ESA Science & Technology - Mission Operations - Getting to Mercury

Artist's impression of BepiColombo.

The Mercury Planetary Orbiter (MPO) is ESA's scientific contribution to the mission. The Institute of Space and Astronautical Science (ISAS) at the Japan Aerospace Exploration Agency (JAXA) has provided the other science spacecraft, the Mercury Magnetospheric Orbiter (Mio). ESA also built the Mercury Transfer Module (MTM), which carries the two orbiters to their destination, and the Magnetospheroc Orbiter Sunshield and Interface Structure (MOSIF), which provides thermal protection and the mechanical and electrical interfaces for Mio. The Mercury Composite Spacecraft (MCS) consists of the MPO, Mio, MTM and MOSIF. ESA has been 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.

Launch

BepiColombo was launched on 20 October 2018, at 01:45:28 UTC, on board an Ariane 5 launch vehicle from Europe's Spaceport in French Guiana.

Launch and Early Orbit Phase

The Launch and Early Orbit Phase (LEOP) started around six hours before launch, with pre-launch operations comprising spacecraft activation, pre-launch checkout and preparation activities for the launch. About 15 minutes before liftoff, the umbilical connector to the external power supply was removed and the spacecraft ran on internal battery power.

The ascent and injection phase lasted 28 minutes and ended with the separation of the MCS from the launcher. Upon on-board detection of separation, an automatic sequence performed priming of the MTM reaction control system and initiate Sun acquisition together with deployment of the MPO and MTM solar arrays while the Attitude and Orbit Control System (AOCS) was temporarily disabled. Finally, Sun re-acquisition with stabilisation in a slow spin around the Sun vector was performed. Telemetry downlinking via one of the low-gain antennas was initiated during this sequence, as soon as a ground station was visible. The antenna with the best view of the ground station was automatically selected when an uplink carrier signal was detected. At 02:20 UTC, ESA's deep space tracking station at New Norcia, Western Australia, picked up these first signals.

After spacecraft stabilisation, the ground operations centre took control and commanded the spacecraft into normal operational mode.

This phase was concluded on Monday 22 October at 11:45 UTC, just 58 hours after launch.

Near Earth Commissioning Phase

The Near Earth Commissioning Phase (NECP) lasted about five months, during which all initial commissioning activities were completed. It included:

• commissioning of the spacecraft (MPO and MTM) including deployment of the medium- and high-gain antennas and the magnetometer boom, release of the electric propulsion system thruster-pointing mechanisms and release of Mio's launch locks;
• MPO payload and Mio activation and functional checkout, as far as this is possible in the MCS configuration;
• solar-electric propulsion functional checkout and in-flight validation.

Final commissioning of the MPO payloads and of Mio will take place in Mercury orbit.

Interplanetary Cruise Phase

The interplanetary cruise phase includes the commissioning of the MTM solar-electric propulsion system, the electric thrust phases, coast arcs and the planetary fly-bys. Periodic MPO payload and Mio health checks are performed, typically every six months. Several solar conjunctions with degraded communications link performance will occur. The Earth distance will reach a maximum of 1.6 AU and the Sun distance a maximum of 1.2 AU and a minimum of 0.298 AU during this phase. The duration of the cruise phase will be 7.2 years.

The spacecraft left Earth with a hyperbolic excess velocity of 3.475 km/s. After one year and a half, it returned to Earth in April 2020 to perform a gravity-assist manoeuvre and was deflected towards Venus.

Two consecutive Venus flybys reduce the perihelion to nearly Mercury distance. A sequence of six Mercury flybys will lower the relative velocity to 1.84 km/s. Four final thrust arcs further reduce the relative velocity between the spacecraft and the planet to the point where Mercury will weakly capture the spacecraft on 5 December 2025, without an orbit insertion manoeuvre being required.

Mercury Approach Phase

The Mercury approach phase starts after the last electric propulsion manoeuvre has been completed, approximately two months before the first Mercury orbit insertion manoeuvre. During this phase, the MTM will separate from the spacecraft stack. The remaining composite of MPO/Mio/MOSIF, the MCS Approach configuration (MCSA), will drift into Mercury's sphere of influence, and will need only a small manoeuvre to get captured in an initial orbit of approximately 674 × 178 000 km. This process is known as a 'weak stability boundary' capture.

The MPO will perform the Mercury capture manoeuvre at pericentre and then the manoeuvres for orbit lowering to Mio's mission orbit.

After final checkout and preparation, Mio will be separated in the required attitude and simultaneously spun up by the separation mechanism.

The MPO with the MOSIF still attached, the MCS Orbit configuration (MCSO), performs another two manoeuvres for pericentre raising and apocentre lowering, and then separates the MOSIF in a safe direction. The MPO then descends further to its own mission orbit.

Mercury Orbit Phase

Once the MPO mission orbit is reached the final commissioning of the MPO and its payload is performed; this will last about one month. The MPO attitude follows a continuous nadir-pointing profile, providing optimum viewing conditions for the payload.

All MPO science data will be stored in the spacecraft's solid-state mass memory and downlinked during daily station passes with ESA's Cebreros ground station. Every half Mercury orbit, about every 44 Earth days, the attitude of the spacecraft will have to be reversed around the nadir direction to keep the radiator pointing away from the Sun.

Mio will communicate with the JAXA/ISAS Sagamihara Space Operations Center via the Usuda Deep Space Center (UDSC) 64-metre antenna in Nagano, Japan.

Nominal mission science operations are scheduled to be performed for one Earth year, with a planned extension of another year.