After nearly eight years and a winding path through the inner solar system, the ESA and JAXA mission BepiColombo has begun its arrival phase at Mercury, with orbit insertion set for November 2026.
After nearly eight years of looping through the inner solar system, a spacecraft bound for the Sun's closest neighbour has begun the final and most delicate stage of its journey. The mission is called BepiColombo, and its target is the planet Mercury.
On the third of September, the mission entered what its teams call the arrival phase. It is the start of a careful sequence of manoeuvres that will end with two spacecraft settling into orbit around the smallest planet in our solar system.
A mission built by two agencies
BepiColombo is a joint effort between the European Space Agency and the Japan Aerospace Exploration Agency. It carries two separate orbiters, each designed to study a different aspect of the tiny, sun-scorched world at the heart of the solar system.
The European craft is the Mercury Planetary Orbiter, which will map the surface and probe the planet's interior. Its Japanese companion, named Mio, is built to study Mercury's magnetic field and the turbulent space environment around it.
Why reaching Mercury is so hard
Mercury may seem close in cosmic terms, but getting there is deceptively difficult. The Sun's immense gravity means a spacecraft must shed a huge amount of speed to avoid simply falling past the planet or plunging inward toward the star itself.
To manage that, BepiColombo has taken the long way round. Since its launch, it has used a series of gravity assists, one flyby of Earth, two passes of Venus and six close encounters with Mercury, to gradually slow itself down along the way.
Those planetary flybys were paired with a gentle but persistent electric propulsion system. Together they have shaped a patient, winding path that has finally carried the spacecraft to the very doorstep of its distant destination.
The steps still to come
The arrival phase began with a major change to the spacecraft itself. On the third of September, the module that supplied much of the propulsion during the long cruise was set to separate, leaving the two orbiters to complete the final approach.
According to the mission plan, the spacecraft are due to enter orbit around Mercury on the twenty first of November. It will mark the culmination of a journey that has lasted the better part of a decade across the inner solar system.
The two orbiters will not remain joined for long after that. Around the ninth and tenth of December, they are scheduled to separate from each other, with each one moving into its own carefully chosen path around the planet.
Two orbits, two missions
Once in place, the orbiters will follow very different loops. The European orbiter is set to travel on a relatively tight polar orbit, while its Japanese partner will swing out on a far wider and more elongated path around Mercury.
Those contrasting orbits are entirely deliberate. Together they allow the mission to study Mercury from close range and from farther out at the same time, building a fuller picture of the planet than either spacecraft could manage alone.
The science that lies ahead
The full scientific campaign is not expected to begin until the spring of 2027, once the orbiters are settled and their instruments have been checked. Only then will the long and patient cruise start to pay off in a stream of new data.
For scientists, the wait promises to be worth it. Mercury remains one of the least explored planets in the solar system, and BepiColombo is designed to help answer why such a small, hot world turned out the way that it did.
Great coverage of Mercury.