Closer to Mercury than ever before


Final system acceptance tests are underway. The mission of the European Space Agency to Mercury is on track for launch in 2018. All eyes are now on BepiColombo.

Aiming to a closer observation of Mercury, the BepiColombo mission of the European Space Agency (ESA) inaugurates a new era in the European Astronomy. In contrast with other planetary missions of ESA, which have visited relatively colder parts of the Solar System, BepiColombo, is going to perform an operation very close to the Sun. In the years to come this planetary mission will enrich our so far knowledge about the least explored planet in the inner solar system with new information about its geophysics, composition, exosphere, magnetosphere and history. The two orbiters of the mission, Mercury Planetary Orbiter-MPO and the Mercury Magnetospheric Orbiter-MMO, will be largely operating in the realm of the unknown, observing and collecting data for one Earth year of a more uncharted territory. Jan van Casteren, Special advisor of BepiColombo mission talks about this astronomical challenge, which will launch in April 2018 and arrive in Mercury in late 2024.

Mr. van Casteren, which are the scientific objectives for the mission?

We go with BepiColombo to the planet Mercury, which is the inner-most planet of our solar system. Due to the fact that it is very close to the Sun Mercury is very difficult to observe. Scientists, have always been interested in understanding how planets are formed, how they evolve and how it is possible for life to originate there. In fact to gain a deeper understanding, we look at the earth-like planets in our solar systems: Mars, Earth, Venus and Mercury. They are actually totally different from each other and the reason is one of the fundamental questions. ESA had missions to Mars and Venus, but not yet to the inner-most planet. Hence, Mercury science became a high priority so far. What we want is to investigate the planet, its interior structure, what it is made of at the surface but also the external environment: the magnetic sphere and the exosphere. To do that, BepiColombo will make an in-depth investigation with two orbiters. It will be the first time that two orbiters simultaneously will orbit the planet Mercury. One very close to the planet and the other one a little bit further away. This means that we can separate the inner and the outer components of the magnetic field and the exosphere around the planet and that we get a better grip on the dynamics and on the gradients. Moreover, the orbiter that is closer to the planet can make a global mapping of the planet in high resolution for the first time.

Artist impression of what I call “the hell of our solar system”

Several tests are conducted all this period before its final launch. In which phase are these tests at the moment?

We have done almost all of the development tests. But before we could do these tests it was important to develop all new technology, necessary for this challenging mission. The surface temperatures at Mercury are extreme, up to 450 degrees Centigrade and the solar radiation is ten times higher than on Earth, so this means that everything on the outside of the spacecraft has to withstand these harsh conditions. We have never had a satellite that goes to such a hot environment, so we had to develop a lot of new materials and equipment in order to do that. An exceptional amount of development and testing was completed over a period of ten years on a lot of equipment, developing new materials, solar cells, mechanisms, antennas, coatings, etc. Also, we developed an electric propulsion system based on ion engines to propel the orbiters to Mercury. All this new technology developments has taken the longest time but it is now successfully completed. All equipment was built, delivered and integrated and we are now at the final stages of the system testing.                    

Are there any unpredictable factors that could put the journey in danger? Which are your safety nets in this case?

The unpredictability could be mainly in the environment and how the materials and equipment behave in that environment. For this reason, we have modeled and tested it in the best possible way. We already know a lot about the Sun, its radiation and how harsh it is at the Mercury environment, so we believe that the remaining uncertainties can be absorbed by the design margins that we have installed for that purpose. For example, when you expose the solar array to the very high intensity at Mercury, close to the Sun, the radiation is about 10 times more intense than in Earth’s proximity. In combination with the high temperatures of the solar cells, this gives uncertainties on the solar array output over the lifetime. Therefore we apply a margin on the solar array performance to account for uncertainties in the degradation due to for example high intensity ultra-violet radiation, high temperature, particle radiation and contamination.

BepiColombo composite spacecraft under test at ESTEC

NASA’s MESSENGER has already visited Mercury. What more will BepiColombo add to our so far knowledge about the planet? Is it about the dual spacecraft?

“Grosso modo” I would say there are three aspects related to this question. The first aspect is the one that you just mentioned: we have two orbiters so we can map in a much better way the magnetic and the exospheric environment. The second aspect is that one of the spacecraft goes into an orbit, which will be all the time very close to the planet in comparison with MESSENGER’s orbit that was only in a limited part close to the surface of Mercury. MESSENGER’s orbit was highly eccentric thus a high resolution mapping of the planet was possible only in the northern hemisphere. Instead, BepiColombo will be able to perform a close surveillance globally with imaging and spectral mapping of the surface at a much better resolution. The third aspect comes from the MESSENGER science community itself. Science progresses with new investigations but often, newly gained insights may also uncover other aspects that are not understood. And this has naturally happened with MESSENGER science as well. MESSENGER has provided us with fantastic new knowledge about Mercury but also scientists have come up with a number of questions for further investigation and these are all areas that BepiColombo will address.

BepiColombo orbits, MPO at low altitude, MMO at higher eccentricity

Why is this mission a cornerstone for ESA?

Maybe I can interpret your question in two ways. The easy one is the financial budget for a cornerstone, which is normally above 650 million euros. With a budget of 1.2 billion, BepiColombo classifies as a cornerstone mission. But then the more difficult question, why does the Agency want to spend that amount of money on Mercury science, why is this so important that it is worth a cornerstone mission; that is really more a scientific question. But I think it comes back to the answer to your first question about BepiColombo’s scientific objectives.  Planetary science is important for scientists to contribute to the understanding how life originates, how planets are formed and how planets evolve. Therefore, the Agency feels that once every so many years we must have a major planetary mission to advance the investigations on the fundamental questions of life.

MPO in thermal vacuum test in the Large Space Simulator at ESTEC
The interview was first published in Science View.