Mercury is a surprisingly difficult planet to explore. Although it is much closer to Earth than the outer planets, a spacecraft traveling toward Mercury falls deep into the Sun’s gravitational well and gains enormous speed. Slowing down enough to enter orbit therefore requires careful use of gravity assists and propulsion.
Only a handful of spacecraft have explored Mercury at close range. NASA’s Mariner 10 made the first visits in the 1970s, MESSENGER became the first spacecraft to orbit the planet in 2011, and the ESA/JAXA BepiColombo mission is now approaching Mercury orbit. Together, these missions have transformed our understanding of the innermost planet.
1. Getting Into Orbit Around Mercury Is Surprisingly Difficult
Sending a spacecraft inward toward Mercury is not simply a matter of flying directly toward the planet. As the spacecraft approaches the Sun it accelerates, making it difficult to slow down enough for Mercury’s relatively weak gravity to capture it.
Mercury also has almost no atmosphere, so a spacecraft cannot rely on conventional aerobraking to shed large amounts of speed. Instead, missions use planetary gravity assists to gradually reshape their trajectories and reduce the amount of propellant required.
MESSENGER, for example, used one Earth flyby, two Venus flybys and three Mercury flybys before finally entering orbit. BepiColombo has followed an even more elaborate route involving Earth, Venus and six Mercury flybys.
2. Mariner 10 Was the First Spacecraft to Visit Mercury
NASA’s Mariner 10 launched on November 3, 1973, and became the first spacecraft to visit Mercury. It was also the first interplanetary mission to use a gravity assist, flying past Venus to alter its trajectory toward the innermost planet.
Mariner 10 did not orbit Mercury. Instead, it made three flybys in 1974 and 1975, approaching to within just 327 kilometers (203 miles) during its final encounter.
The spacecraft revealed a heavily cratered world crossed by enormous scarps and made the unexpected discovery that Mercury possesses its own magnetic field. Because essentially the same hemisphere was illuminated during each encounter, however, Mariner 10 was able to photograph only part of Mercury’s surface.
3. MESSENGER Became the First Spacecraft to Orbit Mercury
NASA’s MESSENGER spacecraft launched on August 3, 2004. Its name stood for MErcury Surface, Space ENvironment, GEochemistry, and Ranging.
After its complex series of gravity assists, MESSENGER became the first spacecraft to enter orbit around Mercury in March 2011. It remained there for more than four years, mapping the planet and studying its geology, composition, magnetic field, interior and extremely thin exosphere.
MESSENGER also photographed regions that Mariner 10 had never seen, eventually providing global coverage of the planet. After exhausting its propellant, MESSENGER’s orbit gradually decayed, and the spacecraft crashed into Mercury’s surface on April 30, 2015.
4. BepiColombo Is the Next Generation of Mercury Exploration
The third mission to explore Mercury at close range is BepiColombo, a joint project between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA).
Launched in October 2018, BepiColombo used one Earth flyby, two Venus flybys and six Mercury flybys to gradually adjust its trajectory. Its final Mercury flyby took place on January 8, 2025.
Unlike the earlier missions, BepiColombo consists of two scientific orbiters: ESA’s Mercury Planetary Orbiter (MPO) and JAXA’s Mio, also known as the Mercury Magnetospheric Orbiter. The pair will study Mercury simultaneously from different orbits.
As of October 2026, BepiColombo is in its Mercury arrival phase. The spacecraft is scheduled to be captured into orbit on November 21, 2026, with MPO and Mio separating in December. Full science operations are currently planned to begin in April 2027.
5. MESSENGER Revealed a Long History of Volcanism
Before MESSENGER, the extent of Mercury’s volcanic history was uncertain. The spacecraft revealed widespread volcanic plains as well as numerous sites where explosive eruptions had occurred.
These explosive deposits were particularly surprising because they require volatile substances capable of expanding rapidly into gas. Mercury had once been expected to be extremely depleted in volatile materials because of its proximity to the Sun.
MESSENGER instead showed that volcanism affected Mercury over a substantial part of its geological history, fundamentally changing ideas about the planet’s formation and evolution.
6. MESSENGER Confirmed Water Ice at Mercury’s Poles
One of MESSENGER’s most remarkable achievements was confirming that water ice exists on Mercury.
Radar observations from Earth had previously detected unusually reflective material inside craters near Mercury’s poles. MESSENGER showed that these deposits coincide with permanently shadowed regions and confirmed that the dominant material is water ice.
Mercury can become intensely hot in sunlight, but its rotational axis is tilted by only a tiny amount. Sunlight therefore never reaches the floors of some deep polar craters, allowing temperatures there to remain low enough for ice to survive for extremely long periods.
7. The Missions Revealed Mercury’s Unexpected Magnetic Field
Mariner 10 produced one of the great surprises of early planetary exploration when it discovered that Mercury has a global magnetic field.
MESSENGER subsequently mapped that field in far greater detail. It showed that Mercury’s magnetic dipole is displaced significantly northward relative to the planet’s center, creating an unusually asymmetric magnetic environment.
The existence of the field provides important evidence that at least part of Mercury’s enormous metallic core remains molten and capable of generating a magnetic dynamo. BepiColombo’s two-orbiter configuration should allow scientists to study the planet’s magnetic field and its interaction with the solar wind in unprecedented detail.
8. MESSENGER Changed Ideas About Mercury’s Giant Core
Mercury has an extraordinarily large metallic core relative to the size of the planet. Mariner 10 data had already indicated that Mercury must contain a large amount of metal, but MESSENGER provided much more information about its interior and surface chemistry.
Older theories suggested that a giant impact might have stripped away much of Mercury’s original rocky mantle, or that intense heating near the young Sun removed the outer material.
MESSENGER complicated those ideas by discovering unexpectedly high abundances of volatile elements such as sulfur and potassium. Such materials should have been strongly depleted by the extreme heating required by some of the earlier formation models.
Mercury’s unusual metal-rich structure therefore remains an important problem in understanding how the terrestrial planets formed.
9. The Missions Revealed a Planet That Has Shrunk
Mariner 10 photographed enormous cliff-like structures known as lobate scarps. MESSENGER later showed that these features are widespread across Mercury.
They formed as the planet’s interior cooled and contracted. As Mercury shrank, its crust was compressed, producing thrust faults that pushed sections of terrain over neighboring regions.
MESSENGER’s global observations showed that the amount of contraction was substantially greater than could be inferred from Mariner 10’s incomplete coverage. The scarps therefore preserve a visible record of billions of years of cooling inside the planet.
10. Caloris Basin Records One of Mercury’s Greatest Impacts
The Caloris Basin is one of Mercury’s most spectacular geological features and one of the largest impact basins in the solar system, measuring roughly 1,500 kilometers (930 miles) across.
Mariner 10 saw only part of it. MESSENGER’s first Mercury flyby in January 2008 provided humanity’s first close views of the previously unseen western portion, and the later orbital mission allowed the entire structure to be mapped in detail.
The basin preserves evidence of an enormous ancient collision, subsequent volcanic flooding and later tectonic deformation. On the opposite side of Mercury lies a region of disrupted terrain traditionally associated with seismic waves generated by the Caloris impact, although the precise history of this terrain continues to be investigated.
BepiColombo will now build on the discoveries of Mariner 10 and MESSENGER. By combining measurements from two orbiters, the mission should provide a much more complete picture of Mercury’s interior, surface, exosphere and magnetic environment—and may finally resolve some of the mysteries raised by the first spacecraft more than half a century ago.