Fully fuelled at launch, the flight stack will weigh over four tonnes

The European and Japanese satellites that make up the BepiColombo mission to the Planet Mercury are being put on display on Thursday.

The two spacecraft will be joined together for both the launch from Earth and the cruise to the innermost world.

Only when they arrive at Mercury will they separate to conduct different but complementary observations.

Thursday’s showcase is the media’s last chance to view the full "flight stack", as it is called.

The event will take place at the European Space Agency’s (Esa) technical centre here in Noordwijk, Netherlands.

Europe’s Mercury Planetary Orbiter (MPO) and Japan’s Mercury Magnetospheric Orbiter (MMO) will shortly after be unbolted from each other for some final individual testing, before onward shipment to the launch site in French Guiana.

The double mission is due to get under way in 2018. An Ariane rocket is set to carry all the equipment skyward in October.

Everyone will have to be patient, however. It is going to take seven years for the satellite duo to get to their destination.

How to build a mission to Mercury

Europe’s Airbus company led production of the MTM, MPO and the sun shield for MMO

  • MTM is a propulsion module to control the cruise to Mercury
  • Europe’s Mercury Planetary Orbiter carries 11 instruments
  • For the cruise phase, a sun shield protects the MMO
  • At Mercury, the Japanese orbiter dispenses with the shield
  • It will simply spin to prevent its surfaces from overheating
  • MMO and MPO will go into different polar orbits at Mercury

The MPO and MMO will be looking to extend and deepen the knowledge gained at Mercury by the US space agency’s recent Messenger mission.

The American probe, which ceased operations in 2015, took some 270,000 images of the planet’s surface and acquired 10 terabytes of other scientific measurements.

It provided remarkable new insights on the composition and structure of the smallest terrestrial planet, and it made the amazing discovery that water-ice is held inside some of its shadowed craters. This on a surface that is generally hot enough to melt lead.

Esa and the Japanese space agency (Jaxa) hope that the more advanced, higher-resolution technology on their satellites will be able to answer some of the questions Messenger could not.

The oddball close to the Sun

Past Mercury visitors were Nasa missions: Mariner 10 and Messenger

  • The planet’s diameter is 4,880km – about one-third that of the Earth
  • It is the second densest planet in the Solar System: 5.4 grams/cu cm
  • The Caloris Basin is the largest surface feature (1,550km across)
  • It is an extreme place: surface temps swing between 425C and -180C
  • There is water-ice in the planet’s permanently shadowed craters
  • Mercury’s huge iron core takes up more than 60% of the planet’s mass
  • Apart from Earth, it is the only inner planet with a global magnetic field

The key conundrum is why the planet contains an outsized iron core and only a thin veneer of silicate rocks.

A favoured theory before Messenger was that Mercury at some point in its history was stripped of its outers layers, either by a big collision with another body or by the erosive effects of being so close to the Sun.

But the American probe observed large abundances of sulphur- and potassium-containing compounds in the surface – volatile substances that should not be present if either of the popular formation processes held true.

Europe’s MPO will have 11 scientific instruments onboard. It will fly in a circular polar orbit around the planet, mapping the terrain, generating height profiles, sensing the interior, and collecting data on surface composition and the wispy "atmosphere".

Japan’s MMO will have five instruments and will investigate the planet’s magnetic field.

Mercury is the only terrestrial planet – apart from Earth – to have global magnetic field. But it is an odd one. The field is roughly three times stronger in the northern hemisphere than it is in the south.

Both Esa and Jaxa are delighted to at last be approaching launch.

The development of the mission, particularly on the European side, has been a torrid learning curve. Hellish, one might even say.

The launch date was repeatedly put back as engineers struggled to find systems and materials that could cope with the intense heat and radiation experienced just a few tens of millions of km from the Sun.

When Esa’s Science Programme Committee originally green-lit BepiColombo in 2000, it had in mind a launch in 2009. Even when the industrial contract to build the MPO was finally signed in 2008, a launch was thought possible in 2013.

Esa says the mission is costing it €1.3bn (£1.1bn; $1.4bn). This figure does not include the costs of the MPO’s instruments which are borne by the national member states that supplied those instruments. Nor does it include the Japanese MMO contribution.

Messenger found hollowed out surface features that indicated the presence of volatiles