheric evolution in the inner solar system. Among the terrestrial planets, Mercury is unique with its high bulk density, low oxygen fugacity, anomalous magnetic field, and evidence of sustained geologic activity [1-3]. MESSENGER and BepiColombo (ongoing) have significantly advanced our understanding of these characteristics, yet gaps persist in spatial resolution, low-altitude access, and orbital flexibility [4-5]. Addressing these gaps requires a mission capable of sustained low-altitude operations beyond what current orbital geometries permit. HERMES builds on the prior Mercury Scout solar sail concept [6] through a novel hybrid architecture that integrates chemical propulsion with solar sail propulsion. This preserves orbital flexibility while reducing transit time and enabling exosphere sample return.Operations at Mercury demand exceptionally high total Δv due to proximity to the Sun’s gravitational well [7]. Chemical propulsion delivers the high thrust required for orbit insertion but is constrained by finite propellant mass, restricting long-duration orbital flexibility. In contrast, solar sail propulsion exploits continuous low thrust from solar photon momentum transfer, removing propellant dependence for sustained trajectory modification. Previous missions (e.g., IKAROS, LightSail, NEA Scout, Solar Cruiser concept) validated sail deployment and heliocentric maneuvering, though sustained sail operations in planetary orbit have not yet been demonstrated.HERMES combines solar sail and chemical propulsion to reduce transit time, maximize operational flexibility, and enable propellant-free sample return. The solar sail provides the majority of the mission delta-v budget through continuous photon-pressure acceleration during heliocentric transfer. The chemical stage delivers the high-thrust impulsive burn required for rapid capture into Mercury orbit. Following insertion, the solar sail supports orbit modification and periapsis repositioning at Mercury (Fig. 1, inset), sustained low-altitude reconnaissance, and propellant-free Earth return of collected samples. Solar sail-enabled sample return concepts have been previously investigated for Mercury, demonstrating the feasibility of propellant-free Earth return trajectories [8] (Fig. 1).  Figure 1. Simplified schematic Earth-Mercury transfer trajectories viewed from the ecliptic plane (not to scale). HERMES travels from Earth to Mercury using chemical and solar sail propulsion along a multi-gravity-assist trajectory with Venus and Mercury flybys (based on MESSENGER [7]). Chemical propulsion is used for orbit insertion. In Mercury orbit, solar sail propulsion enables sustained low-altitude operations and repeated periapsis modification (inset, based on prior concept study [6]). Following orbital reconnaissance, the sail bus separates from the science bus and returns to Earth via a continuous low-thrust outward spiral (trajectory based on Hughes et al. [8]). The spacecraft is comprised of two functional elements:Science and operations bus – Carries the remote sensing instrument suite, including a sub-meter narrow angle camera (