JAXA Japan Aerospace Exploration Agency

Q. What research will the Mercury Exploration Mission BepiColombo carry out?

Structure of BepiColombo. From left to right: MMO, sun shield, MPO, Mercury Transfer Module (Courtesy of ESA and AOES Medialab)


Mercury (Courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Mercury is a rocky planet like Earth, and it has a magnetic field, but it looks totally different from our planet. BepiColombo consists of two satellites, built by Europe and Japan, whose mission is to explore Mercury's surface, internal structure and magnetic field. The goal is to answer questions about the current state of Mercury, its origins and its evolution. Specifically, the European Space Agency's Mercury Planetary Orbiter (MPO) will study the topography and composition of the planet's surface, while the Japanese Mercury Magnetospheric Orbiter (MMO) will observe its magnetic field, magnetosphere and atmosphere. BepiColombo is scheduled to be launched aboard the Arianme 5 rocket in 2014, and to reach the orbit of Mercury after a six year cruise. It will then conduct observations for about a year.

Q. What's the reason for exploring Mercury now?

Mercury is half the size of Earth, and until recently such a small planet was thought to have a solid interior and no magnetic field. However, when NASA's Mariner 10 probe did a swing-by of Mercury in 1974-75, it discovered the planet did indeed have a magnetic field and a magnetosphere. This was an enormous discovery that nobody had ever expected. It created a greater interest in Mercury, and made scientists eager to conduct thorough research.
But reaching the orbit of Mercury, which is located in the innermost part of the Solar System, takes a lot of fuel and time, and it is also technically difficult. The planet is very close to the Sun, so its environment is very harsh, with large amounts of solar radiation and heat. On Mercury's surface, the temperature reaches 400°C on the dayside, but goes down to -180°C on the nightside, so the orbiter needs to be able to cope with severe temperature variations.
Thanks to recent technological progress, we were able to overcome these obstacles, and finally the time has come to travel to Mercury. Even though this could still be a very difficult and challenging mission from a technical perspective, we have huge aspirations as we get ready to travel to a planet that no humans have ever explored before.

Q. What stage is BepiColombo at right now, and what challenges is the project facing?

Mercury Magnetospheric Orbiter (MMO) (Courtesy of the Research Institute for Sustainable Humanosphere)


Mechanical Test Model of the MMO

Currently, we are doing vibration, acoustic and impact testing on a Mechanical Test Model of the MMO in a simulated Mercury environment. We have already started to produce actual science instruments, so designs that existed only on a paper have now started to become real. The launch is set for 2014, but one year before the launch we are scheduled to transport the MMO to Europe and subject it to a full test.
The main challenge is Mercury's harsh environment. For example, the intensity of sunlight there is more than 10 times greater than on Earth, so we are looking at whether solar cells will deteriorate under such conditions.
The most serious problem is protecting the probe from heat. The MMO will use a transmitter to send observation data to Earth. This transmitter itself releases a great deal of heat, and in such a hot environment it's impossible to cool it. So if we operate the transmitter too much, it will overheat and might not be able to communicate. Although we monitor the orbiter's temperature from our control facility, the probe will be very far from Earth, so if we notice any problems, we won't be able to turn off the transmitter quickly enough. Thus we have to calculate in advance how the orbiter's temperature will change, and plan a very precise operation schedule for data acquisition and transmission. In other words, we have to establish a systematic operation method in an environment we know next to nothing about.
At the same time, there is no sunlight on the night side of Mercury, which means the orbiter cannot generate any electricity. So when it moves to the nightside, we have to stop all operations. You might think we could solve this problem by loading up the orbiter with batteries, but there are limits to its size and total mass, so that's not possible. And in addition, whatever payload it does carry also has to be able to withstand the harsh environment. I think this is the hardest part.

Q. What are your expectations for the BepiColombo mission?

Mercury surface (Courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

What we know about Mercury so far defies common sense. When a small planet is created, it should cool down very fast and become rock solid. Then it's not supposed to have a magnetic field. But Mercury does have a magnetic field. In addition, there is very little atmosphere around Mercury, and the magnetic field touches the planet's surface directly, so do ionized gases from space hit the surface directly.
On Earth, we have a phenomenon called aurora [known in the northern hemisphere as northern lights], which produces luminescence when plasma particles from the Sun strike the atmosphere after being captured by a magnetic field. Since Mercury also has a magnetic field, we believe that a similar but different kind of dynamic phenomenon occurs frequently there. I'm interested in the physics of gases in space, so from this perspective I'd like to know what's going on in space around Mercury. It is very important to compare data from the vicinity of Earth and Mercury, which have similar but different environments, in order to understand the physics of space gases in the universe.
Until now, I think we could imagine the conditions on another planet based only on our knowledge of Earth: "Since Earth is like this, this planet should be like that." But once our orbiter reaches the planet, we can actually acquire empirical knowledge, which is completely different. We can study and understand another planet as precisely as we've studied Earth, by actually going there. And when we find something, we shouldn't stop at saying "wow!" It's important to delve deeper, to think about why this phenomenon occurs, what kind of scientific laws are at work, and so on. I'd like to deepen my understanding of planets by using the precise data captured by BepiColombo on Mercury to try to figure out the universal laws of space.