BepiColombo Completes Mercury Flyby on New Trajectory
The joint European/Japanese BepiColombo mission completed its fourth encounter with Mercury on Wednesday, September 4th 2024. The flyby was the latest of many gravity assists the mission needs to decelerate itself into Mercury orbit, and took the spacecraft closer to the planet’s surface than ever before. Critically, this latest flyby has placed BepiColombo onto a new trajectory following issues with the spacecraft’s propulsion system.
BepiColombo, which launched on an Ariane 5 rocket back in 2018, consists of three spacecraft in a stack. JAXA Mercury Magnetic Orbiter (MMO), also known as Mio, and ESA’s Mercury Planetary Orbiter form the scientific payload of the mission and will be deployed into Mercury orbit. The third spacecraft is the Mercury Transfer Module (MTM), and it is the component currently providing propulsion on the way to Mercury. Back in May, ESA reported that the MTM was experiencing a power problem which prevented the spacecraft from firing its ion engines at full power, only partially restoring the spacecraft’s total thrust.
Unable to continue operating the spacecraft’s propulsion system at the minimum level needed for the originally planned trajectory, members of ESA’s Flight Dynamics team devised a new pathway forward, delaying Mercury orbit insertion from December of 2025 back to November of 2026.
Due to its position deeper in the Sun’s gravity well, reaching Mercury, let alone entering its orbit, is a difficult task. Only two spacecraft had visited Mercury prior to BepiColombo’s October 2021 encounter. NASA’s Mariner 10 flew by Mercury three times between 1974 and 1975 and the later MESSENGER spacecraft orbited the planet from 2011 to 2015. This leaves Mercury as the least explored of the terrestrial planets with significant gaps in our understanding. As an example, MESSENGER’s orbit around Mercury was highly elliptical, and approached the planet much closer over its north pole than its south. This means MESSENGER’s data on Mercury’s southern hemisphere is noticeably lower quality than that of its northern regions, and even then the majority of data remains in the range of hundreds of kilometers per pixel. The Mercury Planetary Orbiter aims to provide a significantly improved and global map of the planet’s surface features.
The full BepiColombo spacecraft is by far the most ambitious mission sent to Mercury, being around three times the launch mass of MESSENGER (though over half of that belongs to the MTM), and aims to provide new, near-global data on Mercury’s surface and magnetic field. During the spacecraft’s latest flyby, the majority of the spacecraft’s experiments were active, including four out of five on JAXA’s MMO, which is currently housed in a conical sunshield. Many instruments are unable to operate fully while the orbiters remain attached to the MTM; some instruments are unable to operate at all, but every little piece of data counts in advancing our understanding of Mercury.
In terms of visuals, it is the MTM’s three Monitoring Cameras (M-CAM 1 thru 3), equipment not originally designed with science in mind, which provided the mission’s closest views of the planet to date. The majority of imagery released so far comes from M-CAM 2, which captured BepiColombo’s first images of Mercury’s south pole. M-CAM 2 and 3 have since been turned off, and M-CAM 1 aims to image Mercury receding as the spacecraft departs; this imagery is yet to be released at time of writing.
BepiColombo is now back on track for Mercury orbit, later than planned, but nonetheless determined to reach it. 2024 will also see the mission’s fifth Mercury flyby in December, with a sixth occurring little over a month later in January of 2025. It is up to mission team members now to stay the course, and ensure that all systems maintain functionality necessary to complete this new course to Mercury. Additional time spent in the high temperature environment near the sun poses additional risk to spacecraft health and impacts the mission’s science phase. For the time being all is well, and BepiColomo’s latest trial has been passed; just the latest example of a hard but necessary choice being made in pursuit of completing objectives in spaceflight.
Edited by Beverly Casillas
