On 9 May 2016, everywhere on Earth, millions of people pointed telescopes towards the Sun and very carefully searched for a tiny black dot transiting the huge solar disc. The tiny dot was the shadow cast by Mercury as the planet travels between Earth and the Sun. The Mercury transit is one of the rarest astronomical events, occurring only 13 times each century.
To this day, Mercury remains the most mysterious planet of our solar system. The Sun’s glare makes it impossible to study via telescope and the extreme heat and immense gravitational pull generated by the Sun make it hard to reach. So far, only two NASA missions have visited Mercury: Mariner 10 in the 1970’s and Messenger, which orbited the planet from 2011 until it ran out of fuel in April 2015.
As many questions remain unanswered, ESA and the Japanese Aerospace Exploration Agency (JAXA) have teamed up for the dual spacecraft mission “BepiColombo”. Named after the Italian professor Giuseppe ‘Bepi’ Colombo, who was instrumental in making the Mariner 10 mission such a success, the mission will be made up of two separate orbiters, the Mercury Planetary Orbiter (supplied by ESA) and the Mercury Magnetospheric Orbiter (supplied by JAXA).
As prime contractor for ESA, Airbus Defence and Space is responsible for designing and building the Mercury Planetary Orbiter (MPO) and all other European spacecraft hardware. “We have created a stacked spacecraft, so that both orbiters can travel to Mercury as one unit, powered by a common propulsion module – the Mercury Transfer Module (MTM),” explains Roger Wilson, of the Airbus system engineering team. “When they reach their destination after their seven-year journey, the two orbiters will be separated and operate in their own individual orbits around the planet to conduct the most thorough exploration of Mercury ever undertaken.”
As it is only 58 million kilometres away from the Sun, Mercury presents a special challenge to visiting spacecraft. During the day, the planet’s surface is baked hot enough to melt certain metals. So spacecraft in orbit do not only have to be able to cope with the immense heat of the Sun, but also with the infrared radiation emitted by the hot surface of the planet.
Tasked to observe the planet’s surface, ESA’s Mercury Planetary Orbiter will always be facing Mercury with the same side, as it needs continuously point its instruments at the planet’s surface. In consequence, the Airbus Defence and Space engineers have covered the sun-illuminated surfaces of ESA’s Mercury Planetary Orbiter with high temperature multi-layered insulation. The material, made up of 49 layers of ceramics and aluminium, was especially designed for the BepiColombo mission. The antennas are made of heat-resistant titanium.
As a spin-stabilised spacecraft, the Mercury Magnetospheric Orbiter (MMO) is less sensitive to the solar illumination than the MPO as all of its sides will be evenly exposed to Sun and planet. During the seven-year journey, however, when it has to stay still, it needs to be protected by the MOSIF (MMO Sunshield and Interface Structure). This gives the BepiColombo travel stack an appearance reminiscent of an ice cream cone.