The weekend (October 1), European and Japanese space agency BepiColombo’s joint spacecraft overtook Mercury for the first time, just skimming 199 kilometers above the surface of the planet!
It’s a close shave, but it was on Mercury night, so no images were taken. Soon after, however, when she was 2,418 km away, it took this extremely cool photo of the innermost planet:
Wow. On the left is the original plan and on the right some craters are annotated. You can also see parts of the spaceship in the foreground. I’m still amazed at how much Mercury looks like our own Moon: gray, airless, and just covered in impact craters. But in reality the two are very different, which is why sending probes to Mercury is a good idea.
BepiColombo is an ambitious mission. These are actually three spacecraft, temporarily docked for the trip (which I’ll get to in a moment, and yes, it’s a journey). There are two scientific spacecraft: the Mercury Planet Orbiter (built by Europeans) and the Mercury Magnetospheric Orbiter (nicknamed Mio and built by JAXA, the Japanese agency). These are loaded with instruments to study the geology of Mercury, its surface mineralogy, its magnetic field, and to measure the characteristics of the planet’s enormous liquid / solid iron core, which extends to 85% of its center on its surface.
The Mercury transfer module is the delivery vehicle for the two science ships. It has solar panels for power – useful when diving so close to the sun; Mercury’s elliptical orbit causes it to pass from about 46 to 70 million km from the Sun.
The module uses ion propulsion for thrust. Unlike large, loud, heavy chemical rockets (the kind you usually see), this uses an electric field to accelerate the xenon ions out the rear to an extremely high speed, much faster than chemical rocket exhaust. The thrust is incredibly low, as it uses very little xenon at any given time, but it can push for months, as chemical rockets travel through their fuel supply much faster. It therefore uses very low accelerations over very long periods of time to change course, with the advantage of using much less valuable mass during the initial launch.
BepiColombo launched on October 20, 2018. Here’s the fun part: getting to Mercury is hard. You can’t just throw something at the Sun and hope to get there! The Earth orbits the Sun at more than 30 km / second, so you have to cancel this movement to descend towards Mercury. It takes a lot of thrust, usually more than what our current technology can easily provide to a spacecraft (even with ion engines).
So BepiColombo shoots a Robin Hood over the planets.
When we talk about orbital mechanics, we have to think in terms of energy. The Earth moves around the Sun in an orbit of higher energy than Mercury (or, in other words, it takes a lot of energy to move away from the Sun, from Mercury to Earth, so going into the other way you have to to delete this energy). At launch, BepiColombo has too much energy, so engineers have carved out a devilishly intelligent path for it over time which takes energy from the spaceship and gives it to the planets!
It launched into an elliptical orbit that took it a little farther from the Sun than Earth, then dropped it a little closer. It then passed Earth on April 10, 2020, giving some of its energy to Earth. This caused the spacecraft to fall towards the Sun … and in turn moved the Earth a little further from the Sun! It depends on the mass ratio of the two objects, and since the Earth is a bit more massive than BepiColombo – about a sextillion times (1021) – our planet has not moved enough to even measure.
It then fell just inside the orbit of Venus and on October 15, 2020 did essentially the same. He used gravity and the motion of Venus to place himself in an elliptical orbit that passed Venus a second time on August 11, 2021. He gave Venus enough energy to make it descend even further towards the Sun.
But it still has too much energy, so on October 1, 2021, it flew over Mercury to dump more of its energy – and send back those amazing images. But this is only the first passage! It is still on a path that is too elliptical, so it will pass through Mercury five more times, finally making a circularization of its orbit around the Sun enough so that it can then slowly approach the planet and go into orbit. It is expected to achieve this goal on December 5, 2025.
Mercury is a strange place. It is the smallest planet, but also very dense. The surface is rocky and the rock is of low density, so it must have a dense iron core. NASA’s phenomenal MESSENGER spacecraft orbiting Mercury for four years, finding out quite a bit about it, including finding that the nucleus was larger than expected and had to be at least partially liquid to generate Mercury’s magnetic field. BepiColombo will follow this goal by obtaining more data to help scientists understand why Mercury is like this.
One idea is that Mercury was originally bigger then was hit, hard, by a smaller planetesimal (objects of the early solar system measuring between 1 and 100 km from which the planets were formed). This impact stripped much of Mercury’s crust and mantle, leaving behind a planet with what appears to be an oversized core. BepiColombo data could support or refute this hypothesis.
Mercury is difficult to study from Earth because it is always near the Sun in the sky, making it difficult to observe. But it is also difficult to get there, so only two spacecraft (Sailor 10, who made three overflights, and MESSENGER), visited it. BepiColombo will be the third and hopefully answer a lot of questions about this enigmatic planet.
And I hope to raise more questions. In fact, it is a guarantee; the more we learn about anything in the Universe, the more we find that we need to learn.
Mercurius Announcement, BepiColombo. Only five passes to go.