Key Takeaways
- BepiColombo will enter Mercury orbit in late 2026 after seven years and nine gravity assists
- The joint ESA-JAXA mission carries two separate orbiters that will separate after orbit insertion
- Mercury’s extreme thermal environment and proximity to the Sun make orbital operations a unique challenge
The Most Complex Interplanetary Journey Yet
BepiColombo launched on October 20, 2018, from the Guiana Space Centre in Kourou, French Guiana, aboard an Ariane 5 rocket. Getting to Mercury is harder than getting to Mars, despite Mercury being much closer in the solar system. The problem is energy: a spacecraft moving outward from Earth already carries most of the orbital velocity it needs for an outer planet trajectory. Falling inward toward Mercury requires shedding an enormous amount of velocity relative to the Sun, and the rocket fuel required to do that directly would make the spacecraft prohibitively heavy.
The solution is gravity assists: repeated flybys of planets and Mercury itself, using each encounter to bleed off relative velocity in small increments. BepiColombo executed one Earth flyby in April 2020, two Venus flybys in October 2020 and August 2021, and six Mercury flybys between 2021 and 2025 – a total of nine gravity assists spanning seven years. Each Mercury flyby brought the spacecraft closer to the orbital configuration needed for capture, and each was used to collect scientific data in preview of the main mission.
The Mercury orbit insertion (MOI) burn is scheduled for late 2026. During MOI, the spacecraft’s main engine fires to slow it enough that Mercury’s gravity captures it into a highly elliptical initial orbit. Subsequent orbit reduction manoeuvres over the following months will lower the orbit to the operational configuration for science operations.
A Mission in Three Parts
BepiColombo is actually three spacecraft stacked together for the cruise phase: the Mercury Planetary Orbiter (MPO), the Mercury Magnetospheric Orbiter (MMO), and the Mercury Transfer Module (MTM). The MTM provides propulsion and solar power during the cruise and will be jettisoned before orbit insertion. MPO is ESA’s contribution; MMO (also called Mio) is JAXA’s contribution.
After MOI, the two science orbiters will separate from each other and enter their respective operational orbits. MPO will settle into a polar orbit with a pericentre of approximately 480 kilometres and an apocentre of approximately 1,500 kilometres, well-suited for surface mapping and close planetary observations. MMO will occupy a more distant polar orbit with pericentre of approximately 590 kilometres and apocentre of approximately 11,640 kilometres, covering Mercury’s magnetospheric environment from multiple distances.
The separation of the science mission into two complementary orbiters is a direct consequence of the partnership structure. ESA’s MPO is optimized for surface and geophysics science, while JAXA’s MMO is optimized for magnetospheric and plasma science. Flying them simultaneously in different orbits allows simultaneous in-situ measurements of Mercury’s magnetosphere at different distances, creating the kind of multi-point observation geometry that single-spacecraft missions cannot provide.
Mercury’s Surface as a Scientific Target
Mercury is the least explored of the four inner rocky planets despite being the closest planet to the Sun. NASA’s MESSENGER spacecraft, which orbited Mercury from 2011 to 2015 before impacting its surface, revolutionized understanding of the planet. MESSENGER revealed that Mercury’s geological history is far more diverse and violent than previously known: large volcanic plains, enormous tectonic scarps indicating planetary contraction as the core cooled, and – most surprisingly – permanently shadowed craters at the poles containing water ice.
BepiColombo’s MPO carries 11 instruments with considerably broader spectral coverage than MESSENGER, designed to build on and extend those discoveries. The BELA (BepiColombo Laser Altimeter) will produce global topographic maps at significantly higher spatial resolution than MESSENGER’s laser altimeter, refining the shape and crater density statistics of terrains that MESSENGER only partially imaged. The SIMBIO-SYS imaging suite will cover the hemisphere that MESSENGER’s orbital geometry left poorly imaged during the 2011-to-2015 mission.
The north polar water ice deposits confirmed by MESSENGER represent one of Mercury’s most scientifically provocative features. Ice on the innermost planet, where daytime surface temperatures exceed 430 degrees Celsius on the Sun-facing side, is a product of extreme local environment: permanently shadowed crater floors at the poles where sunlight never reaches can be as cold as minus 180 degrees Celsius, cold enough to trap and retain water ice for billions of years. MESSENGER data suggested the ice might be mixed with organic material. BepiColombo’s higher-resolution observations of the polar regions could confirm or refine that interpretation.
Mercury’s Magnetic Field and Interior
Mercury’s interior presents one of planetary science’s most persistent puzzles. The planet has a global magnetic field, making it the only inner rocky planet besides Earth to do so. The magnetic field is much weaker than Earth’s – about one percent of its strength – but it is dipolar and clearly generated by a dynamo operating in a liquid metallic core.
The puzzle is that Mercury should not have a liquid core. A planet of Mercury’s small size – 4,879 kilometres in diameter compared to Earth’s 12,756 kilometres – should have cooled and solidified its core billions of years ago. That it has not implies either a specific composition that suppresses the solidification temperature, or some additional heat source, or both. High-precision gravity measurements and magnetic field mapping by MPO will constrain the core size, structure, and heat flux in ways that could resolve this inconsistency.
MESSENGER’s gravity data suggested that Mercury’s core radius is about 83% of the planet’s radius – exceptionally large compared to Earth’s 55% – and that the core likely contains a significant fraction of sulfur or other light elements that suppress its freezing point. BepiColombo will extend those measurements to higher precision, potentially detecting the inner solid core within the liquid outer core if one exists, and characterising the depth and thickness of the mantle.
Operating in Mercury’s Extreme Environment
Engineering a spacecraft for Mercury orbit is among the most demanding thermal challenges in solar system exploration. At Mercury’s orbital distance of approximately 0.39 astronomical units from the Sun, the solar flux is about 6.5 times greater than at Earth. Surface temperatures on Mercury’s day side reach 430 degrees Celsius; temperatures on the night side drop to minus 180 degrees Celsius. The spacecraft must handle both extremes, often in rapid succession as it orbits.
MPO’s thermal design uses a combination of multi-layer insulation, radiators pointed away from both the Sun and Mercury’s hot surface, and a thermal shield that keeps the spacecraft body in permanent shadow during close orbital operations. Some instruments are operated only during specific orbital geometries to avoid thermal damage. The solar panels are tilted away from direct sunlight for most of the orbit, accepting reduced power generation in exchange for thermal survival.
The MTM’s solar electric propulsion system – ion thrusters powered by the solar panels – has performed reliably throughout the cruise. Ion propulsion is essential because a chemical rocket carrying enough propellant for the extended Mercury approach would have been far too heavy for an Ariane 5 to launch. The trade-off was time: seven years of slow spiral rather than a direct fast transit.
What BepiColombo Adds to the MESSENGER Legacy
MESSENGER spent four years in Mercury orbit and answered fundamental questions about the planet’s composition, geological history, and polar ice. But MESSENGER had limitations: its orbital geometry gave better coverage of the northern hemisphere than the south, its instruments had gaps in spectral coverage, and it was not designed for the kind of high-precision gravity and magnetic field mapping that BepiColombo can achieve.
BepiColombo’s science return will come in two phases. First, the initial months of operations from the highly elliptical post-insertion orbit provides the first science data from the new instruments, including a comparative benchmark against MESSENGER’s existing dataset. Second, once the orbit is lowered to operational altitude, a systematic global mapping campaign will produce the complete, high-resolution, multi-wavelength dataset that planetary scientists have been waiting for since MESSENGER’s 2015 mission end.
Summary
BepiColombo’s late 2026 Mercury orbit insertion is the payoff on an 8-year investment in one of the most technically demanding planetary missions ever flown. The combination of two orbiters in complementary orbits, the full suite of spectroscopic and geophysical instruments, and the thermal engineering that makes proximity operations survivable at Mercury’s distance from the Sun represents the state of what international collaboration in planetary science can achieve. Whether the results clarify Mercury’s persistent interior puzzle or deepen it, the planet will be substantially better understood by the time BepiColombo exhausts its fuel and joins MESSENGER in the impact crater roster.
Appendix: Top 10 Questions Answered in This Article
What is BepiColombo?
BepiColombo is a joint ESA-JAXA mission to Mercury consisting of two science orbiters: ESA’s Mercury Planetary Orbiter and JAXA’s Mercury Magnetospheric Orbiter. It launched in October 2018 and is expected to enter Mercury orbit in late 2026 after a seven-year journey involving nine gravity assists.
Why does it take seven years to reach Mercury?
Getting to Mercury requires shedding a large amount of orbital velocity relative to the Sun, which would require prohibitively heavy chemical propellant if done directly. BepiColombo uses a combination of solar electric propulsion and nine gravity assists at Earth, Venus, and Mercury itself to gradually reduce its velocity over seven years.
What are BepiColombo’s two science orbiters?
The Mercury Planetary Orbiter (ESA) will operate in a lower, closer orbit optimized for surface mapping and geophysics. The Mercury Magnetospheric Orbiter (JAXA), also called Mio, will operate in a more distant orbit to study Mercury’s magnetosphere and plasma environment from multiple distances.
What did NASA’s MESSENGER mission discover about Mercury?
MESSENGER discovered that Mercury has extensive volcanic plains, large tectonic scarps from planetary contraction, a surprisingly large metallic core, and water ice in permanently shadowed polar craters. These discoveries fundamentally changed understanding of Mercury’s geological and thermal history.
Why does Mercury have water ice if it is the closest planet to the Sun?
Mercury’s permanently shadowed polar craters never receive direct sunlight and remain as cold as minus 180 degrees Celsius. Water ice delivered by comets and asteroids over billions of years can be trapped and preserved in these cold traps indefinitely despite the extreme heat on Mercury’s sun-facing surface.
Why does Mercury’s liquid core puzzle planetary scientists?
A planet as small as Mercury should have cooled and solidified its core billions of years ago. The presence of an active liquid metallic core generating a global magnetic field implies a specific composition – possibly including sulfur or other light elements – that suppresses the core’s freezing temperature, or an additional internal heat source.
What instruments will BepiColombo use at Mercury?
MPO carries 11 instruments including the BELA laser altimeter for topographic mapping and the SIMBIO-SYS imaging suite. The instruments cover spectral ranges from X-ray to infrared, enabling mineralogical mapping of the surface in greater detail than MESSENGER’s instrument suite.
How does BepiColombo handle Mercury’s extreme temperatures?
MPO uses multi-layer insulation, Sun-facing thermal shields that keep the spacecraft body in shadow, and radiators pointed away from Mercury’s hot surface. Solar panels are tilted to reduce solar flux exposure, accepting reduced power generation in exchange for thermal survival. Some instruments are only operated in specific orbital geometries to avoid thermal damage.
When did BepiColombo begin its Mercury flyby series?
BepiColombo conducted its first Mercury flyby in October 2021, with subsequent flybys through 2025. The six Mercury flybys progressively reduced the spacecraft’s velocity relative to Mercury in preparation for orbit insertion in late 2026.
What will BepiColombo measure that MESSENGER could not?
BepiColombo will improve on MESSENGER’s coverage of Mercury’s southern hemisphere, provide higher-precision gravity and magnetic field mapping to better characterise the core structure, and extend spectral coverage for mineralogical mapping. The two-orbiter configuration also enables simultaneous multi-point magnetospheric measurements that a single spacecraft cannot achieve.
