The European Space Agency‘s BepiColombo Mercury arrival took its opening step on 3 September, when the mission’s Mercury Transfer Module separated successfully from the spacecraft stack after a journey covering 6 billion miles and spanning more than eight years. Only two spacecraft had visited Mercury before BepiColombo, and the separation was described by ESA as ‘one of the most complex planetary arrival sequences ever attempted’ by the agency.
Why Getting to Mercury Takes Longer Than Getting to Pluto
Mercury’s proximity to the Sun creates a navigation problem that trips up any intuitive sense of distance. The Sun’s gravitational pull is so intense that a spacecraft cannot take a direct route and slow down without propulsion systems far beyond what a science probe can carry. BepiColombo’s solution was a sequence of nine planetary flybys, using the gravity of other worlds to gradually bleed off speed. Arrive too fast and the probe overshoots Mercury entirely; arrive too slow and solar gravity bends it permanently off course.
All nine flybys were completed successfully, and at noon local time from ESA’s mission control in Darmstadt, Germany, the first phase of arrival was confirmed. ‘BepiColombo’s Mercury Transfer Module successfully separated from the spacecraft stack,’ ESA said in a statement. ‘This landmark achievement for the ESA and Japan Aerospace Exploration Agency (JAXA) mission marks the first step of BepiColombo’s long-awaited arrival at Mercury.’
The MTM, which combined solar and gas propellant propulsion to fight the Sun’s constant interference, had brought the craft to within 1.8 million miles of Mercury before its job was done. It was then left to drift away. In the control room, relief was immediate. ‘We heard it loud and clear in the voice loop from Flight Dynamics Manager, Frank Budnik, that they could clearly see from the Doppler data that MTM had separated. After all this waiting and preparation, we all looked at each other and hugged. It was a very powerful moment,’ said Emmanuela Bordoni, ESA BepiColombo B-shift Spacecraft Operations Manager.
BepiColombo Mercury Arrival Sets Up a Historic First in Orbit
What comes next is where BepiColombo breaks new ground. According to ESA’s mission briefing, the two remaining science modules, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (Mio), built by JAXA, will enter orbit around Mercury together on 26 November, before separating from each other in December 2026. That will make BepiColombo the first mission ever to place two spacecraft simultaneously in orbit around Mercury, a distinction neither NASA’s Mariner 10 nor its MESSENGER mission could claim.
The primary science phase is scheduled to begin in April 2027. MPO carries 11 science instruments and will eventually settle into its final orbit in March. Mio will study the magnetosphere. The science goals are ambitious: understanding how a rocky planet can form so close to its host star, probing whether Mercury has plate tectonics or volcanism, and mapping both its exterior and interior magnetic fields in detail no previous mission has achieved.
What Earlier Missions Left Unanswered
The two prior Mercury missions gave scientists a foundation, but also a long list of open questions. NASA’s Mariner 10 flew past Mercury three times in the mid-1970s, taking around 2,700 images and making the first ever use of a gravity assist, using Venus to reach the planet. Its magnetometers revealed a weak magnetic field, and radiometer readings returned nighttime temperatures of minus 297 degrees Fahrenheit (minus 183 degrees Celsius) and daytime peaks of 369 degrees Fahrenheit (187 degrees Celsius).
NASA’s MESSENGER mission orbited Mercury for more than four years and took around 200,000 photographs, visually mapping the entire planet. Among its findings were high concentrations of magnesium and calcium on Mercury’s night side, large amounts of water in its exosphere, evidence of past volcanic activity, and confirmation that the polar deposits are dominantly water-ice. MESSENGER also found that Mercury’s magnetic field, though broadly stable and strong enough at the equator to deflect the solar wind, is ‘leaky’: twisted bundles of magnetic fields, some up to 480 miles wide, created gaps through which solar wind could reach the planet’s surface directly.
Unresolved is why Mercury holds a higher iron content in its core than any other planet in the solar system, and what created the planet’s ‘lobate scarps’, uniform ridges that can extend hundreds of miles and reach 1,500 metres at their tallest. Two formation theories exist, but neither has been confirmed. BepiColombo’s twin-orbiter setup is built specifically to gather the data needed to test them.
‘Although we already have great science from the cruise phase and nine planet flybys, it’s fantastic that we’ve taken this first important step towards finally being able to use all the powerful instruments on both MPO and Mio to study Mercury,’ said Geraint Jones, ESA BepiColombo Lead Project Scientist. The mission is named in honour of Italian mathematician and engineer Giuseppe ‘Bepi’ Colombo, and the December 2026 orbital separation will be the moment it truly begins to deliver on that legacy.
