content 22-November-2017 00:02:54

Mercury Magnetospheric Orbiter - spacecraft

BepiColombo, an ESA mission in cooperation with Japan, will explore Mercury, the planet closest to the Sun.

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

The Mercury Magnetospheric Orbiter (MMO), which is being provided by the Institute of Space and Astronautical Science (ISAS) at the Japan Aerospace Exploration Agency (JAXA), is one of two spacecraft that will orbit Mercury as part of the BepiColombo mission. The other spacecraft, the Mercury Planetary Orbiter (MPO); the Mercury Transfer Module (MTM), which will carry the two orbiters to their destination; and the MMO Sunshield and Interface Structure (MOSIF), which provides thermal protection and the mechanical and electrical interfaces for the MMO, are being provided by ESA. The Mercury Composite Spacecraft (MCS) consists of the MPO, MMO, MTM and MOSIF. ESA is responsible for the overall mission design, the design, development and test of the MPO, MTM and MOSIF, the integration and test of the MCS and the launch.

Mechanical and thermal design

The primary structure of the MMO is an octagonal prism, measuring 1.8 metres between opposing faces and with an overall height of 0.9 metres. Its total mass will be around 275 kg, of which the instruments make up about 45 kg. A circular central core carries the separation interface at one end and supports the pylon for the high-gain antenna at the other. The 'top' and 'bottom' panels are recessed within the octagonal outer body; instruments and spacecraft subsystems mounted on the exterior of the top and bottom panels are shielded from the Sun by the walls of the octagon. Each of the eight side panels is fitted with solar cells; the aggregate power generation capacity will be ~350 W. The area of the panels not covered by solar cells has a mirror finish to reflect solar radiation.

A test model of the Mercury Magnetospheric Orbiter in the Large Space Simulator in ESTEC, the Netherlands. Credit: ESA/Anneke Le Floc'h

The MMO will be spin stabilised, with a rotation rate of 15 RPM and a spin axis almost perpendicular to the orbital plane of Mercury around the Sun. This choice of spin axis ensures that the top and bottom of the spacecraft are never Sun-pointed, and enables Earth-pointing of the high-gain antenna with only one degree of freedom mechanism. The centrifugal effect of the spin will enable the deployment of four 15-metre-long wire antennas for electric field and radio wave measurements. The orbiter will also be equipped with two five-metre-long masts to provide a suitable environment for magnetic field measurements, clear of the influence of the spacecraft structure and electrical equipment.

The MMO will travel around Mercury in a polar orbit with a period of approximately 9.3 hours, a perihermion (periapsis) of 590 km and an apohermion (apoapsis) of 11 640 km. The orbit will be coplanar with that of the MPO, which will have an orbital period of around 2.3 hours.

Launch, cruise and arrival at Mercury

For launch and the journey to Mercury, the MMO will be carried with the MPO 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.

The flight model of the BepiColombo Mercury Magnetospheric Orbiter (MMO) being lifted off of the base of its transport container, at ESTEC, The Netherlands, in April 2015. The protoflight model of the Mercury Transfer Module (MTM) is visible on the right. Credit: ESA – Anneke Le Floc'h

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 MMO is equipped with a 0.8-metre high-gain antenna, which will communicate with the JAXA/ISAS Sagamihara Space Operations Center via the Usuda Deep Space Center (UDSC) 64-metre antenna in Nagano, Japan. A medium-gain antenna is available for backup communications.

Last Update: 17 June 2015

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