Two threads of recent reporting — planetary geology and mission operations — are converging on Mercury this month. Independent studies revising how much the planet has contracted over geologic time now sit alongside milestone mission activity from the joint European Space Agency (ESA) and Japan Aerospace Exploration Agency (JAXA) BepiColombo mission, which has just completed separation from its transfer module en route to orbit.
Reassessing how much Mercury has shrunk
New analyses reported by multiple outlets conclude Mercury may have contracted far more than scientists previously thought. According to coverage of a recent study, Mercury’s total diameter loss since formation could be nearly 12 miles (19 kilometers). That represents a 10% to 30% increase in estimated contraction compared with earlier assessments, driven in part by the recognition that impact ejecta can obscure key surface features used to measure planetary shrinkage.
Mercury’s wrinkled surface — previously interpreted as evidence of global contraction as the planet cooled and interior volume decreased — now appears to record an even larger cumulative shrinkage once the masking effect of crater debris is taken into account. Reporting notes that Mercury formed about 4.5 billion years ago through energetic collisions, and that the heat from those impacts has been radiated away over time, a process linked to the long‑term contraction recorded on the surface.
BepiColombo arrives as new findings emerge
Those scientific developments coincide with operational milestones for BepiColombo. After an eight‑year cruise, the mission has entered the Mercury arrival phase: in early September the spacecraft’s two science orbiters — the Mercury Planetary Orbiter and the Mercury Magnetospheric Orbiter — separated from the Mercury Transfer Module (MTM). The orbiters are scheduled to enter Mercury orbit in November and to separate from each other in December.
BepiColombo’s journey included multiple close approaches and gravity assists: reports summarize that the spacecraft performed nine gravity assists during the cruise — one at Earth, two at Venus and six at Mercury — and flew past Mercury at close range six times overall. During its fourth flyby in September 2024 the mission approached within 165 kilometers (103 miles) of the surface.
Those flybys already yielded science. Instruments recorded space‑weather effects during solar activity: the SIXS particle and X‑ray detector, built in Finland for BepiColombo, detected electrons and protons accelerated by a solar eruption that penetrated Mercury’s magnetic field and struck the surface.
Why the timing matters
The alignment of fresh geological interpretation with BepiColombo’s arrival is important because the spacecraft will study Mercury in ways complementary to orbital imaging and remote sensing from earlier missions. BepiColombo’s orbiters will begin their dedicated science phase after orbital insertion and inter‑orbiter separation later this year, following the MTM separation on September 3, 2026.
While the recent contraction estimates come from reanalysis of surface expression and impact processes, BepiColombo’s close observations and in situ measurements during its operational phase should provide additional context for Mercury’s thermal and tectonic history, along with better constraints on how impact gardening and explosion ejecta have modified the planet’s surface record.
Taken together, the revised shrinkage numbers and BepiColombo’s imminent science campaign underline that Mercury’s geologic story is still being rewritten. As the orbiters begin their months‑long sequence of maneuvers and separations, researchers will have new opportunities to reconcile surface morphology, impact history and interior evolution on the solar system’s smallest planet.
