BepiColombo is a revolutionary mission that will study the smallest and innermost planet in the Solar System like never before. But its fate became uncertain a few months ago following a glitch that has prevented its thrusters from operating at full power. The mission team has recently made two important decisions. The spacecraft will get into orbit almost a year later than expected and the fourth flyby will have to be more daring than they had anticipated.

BepiColombo is a joint mission of the European Space Agency (ESA) and the Japanese Space Agency (JAXA). On September 4, the spacecraft will perform its fourth flyby of the planet. This will be the closest BepiColombo will ever get to the planet, closer than was originally deemed safe. It will reach 165 kilometers (103 miles), less than the safe 200 kilometers (120 miles) planned. But the thruster issues have forced the team to make a bold decision that will give them more flexibility in the coming months.

“Originally, all flybys at Mercury [are] at 200-kilometer altitude. There are many reasons you have to navigate at this altitude and this is already quite demanding. One of them is the thermal issues, so it was always set as a limit,” Frank Budnik, who leads the flight dynamics team for BepiColombo, told IFLScience.

The flyby will take place on the night side and only partially across the terminator, making sure that the spacecraft is as safe as possible in its passage. Still, it will be an opportunity to conduct some exciting science and the team will share some images in the coming days.

It was clear very quickly that with this reduced power availability, we will not be able to maintain the old mission.

Frank Budnik

The next two flybys will take place in December and January. They can’t be altered due to orbital mechanics. But the spacecraft has to perform some maneuvers before the fifth flyby and this new altitude will give the spacecraft some more leeway. Basically, it has bought the team time in case the thrusters need a bit longer to slow down BepiColombo.

If you only know space from science fiction, it might seem straightforward to go from one place to another. But in reality, it is not like that. Things launched from Earth have a lot of orbital speed, and going towards the inner Solar System means that you need to slow down massively. You could do that if you had a massive rocket firing its engine, but that’s not efficient; so, spacecraft go on peculiar orbits and do calculated thruster bursts.

The lack of full power is what ultimately led to the decision to postpone entering into orbit around Mercury from December 2025 to November 2026. When that happens, the spacecraft will split into two orbiters that will study the planet in unprecedented detail. There will be some disappointment, but space is tough and sometimes it is better to delay things to guarantee the safety of a mission.

“It was clear very quickly that with this reduced power availability, we will not be able to maintain the old mission,” Budnik told IFLScience.

ESA’s Flight Dynamics team was extremely clever in developing this new maneuver for BepiColombo. Sure, it is daring, but it also finds a solution for a problem they did not expect to have. And both decisions, the change in long-term trajectory and the change in altitude, were taken in the last few weeks.

“It was quite a tough decision, but on the other hand, this is the fourth Mercury swing-by, the seventh in total,” Budnik explained when we asked about the challenge of making the altitude change. “We had quite some navigation experience with the previous swing-bys. And we felt, okay. We did, of course, some computations before, some rule of thumbs… And this is why we said, okay, let's do it. Because in the end, we can only gain from it.”

The next swings past Mercury will be significantly further away from the planet and there are no plans to change those. The spacecraft will spend the next two years and a bit getting closer and closer to Mercury’s speed and orbit until the spacecraft can split and the orbiters can begin their work. But first is the fourth flyby and we are all rooting for a great success.