BepiColombo: ESA Shares Third Mercury Flyby Exciting Results

BepiColombo, a joint space mission of the European Space Agency (ESA) and the Japanese Space Agency (JAXA), continues to make significant progress. During its third Mercury flyby in June 2023, the mission collected a substantial amount of data which has now been analyzed, providing many new discoveries.

The BepiColombo mission

Launched in 2018, this mission aims to provide a better understanding of the inner planet of the Solar System: Mercury. Close to the Sun and more difficult for an orbiter to reach than Saturn, this small desert world is the least explored planet of the inner Solar System.

BepiColombo is composed of two distinct science orbiters currently staked on a single spacecraft: the ESA-led Mercury Planetary Orbiter (MPO) and the JAXA-led Mercury Magnetospheric Orbiter (MMO). These orbiters will be separated and deployed in complementary Mercury orbits in 2026 to provide the essential dual-spacecraft measurements needed to paint a complete picture of Mercury’s dynamic environment.

To date, the BepiColombo spacecraft has made four Mercury flybys, the last of which occurred on September 4, 2024, and was the closest flyby of a planet ever.

This spacecraft has collected a lot of data and will continue to collect much more with the goal of answering some questions: Why is there ice in the polar craters of the scorched planet? Why does Mercury have a magnetic field? And what are the mysterious “hollows” on its surface?

Data from the third flyby

After analyzing the data collected during the third flyby, ESA scientists are one step closer to answering these questions, in particular understanding the nature of Mercury’s magnetosphere.

During the flyby, the spacecraft passed through Mercury’s magnetosphere in just 30 minutes, moving rapidly with a closest approach of just 235 km. During this short time, Bepicolombo sampled different types of particles, measuring their temperature and motion.

Using computer models, the team combined BepiColombo’s measurements to determine the origin of the detected particles, revealing the characteristics of the magnetosphere.

Revealing Mercury’s magnetic secrets

Among the predicted structures was the “shock” boundary between the solar wind and the magnetosphere, as well as the “horns” of the tail of the plasma sheet, a region of hotter and denser ionized gas extending from the back of Mercury in the opposite direction from the Sun.

But this flyby also brought surprises: after the bow shock, BepiColombo crossed the magnetopause, which separates the shocked solar wind in the magnetosheath from the rest of Mercury’s magnetosphere. Measurements made in this area resulted in a relevant discovery: the detection of a low latitude boundary layer, a region of turbulent plasma at the edge of the magnetosphere.

This zone contained particles with a wider range of energies than previously observed on Mercury, observable due to the sensitivity of BepiColombo’s instruments, especially the mass spectrometer designed specifically for Mercury’s complex environment.

Subsequently, after passing through the plasma sheet horns resulting from electrons being accelerated from the distant plasma tail toward the planet, the spacecraft made another surprising detection. An energy dispersion characteristic of the plasma mantle in the polar regions was observed at the equator.

The presence of these energetic ions both near the equator and at the low altitude at which BepiColombo transited suggests that the spacecraft passed through a weak ring current around Mercury.

A ring current consists of charged particles trapped in a planet’s magnetosphere. Earth has a well-known ring current located far above its surface, but it’s unclear how particles can be trapped so close to Mercury.

This mystery may be solved with ongoing data collection from the MPO and MMO spacecraft in 2026.

Cold Plasma Interactions

In addition, as the spacecraft crossed the planet’s shadow, it was able to interact directly with the surrounding space plasma.

When BepiColombo is illuminated by the Sun, it generates an electrical charge that prevents the detection of colder, heavier ions because they are repelled by the spacecraft’s own charge. However, as the spacecraft moves into the shadowed region, this charge changes, allowing a significant influx of cold plasma ions to be detected.

In particular, oxygen, sodium, and potassium ions have been observed, likely released from Mercury’s surface by micrometeorite impacts or solar wind interactions.

“It’s like we’re suddenly seeing the surface composition ‘exploded’ in 3D through the planet’s very thin atmosphere, known as its exosphere. It’s really exciting to start seeing the link between the planet’s surface and the plasma environment.”

— Dominique Delcourt, former instrument lead of MPPE

Future expectations

The results of BepiColombo’s third flyby mark a significant step forward in understanding Mercury’s complex environment. In addition, BepiColombo’s mission continues, with two final flybys scheduled for December 1, 2024, and January 8, 2025, respectively.

Finally, the dual approach entry into orbit of the MPO and MMO orbiters promises to deepen everything we have seen in a few minutes of flyby, and probably more.

“The observations underscore the need for the two orbiters and their complementary instruments to tell us the whole story and build a complete picture of how the magnetic and plasma environment changes over time and space”

— Geraint Jones, ESA’s BepiColombo project scientist