Science

ESA’s BepiColombo Orbiters Fire Up Instruments On First Mercury Flyby

Planetary missions are torturous affairs at best; typically, they take decades from conception to fruition. The European Space Agency’s (ESA) BepiColombo Mission to Mercury is a classic case in point. The 650 million euro ($754 million) eight-year mission, with the Japanese Space Agency (JAXA) as a partner, finally made it to within striking distance of our solar system’s innermost planet, which orbits our star once every 88 days. 

Mercury is among the least explored planets in the solar system not because there’s no scientific interest in the tiny planet, but because it’s so difficult to get to. It’s not even easy for amateur astronomers to see from Earth.

But the planetary science community understands that Mercury is a potential Rosetta stone for understanding the innermost planets of solar systems that vaguely resemble our own. 

“Understanding exoplanets requires that we understand Mercury; understanding Mercury requires that we understand exoplanets,” Paul Byrne, a planetary scientist at Washington University in St. Louis, told me. 

Is our own Mercury an oddball?

It sure seems that way; it’s an iron-rich oddity that appears to have lost much of its outer surface layers. Either that, it simply formed in an inner region of the solar nebula that was much more iron-rich than others. But more likely, it was stripped of its initial outer surface in some sort of catastrophic encounter with another nascent planetary body (or bodies).

We don’t know if Mercury’s iron core is the result of a giant collision or simply a consequence of that protoplanetary disk chemistry, says Byrne.

Yet, the impact history of Mercury, and particularly the potential stripping of its mantle and loss of volatile elements —- elements that evaporate easily at room temperature —- can teach us a lot about terrestrial planet formation near the host star, Stephen Kane, a planetary scientist at the University of California, Riverside, told me.

When BepiColombo finally begins its science operations in 2026, it should answer many such remaining questions that weren’t answered by NASA’s MESSENGER Mission.

Early this month, as BepiColombo swooped past in a close gravity assist flyby at less than 200km, the mission was able to sample the magnetic and particle environment around Mercury, while the gravitational pull of the planet was felt by its accelerometers, notes ESA.

When it finally arrives to stay at Mercury, its two orbiters, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (Mio) will begin science operations and map the surface of Mercury and analyze its composition to learn more about its formation.  

MESSENGER saw a world heavily scarred by impact bombardment but also surprisingly volatile rich for a rocky planet so close to its parent star, Kane and co-authors write in a recent review paper appearing in the Journal of Geophysical Research: PLANETS.

One of BepiColombo’s goals is to use Mercury as a window onto our own solar system’s earliest solar nebula to learn more about our early solar system’s composition and formation.  Mercury seems to harbor an intrinsic magnetic field while the Mars, Moon and Venus have none. And do this tiny planet’s permanently shadowed craters harbor sulfur or water ice? 

And did it actually form where it currently lies or much farther out in the solar nebula and then migrate inward?

We don’t know much about Mercury’s early orbital evolution, says Byrne. But we don’t have any solid evidence that Mercury formed farther out, he says.

As Kane and co-authors point out in their review paper, there currently is a dearth of known terrestrial type extrasolar planets with orbital periods less than 100 days.  

Mercury offers us a natural laboratory for understanding how a rocky planet close to its star can form with and retain substantial inventories of moderately volatile species such as carbon and sulfur, Kane and colleagues note in their paper.

But to fully characterize how rocky planets in inner solar systems form and evolve over cosmic time may take another two decades or more. Patience is no longer seen as a virtue in today’s get it done yesterday society. But patience is what it will take to finally garner answers from planets like Mercury. 

BepiColombo is going to return critical chemical and geophysical measurements of Mercury that will help us take a major step forwarding in understanding why the planet is the way it is, says Byrne.

As for potential extrasolar Mercury-type planets out there?

“Establishing whether planets with similar interior structures and with volatile-rich compositions are present in other planetary systems will help us make more sense of Mercury and our own Solar System,” said Byrne.


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