The joint European Space Agency (ESA) and Japanese Aerospace Exploration Agency (JAXA) modular space probe, BepiColombo, has passed its final test in its launch and transit flight ‘stack’ configuration, the ESA announced on July 6. BepiColombo will become only the third spacecraft to explore the planet Mercury, after two US National Aeronautics and Space Administration missions, Mariner 10 (1973–1975 – this probe did not orbit Mercury but executed three fly-bys of the planet) and Messenger (2011–2015).

The ESA is the lead agency for the mission and the prime contractor for the spacecraft is Airbus Defence & Space (Airbus DS). It is scheduled for launch in October next year. BepiColombo is composed of four specialised modules and has a mass of 4 t; the complete stack is 6.4 m high. The modules are the Mercury Transfer Module (MTM), the Mercury Planetary Orbiter (MPO), the Magnetospheric Orbiter Sunshield and Interface Structure (MOSIF) and the Mercury Magnetospheric Orbiter (MMO).

The MTM is the interplanetary propulsion unit and is fitted with a specially developed solar-electric ion propulsion system. The MTM will not drive BepiColombo towards Mercury, which is the closest planet to the sun, the smallest planet in the solar system and the least explored and least understood of the inner, rocky planets (Mercury, Venus, earth and Mars). Rather, it will decelerate the spacecraft, allowing the sun’s gravity to pull it towards itself and so, also, towards Mercury. The deceleration required will be 7 km/s, which is seven times the acceleration needed to reach Mars. Of the required deceleration, 4 km/s will be provided by the MTM, while the remaining 3 km/s will be derived from gravity braking achieved through nine planetary fly-bys: one of earth, two of Venus and six of Mercury. Finally, the spacecraft will be flying slow enough to be captured by Mercury’s gravity and enter orbit around the planet.

MPO and the MMO are actually separate spacecraft with different missions that will occupy different orbits around Mercury. The MPO, which is an ESA probe, will study the surface and internal make-up of Mercury, while the MMO, which is the JAXA probe, will examine the planet’s magnetosphere (the zone of space around Mercury which is affected by the planet’s magnetic field). The MOSIF links the MPO and the MMO during the interplanetary flight, and will allow the MMO to receive essential services and resources from the MPO during the last stage of the flight, as well as provide thermal protection for the MMO.

Just before the mission achieves orbit around Mercury, the MTM will be jettisoned. The MPO will then provide support for the MMO until the latter is released (‘spin-ejected’ is the term used by Airbus DS) into its orbit, at which point JAXA will assume command of the MMO. The MOSIF will then be jettisoned. Finally, the MPO will use a chemically-powered propulsion system to manoeuvre itself into a lower orbit, where it will start its mission. The flight will take seven years and the observation mission is planned to last one earth year, although it could be extended by another year.

The design and construction of BepiColombo presented special problems, Airbus DS pointed out. Mercury orbits only 58-million kilometres from the sun and its daytime surface temperatures reach 450 ºC or higher. BepiColombo will have to cope with both this great heat and the infrared radiation being re-emitted by the planet itself. “In consequence,” stated the company, “Airbus engineers have covered every external surface of the ESA’s MPO except for the single radiator side, with high-temperature multilayered insulation. The material, made up of 50 layers of ceramics and aluminium, was especially designed for the BepiColombo mission. The antennas are made of heat-resistant titanium, covered by a newly developed coating.”

The next steps in the programme will be the testing of the stack connection and separation system with the launch vehicle (rocket) payload adaptor and then the testing of the module separation systems (which will disassemble the stack). Thereafter, there will be further performance and functional tests and, in November, a final heat test for the MTM. The modules will be transported to Kourou, Guiana, in March next year. There they will be reassembled into a stack and then integrated onto an Ariane 5 rocket.

The development and construction of BepiColombo has involved Airbus DS facilities in Germany (Friedrichshafen – project lead – and Ottobrunn), the Netherlands (Leiden), Spain (Madrid-Barajas and Tres Cantos) and the UK (Stevenage). The spacecraft is named after Italian mathematician and engineer Giuseppe ‘Bepi’ Colombo (1920–1984), whose calculations played a key role in the success of Nasa’s Mariner 10 mission to Mercury.)