Getting to Mercury is harder than getting to Jupiter. That is not intuitive — Jupiter is five times farther from Earth — but orbital mechanics is perverse, and the inner solar system is a gravitational trap. To reach Mercury, a spacecraft must shed an enormous amount of energy, fighting the Sun's pull every kilometer of the way inward. The fuel required to slow down directly is prohibitive, so mission designers instead thread their spacecraft through years of planetary flybys, trading time for delta-v. BepiColombo, the joint ESA-JAXA mission now closing on Mercury, has spent the better part of six years doing exactly that — one Venus flyby, six Mercury flybys, and one of the most elaborate gravity-assist sequences in planetary science history.
When it finally settles into Mercury orbit in late 2026, it will be the first new spacecraft to study the planet from orbit since MESSENGER, which crashed into the surface in April 2015 after four years of science operations. And before MESSENGER, nothing had visited Mercury since Mariner 10's three flybys in 1974 and 1975. Mercury has been the neglected planet, sitting in the Sun's glare, technically close but practically inaccessible. BepiColombo is set to change that in ways that matter.
Two orbiters, one stack
What makes BepiColombo unusual is that it is carrying two separate spacecraft to Mercury simultaneously, stacked together for the cruise phase. ESA's Mercury Planetary Orbiter (MPO) will conduct high-resolution surface mapping and study the planet's tenuous exosphere and geological history. JAXA's Mercury Magnetospheric Orbiter (MMO, renamed Mio) will characterize Mercury's magnetic field and its interaction with the solar wind. They travel together until orbital insertion, then separate into their independent science orbits.
This division of labor is deliberate. Mercury's magnetic field is one of the most puzzling features of the inner solar system. Earth has a robust magnetosphere driven by its liquid iron outer core. Venus and Mars have essentially none. Mercury, which is the smallest rocky planet and should have a cold, solidified interior, has a dipole magnetic field that is weak but unmistakable — about 1% as strong as Earth's, detected first by Mariner 10. MESSENGER confirmed it and found that the field is offset northward from Mercury's geometric center by about 480 kilometers, an asymmetry that no planetary dynamo model fully explains. Mio's 16 science instruments, optimized for magnetic field measurements and plasma physics, are designed to characterize this field in detail that was not possible with MESSENGER's instrumentation.
The dark surface problem
Mercury's surface is unexpectedly dark. The planet reflects only about 6% of incoming sunlight — roughly the same albedo as coal — which makes it one of the darkest rocky bodies in the inner solar system. MESSENGER's spectral data confirmed that the dark material is not a thin veneer but a primary component of the crust, and that it correlates spatially with volcanic plains. The leading hypothesis is that carbon-rich material, delivered over billions of years by cometary and asteroid impacts, has been mixed into Mercury's crust and darkened by space weathering. But this is contested.
MPO carries a hyperspectral imager and X-ray, gamma-ray, and neutron spectrometers capable of mapping elemental abundances across the surface. The combination will let scientists test the carbon hypothesis directly: if the dark material is carbon-rich, the gamma-ray and neutron data will show it. If the darkening is instead due to iron nanoparticles produced by solar wind bombardment — as happens on the Moon — the spectral signatures will look different. Resolving this matters not just for understanding Mercury, but for calibrating models of how all rocky planetary surfaces evolve under space weathering.
Craters that never see the sun
Mercury rotates so slowly relative to its orbital period that some craters near the poles sit in permanent shadow. The interior of these craters has never seen direct sunlight in billions of years. MESSENGER's neutron spectrometer detected hydrogen signatures consistent with water ice in these permanently shadowed regions, confirmed by radar data. But whether the ice is surface frost, buried beneath a thin insulating layer of regolith, or mixed into the soil in a more complex way remains unclear.
MPO's thermal infrared spectrometer and its laser altimeter — BELA, the BepiColombo Laser Altimeter — will map the topography and thermal properties of these polar regions at resolutions MESSENGER could not achieve. BELA's range measurements will also help constrain Mercury's interior structure by detecting tidal deformation: the degree to which Mercury flexes in response to the Sun's gravity tells scientists whether the outer core is liquid or solid, which in turn constrains the planet's thermal history.
What arrival looks like
The orbital insertion sequence for BepiColombo is not a single engine burn. The mission profile uses a series of Mercury orbit captures and resonant orbits to progressively lower the spacecraft's orbit over many months, using each Mercury periapsis pass to shed more velocity. The full separation of MPO and Mio and their transition to final science orbits will take until late 2026. During the cruise phase, the stacked configuration means MPO's nadir-pointing instruments have been partially obscured by Mio, so the science taken during the six Mercury flybys has been limited — a preview, not the main event.
The main event, when it arrives, will be the most comprehensive survey of Mercury ever conducted. Two independent spacecraft, thirteen years of engineering work, and a flight path that has looped through more than 8.5 billion kilometers of interplanetary space — all of it converging on a planet that, for decades, seemed too difficult to bother with.
