Mercury is the least explored rocky planet of the Solar System, with one of the main reasons being that getting there is really difficult. As BepiColombo gets closer to the Sun, the powerful gravitational pull of our host star accelerates the spacecraft towards it.
Gravity assist flybys are a great way to change course using very little fuel, but they are far from simple.
Flight controllers precisely guided BepiColombo so that it passes Mercury at exactly the right distance, from the right angle, and with the right velocity. All of this was calculated years ago, but had to be as close to perfect as possible on the day.
On 19 May, teams at mission control performed the largest chemical propulsion manoeuvre the mission has seen. The purpose was to correct errors in BepiColombo’s orbit that had accumulated as a result of thruster outages during the previous one-and-a-half month-long, slow electric propulsion arc. Correction manoeuvres on the approach to a flyby are part of normal operations; without this one BepiColombo would be 24 000 km too far from Mercury and on the wrong side of the planet!
To be on the safe side, and to ensure no chance the mission could end up on a collision course with Mercury, the latest manoeuvre was designed so that BepiColombo would pass the rocky planet at a slightly higher altitude than needed. The extra margin was a good bet and cancelled out previous errors that had crept in as the spacecraft traversed millions of kilometres through space. One week out from the flyby BepiColombo is now predicted to pass the planet’s surface at an altitude of 236 km (+/- 5 km).
At the moment of close approach BepiColombo will have accelerated to 5.4 km/s with respect to Mercury courtesy of the planet’s gravitational pull, but the flyby will overall reduce the spacecraft’s velocity magnitude compared to the Sun by 0.8 km/s, and change its direction by 2.6 degrees.
“This is the first time that the complex solar electric propulsion method is being used to get a spacecraft to Mercury, and it represents a big challenge during the remaining part of the cruise phase,” said Santa Martinez Sanmartin, ESA’s BepiColombo mission manager.
“We have already adapted our operations concept to have additional communications passes with our ground stations, enabling us to recover faster from thruster interruptions and to improve orbit determination. And all the while this is working with communications delays of more than ten minutes due to the time it currently takes light signals to travel between Earth and the spacecraft.”
Flight dynamics is both a science and an art. Orbits, manoeuvres and flybys are determined years in advance, but spacecraft are not perfect mathematical objects. This is why teams always err on the side of caution, factoring in multiple opportunities for manoeuvres to hone and correct a spacecraft’s actual path.
Tastes of science
While many instruments have been activated during the cruise phase, some also operated during the flyby, providing another tantalising glimpse of the Mercury science expected during the main mission. Magnetic, plasma and particle monitoring instruments will sample the environment before, during and after closest approach.
BepiColombo science themes. Image credit: ESA
This was the first flyby for which the BepiColombo Laser Altimeter (BELA) and Mercury Orbiter Radio-science Experiment (MORE) was switched on, albeit in the case of BELA for functional test purposes only. Once in Mercury orbit, BELA measured the shape of Mercury’s surface, and MORE investigated Mercury’s gravitational field and core.
“Collecting data during flybys is extremely valuable for the science teams to check their instruments are functioning correctly ahead of the main mission,” says ESA’s BepiColombo project scientist Johannes Benkhoff.
“It also provides a novel opportunity to compare with data collected by NASA’s MESSENGER spacecraft during its 2011–2015 mission at Mercury from complementary locations around the planet not usually accessible from orbit. We are delighted to already have data published based on our previous flybys that generated new science results, which makes us even more excited to get into orbit!”
Upon arrival at Mercury in December 2025, BepiColombo’s two science modules – ESA’s Mercury Planetary Orbiter (MPO) and JAXA’s Mercury Magnetospheric Orbiter (MMO) – will separate from the Mercury Transfer Module (MTM) and enter complementary orbits around the planet.
The main science camera is shielded until the spacecraft modules separate but during flybys snapshots are taken by BepiColombo’s monitoring cameras.
A unique selfie
During the closest approach, BepiColombo was in Mercury’s shadow. The illuminated part of the planet entered the spacecraft’s field of view around 13 minutes later, when BepiColombo was at a distance of about 1840 km.
That means there were no illuminated images from closest approach itself. The most visually appealing images showing the details of Mercury’s surface were captured between about 13 and 23 minutes after close approach.
The cameras provide black-and-white snapshots in 1024 x 1024 pixel resolution. Because of their position on the spacecraft, they also capture one of MTM’s solar arrays and the MPO’s antennas in the foreground of the images. As BepiColombo passes Mercury, the planet appears in the top right of the M-CAM 3 images and moves towards the bottom left.
BepiColombo monitoring cameras. Image credit: ESA
The first images were downlinked within a couple of hours after closest approach and are expected to be available for public release from the afternoon of 20 June onwards.
Source: European Space Agency
