A European-Japanese mission to Mercury is in the final stages of preparation for launch on Saturday 20 October. The mission, known as BepiColombo, will lift-off from the European Space Agency’s space port in Kourou, French Guiana, at 01.45 GMT.

Mercury is the innermost planet to the Sun and is, unusually, mostly made of metal. Only 30% of the planet is rocks. BepiColombo will investigate how it could have formed. It will study the internal structure, the magnetic field and the surface composition. The mission is made up of four spacecraft, which are stacked together for launch. Two science spacecraft are mated to a thruster module and a heatshield. Together, this quartet is 6.4-metres tall (21ft).

Following a seven-year voyage to reach Mercury’s orbit, the spacecraft stack will separate. The thruster module and the sunshield will be discarded, and the two science orbiters will begin work.

The Japan Aerospace Exploration Agency (JAXA) has built one spacecraft, ESA has built the other. Both will go into different orbits around the planet. The JAXA module will concentrate on the planet’s magnetic field and its interaction with the Sun, the ESA module will focus on the planet itself.

The mission is named after the Italian mathematician and engineer Giuseppe Colombo, who derived orbital trajectories to Mercury in the 1970s, and it is expected to last at least one year.