ESA’s BepiColombo probe conducts third flyby of Mercury

by Justin Davenport

The European Space Agency’s (ESA) BepiColombo spacecraft, which is in the midst of its seven-year journey to Mercury, has successfully completed its third flyby of the solar system’s innermost planet.

BepiColombo passed 236 kilometers over the night side of Mercury on Monday, June 19 at 19:34 UTC, and started imaging with its monitoring cameras as it passed into sunlight 13 minutes after its closest approach. The imaging opportunity lasted until 44 hours after closest approach. Beagle Rupes, along with a 218-kilometer diameter peak-ring impact basin that was newly named after Jamaican artist Edna Manley, were notable features in the images.

During the flyby, most of BepiColombo’s science instruments were active. This was the first flyby where BELA (BepiColombo Laser Altimeter) and MORE (Mercury Orbiter Radio-science Experiment) were active, though BELA was only on for functional test purposes. BELA will measure the shape of Mercury’s surface, while MORE will measure Mercury’s core and gravitational field.

BepiColombo’s active science instruments collected magnetic, plasma, and particle monitoring data before, during, and after the flyby. This flyby was another opportunity for the BepiColombo team to check instrument function and to gather useful science prior to the spacecraft’s orbital mission, which starts in late 2025.

Image taken during BepiColombo’s third flyby of Mercury, showing surface features. (Credit: ESA)

The 4,100-kilogram spacecraft has made one flyby of Earth, two of Venus, and three of Mercury, with three more flybys of the innermost planet planned before entering Mercury orbit on Dec. 5, 2025.

These flybys are necessary due to the intense gravitational field of the Sun. After its launch from Kourou, French Guiana on Ariane 5 mission VA245 in 2018, BepiColombo was traveling at an extremely fast speed relative to the Sun.

Nine planetary flybys over seven years would be needed to slow BepiColombo enough, relative to the Sun, to allow the spacecraft to enter orbit around Mercury with its engine and fuel supply. Attempting to enter orbit around Mercury within a few months of launch would have required an extremely large amount of fuel and thrust. 

In fact, the propulsive energy required for a Mercury orbital mission is greater than that of a mission to fly by Pluto. Only one other spacecraft has orbited Mercury, and that was NASA’s MESSENGER probe, which orbited the planet from 2011 to 2015. 

Infographic showing the timeline for BepiColombo’s journey to orbit Mercury. (Credit: ESA)

Like BepiColombo, MESSENGER used a trajectory utilizing multiple planetary flybys to slow the spacecraft enough, relative to the Sun, to enter orbit around Mercury. MESSENGER conducted one flyby of Earth, two flybys of Venus, and three flybys of Mercury before entering Mercury orbit on March 18, 2011. 

The first probe to Mercury was Mariner 10, which launched on Nov. 3, 1973. Mercury had been completely unexplored before Mariner 10’s first flyby on March 29, 1974. Prior to the spacecraft’s first flyby of Mercury, Mariner 10 flew past Venus on Feb. 5, 1974, and became the first spacecraft to use an interplanetary gravity assist maneuver to change its trajectory.

The gravity assist maneuver that Mariner 10 used was developed by Italian scientist Giuseppe “Bepi” Colombo, and it allowed mission planners to use an Atlas-Centaur rocket instead of the larger and costlier Titan III. The gravity assist maneuver also allowed the probe to visit Mercury three times instead of just once.

Gravity assist maneuvers have been a staple of interplanetary missions ever since, and have enabled many missions that would not have been possible without it. BepiColombo’s name honors Mariner 10’s legacy and the scientist who made the mission possible.

Artist’s impression of Mariner 10 on its way to Mercury. (Credit: NASA)

Besides the gravity assist maneuvers, BepiColombo also needed a state-of-the-art propulsion system to make its mission possible. The spacecraft’s solar electric propulsion system needs to function for 15,000 hours in order to conduct all the flybys and to get into orbit around Mercury.

The flybys BepiColombo has performed thus far have already allowed scientists to publish research in a variety of journals. In a conversation with NSF, ESA BepiColombo project scientist Johannes Benkhoff discussed early science returns from the probe.

One example of positive early science returns from BepiColombo involves the solar wind and its interaction with Mercury.

“The coordinated observations with many sensors on both spacecraft monitoring the solar wind interaction with Mercury, passing through boundaries where the solar wind reach the magnetic field etc. has already led so [sic] several publications,” Benkhoff said.

BepiColombo has also made measurements related to Einstein’s Theory of Relativity. Benkhoff mentioned the measurements during the conversation.

“With our radio science instrument MORE we try to measure relativistic effects on our signal during solar conjunctions. Measurements are currently being analyzed.”

ESA infographic of BepiColombo’s science objectives during the Venus flybys. (Credit: ESA)

Scientific measurements were even collected during the spacecraft’s flybys of Venus. As Benkhoff mentioned, “At Venus Moa, Persson et al. confirmed a predicted stagnation region experimentally.” This is referring to a region of Venus’ magnetosheath well above the atmosphere and how it interacts with the solar wind.

BepiColombo’s remaining flybys of Mercury are scheduled for Sept. 5, 2024, Dec. 2, 2024, and Jan. 9, 2025. The September 2024 flyby will come to within around 200 kilometers of Mercury’s surface, while the flyby that December will see BepiColombo pass 40,000 kilometers altitude. 

The January 2025 flyby will place the spacecraft around 345 kilometers from Mercury’s surface and will be the last flyby before Mercury orbital insertion late that year. Before Mercury insertion, two orbiting probes will separate from BepiColombo’s MTM (Mercury Transfer Module). The MTM contains the solar electric propulsion system.

The ESA Mercury Planetary Orbiter and the JAXA Mio, also known as the Mercury Magnetospheric Orbiter, will separate from the MTM and explore the innermost planet from orbit in ways that they could not do with flybys.

The separation of the orbiters will enable the SIMBIO-SYS main mission imager to function. Since the imager is shielded by the spacecraft configuration during the cruise to Mercury, three 1024 x 1024 monochrome monitoring cameras on the MTM are instead used for imagery on all flybys prior to Mercury orbital insertion. 

An illustration showing the monitoring camera angles and the breakdown of the spacecraft’s parts. (Credit: ESA)

The three monitoring cameras are intended for outreach and spacecraft health monitoring, and it is not expected that they will deliver science returns. The resolution of these camera images on the surface of Mercury is no greater than tens of meters, according to Benkhoff.

The MESSENGER mission to Mercury mapped the entire planet for the first time and gave scientists a wealth of data and imagery far beyond anything Mariner 10 could have ever done. Among MESSENGER’s discoveries were water ice in shadowed craters on the poles and past volcanic activity on Mercury’s surface. 

BepiColombo’s mission is to follow up on these discoveries in greater detail. As Benkhoff noted: “MESSENGER has helped us to better plan our science strategy and pointed out to new features not known [sic] when the mission was initiated.”

Image of a portion of Mercury’s surface taken during BepiColombo’s June 2023 flyby. (Credit: ESA)

Key points of difference between MESSENGER and BepiColombo include having two separate spacecraft in orbit around Mercury instead of one, many more instruments, and greater instrument coverage and resolution by BepiColombo’s Mio and MPO probes. What’s more, Mercury’s southern hemisphere will be studied in greater detail as compared to MESSENGER, and BepiColombo has a more sensitive radio science capability, using the MORE and Italian Spring Accelerometer instruments, as opposed to MESSENGER.

Instruments like the MIXS-T high-resolution X-ray imager and the MERTIS thermal IR spectrometer for mineralogy and temperature mapping are also part of BepiColombo’s science package. 

BepiColombo was able to take advantage of newer technology than was available when MESSENGER launched in 2004, but the expected scientific return of these images hinges on the mission’s safe arrival at Mercury at the end of 2025.

(Lead image: BepiColombo’s image of Mercury, taken during the spacecraft’s flyby of the planet on June 19, 2023. Credit: ESA)

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