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 proximity to 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. This mission has a spacecraft made up of two separate orbiters, the Mercury Planetary Orbiter (supplied by ESA) and the Mercury Magnetospheric Orbiter (supplied by JAXA), and a propulsion module.
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 separate 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 enough to melt certain metals, so spacecraft in orbit 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 with observing the planet’s surface, ESA’s Mercury Planetary Orbiter will keep the same spacecraft face towards Mercury as it needs continuously point its instruments at the planet’s surface. As a consequence, the Airbus engineers have covered the external surface with very high temperature multi-layered insulation. This material, made up of 49 layers of ceramics and aluminium, has been designed especially for the BepiColombo mission. Another difference to standard spacecraft is the antenna material, which are made from heat-resistant titanium.
How to get to Mercury?
As a spin-stabilised spacecraft, the Japanese-built 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.
To travel from Earth to Mercury involves slowing the spacecraft and allowing the Sun’s gravity to pull the spacecraft towards it – thereby reducing the size of the orbit around the Sun. To arrive at the correct speed to be captured into orbit by Mercury’s gravity, the spacecraft must slow by 7 km/s – this is 7 times the thrust needed to reach Mars. BepiColombo achieves this braking by means of 9 planetary fly-bys (one x Earth, two x Venus, six x Mercury) and the use of an Electric Propulsion System (especially developed for the mission) to provide 4 km/s of the braking. to reach its final scientific orbit around Mercury. During the 7 year cruise phase, BepiColombo will complete 18 orbits around the Sun.
Once in orbit, BepiColombo will study Mercury at different wavelengths, map the planet’s mineralogy and elemental composition and explore the magnetosphere and polar deposits as well as the origin of Mercury’s magnetic field. Further experiments will test Einstein’s theory of general relativity.