Artist's impression of the MPO spacecraft in orbit around Mercury. Credit: ESA

Mechanical and thermal design

The Structural and Thermal Model of the MPO in the Large Space Simulator at ESTEC. Credit: ESA/Anneke Le'Floch

The spacecraft structure uses a double-H configuration, designed to harmonise with the single radiator plane necessitated by the Mercury orbit. Heat generated by spacecraft subsystems and payload components, as well as heat that is coming from the Sun and Mercury as it "leaks" through the blankets into the spacecraft, is carried to the radiator by panel-embedded heat pipes. The structural design provides free access to all equipment and instruments during the Assembly, Integration and Test (AIT) programme. The design is mass efficient, with the primary structure serving as the mounting surface for all equipment; it will remain permanently assembled during AIT, avoiding the need for connector brackets and preventing alignment disturbances. MPO has four-point bolted interfaces to both the Mercury Transfer Module (MTM) and the MMO Sunshield and Interface Structure (MOSIF), which provides thermal protection and the mechanical and electrical interfaces for the MMO during the journey to Mercury.

The configuration and thermal design provide a classical thermal environment for internally mounted instrument equipment – avoiding costly development programmes by re-use of available hardware – while employing dedicated high temperature technologies for external items such as antennas, the solar array, the sun sensors and Multi-Layer Insulation (MLI), which are exposed to the harsh thermal environment around Mercury.

Power generation

Artist's impression of the MPO spacecraft. Credit: ESA

Communications

The MPO is equipped with two fixed Low-Gain Antennas (LGAs), a steerable Medium-Gain Antenna (MGA) and a steerable one-metre-diameter High-Gain Antenna (HGA). The two X-band LGAs will provide omnidirectional coverage at small distances from Earth and can also be used for emergency commanding at any distance. The X-Band MGA will be used primarily during the interplanetary cruise phase and in safe and survival modes. The HGA will provide X-band uplink and downlink and Ka-band downlink communications for spacecraft and science operations. The HGA will also be used during the cruise phase to enhance communications and data dump capabilities whenever needed.

ESA's Cebreros 35-metre ground station is planned to be the primary ground facility for communications during all mission phases. The ground stations at Kourou (LEOP), New Norcia (critical phases during cruise and Mercury capture), Perth (LEOP), Usuda (backup) and Uchinoura (backup) will be available for backup during critical flight phases and/or for use during special campaigns.

Attitude and orbit control system

The principal Attitude and Orbit Control System (AOCS) equipment consists of:

• three Star Trackers (STRs), each comprising a star tracker unit housing the optics and electronics, a shutter ­– which can be closed in the event of a major attitude control anomaly ­– and a baffle;
• two Inertial Measurement Units (IMU), including four high-accuracy rate-integrating gyros and four accelerometers in a tetrahedral configuration, together with the processing electronics;
• two redundant sets of two Fine Sun sensors;
• four reaction wheel assemblies, controlled by two sets of wheel drive electronics;
• two redundant sets of four 22-Newton hydrazine / MON-3 (Mixed Oxides of Nitrogen, a mixture of nitrogen tetroxide and 3 per cent of nitric oxide) thrusters to provide the change in velocity (ΔV) needed for orbit capture and orbit lowering to the MMO and MPO operational orbits;
• two redundant sets of four 10-Newton monopropellant (hydrazine) thrusters for attitude control and reaction wheel momentum off-loading.

The reaction wheels are mounted in a tetrahedral configuration; attitude control can be achieved with four wheels operating simultaneously (the nominal operational scenario) or any combination of three wheels.

During science operations, at least two STRs will be used in combination. In the event of major system anomaly on the spacecraft and consequent loss of attitude control, dedicated shutters will protect the STR optical paths to prevent damage due to accidental Sun pointing.

Launch, cruise and arrival at Mercury

For launch and the journey to Mercury, the MPO will be carried with the MMO as part of the MCS.

During interplanetary cruise, the MMO is dormant, with the exception of periodic checkouts. The MPO spacecraft commands the MMO and MTM and communicates with Earth. The MTM supplies electrical power for its passengers, as well as for its electric propulsion system.

Shortly before Mercury orbit insertion, the MTM is jettisoned from the spacecraft stack. The MPO then provides the MMO with the necessary resources and services until it is delivered into its mission orbit, when control is assumed by JAXA.

The MPO will travel around Mercury in a polar orbit with a period of approximately 2.3 hours, a perihermion (periapsis) of 400 km and an apohermion (apoapsis) of 1508 km. The orbit will be coplanar with that of the MMO, which will have an orbital period of around 9.3 hours.