Missions to Mercury: From Mariner 10 to BepiColombo

Mercury is the closest planet to the Sun, but it's also one of the hardest planets to visit. Despite being just 30 to 75 million miles from Earth, depending on where Mercury is in its orbit, only three spacecraft have ever reached it.

Here's the strange thing: it's harder to get to Mercury than to get to Mars or Saturn. The Sun's gravity makes it really tricky to slow down and enter orbit around a small planet that close in. Every Mercury mission so far has had to use clever tricks — including looping past Earth, Venus, and Mercury itself multiple times — just to get into orbit.

Why Are Missions to Mercury So Hard?

This is the part most people don't expect. Mercury is "down" in the gravity well of the Sun. The closer you get to the Sun, the faster you have to be moving to keep from falling in. So a spacecraft heading to Mercury actually speeds up as it falls toward the Sun.

That's a problem. To enter orbit around Mercury, you don't just need to arrive there — you need to slow down enough that Mercury's small gravity can capture you. If you arrive going too fast, you'll just whip past the planet and keep going.

The trick spacecraft use is called a gravity assist or flyby. By passing close to Earth, Venus, or Mercury itself, a spacecraft can use those planets' gravity to gradually slow it down and bend its path. Each flyby steals a tiny bit of the planet's orbital momentum and gives a tiny boost (or in this case, a tiny brake) to the spacecraft. Mercury missions need a lot of these to get into orbit safely.

Mariner 10 (1974–1975) — The First Visit

NASA's Mariner 10 was the first spacecraft ever to reach Mercury. It launched on November 3, 1973, and made three flybys of the planet in 1974 and 1975. Mariner 10 was a pioneer in another way too — it was the first space mission ever to use a gravity assist (around Venus) to reach its target. That technique is now used on almost every long-distance space mission.

What Mariner 10 found:

• Mercury looks a lot like the Moon — gray, rocky, and covered in craters.
• Mercury has a magnetic field, which surprised everyone.
• Mercury has a thin exosphere instead of a real atmosphere.
• Mariner 10 photographed about 45% of Mercury's surface.

The other 55% remained unseen for the next 33 years.

MESSENGER (2011–2015) — The First Orbiter

MESSENGER was NASA's second mission to Mercury, and the first ever to enter orbit around the planet. The name stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. (Yes, NASA spelled it like that on purpose, to spell "messenger.") The MESSENGER probe was about the size of a small car and packed with seven scientific instruments.

MESSENGER launched in August 2004 and took 6.5 years to reach Mercury, using flybys of Earth, Venus, and Mercury itself to slow down. It finally entered orbit on March 18, 2011.

What MESSENGER discovered:

• It mapped 100% of Mercury's surface for the first time.
• It confirmed there's water ice in the permanently shadowed craters at Mercury's poles — one of the biggest planetary science finds of the 21st century.
• It photographed the Caloris Basin in fine detail and discovered the strange spider pattern at its center, called Pantheon Fossae.
• It spotted magnetic tornadoes — huge funnels of solar wind plasma that twist down to Mercury's surface.
• It discovered Mercury's mysterious hollows — bright, shallow depressions found nowhere else in the solar system.

MESSENGER orbited Mercury for over four years, far longer than its planned mission. When it finally ran out of fuel, on April 30, 2015, NASA deliberately crashed it into the surface, where it added one final small crater of its own.

BepiColombo (2018–present) — Mercury's Newest Visitor

BepiColombo is the third mission to Mercury, and it's still on its way. It launched on October 20, 2018, and it's expected to enter orbit around Mercury in November 2026.

BepiColombo is a joint mission between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). It's the first Mercury mission ever launched by a country other than the United States.

BepiColombo is unusual in another way too: it's actually two spacecraft stacked together. They'll separate when they arrive at Mercury and orbit the planet from different angles:

• Mercury Planetary Orbiter (MPO) — built by ESA. Will study Mercury's surface, interior, and exosphere.
• Mio (Mercury Magnetospheric Orbiter) — built by JAXA. Will study Mercury's magnetic field and the way it interacts with the solar wind.

BepiColombo's Long Journey

BepiColombo's trip to Mercury has used nine planetary flybys to slow itself down: one of Earth, two of Venus, and six of Mercury itself. Each flyby gave the spacecraft a chance to take spectacular up-close photos.

The sixth and final Mercury flyby happened on January 8, 2025, when BepiColombo passed just 295 km (183 miles) above Mercury's north pole. The pictures it sent back showed the dark, possibly icy craters at the pole — including the craters Prokofiev, Kandinsky, Tolkien, and Gordimer — in better detail than ever before.

BepiColombo was originally supposed to arrive in orbit around Mercury in December 2025. But in May 2024, ESA reported a problem with its electric thrusters that meant it couldn't push as hard as planned. The team designed a new flight path, and BepiColombo's arrival was pushed back to November 2026. The science mission begins in early 2027.

Who Was Bepi Colombo?

The mission is named after a real person. Giuseppe "Bepi" Colombo (1920–1984) was an Italian mathematician and engineer who worked at the University of Padua. He's the scientist who first figured out Mercury's strange 3:2 spin-orbit pattern in the 1960s.

Bepi Colombo also invented the gravity-assist trajectory technique that Mariner 10 used to reach Mercury, and that almost every space mission since has used to reach distant planets. The BepiColombo mission is named in his honor.

What Will We Learn When BepiColombo Arrives?

When BepiColombo enters orbit in November 2026, it will start answering questions that MESSENGER couldn't fully solve, including:

• Why does such a small planet have a magnetic field at all?
• What exactly are Mercury's hollows, and how do they form?
• How much water ice is really inside the polar craters, and where did it come from?
• What is Mercury's iron core actually like? Is some of it still liquid?
• How are Mercury's surface, exosphere, and magnetic field connected?

It's the most important Mercury mission ever launched. After BepiColombo, the smallest planet won't feel quite so mysterious anymore. Back to the Mercury overview.

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