“A panoramic view of the Sun”

Sami K. Solanki, Director at the Max Planck Institute for Solar System Research on the mission of the Solar Orbiter space probe

The researchers have great expectations for the Solar Orbiter space probe. From its elliptical orbit, it will explore the Sun over several years and approach the surface to within approx. 42 million km. The probe has 10 scientific instruments on board. The Max Planck Institute for Solar System Research in Göttingen has been significantly involved in four of them. Sami K. Solanki is Director of the “Sun and Heliosphere” Department and explains the special features of Solar Orbiter.

Professor Solanki, more than half a dozen space probes have explored the Sun more or less up close. Solar Orbiter is yet another. Is this still necessary?

The space-based solar research of the past decades has indeed been a huge success. We owe much of our current knowledge of the Sun to these missions. Each of them had specific objectives and questions. In sum, they provide a picture of the Sun as we know and understand it today. But – as is so often the case in science – findings lead to further questions. Many basic mysteries were not – and cannot – be solved by previous missions. For example, the view of the polar regions of the Sun is still missing. That’s where  Solar Orbiter comes in – because no other mission can do that. And there are other special features. The orbit of Solar Orbiter has been chosen so that the probe flies close to the Sun in synchronism with its rotation. This allows us to look at the same part of the Sun for much longer than ever before and thus to follow how certain structures develop and change. Solar Orbiter will also combine its observations with telescopes on Earth. These then look at the earth-facing side of the Sun while the Solar Orbiter looks at the reverse side. This creates a panoramic view of the Sun.

You’ve mentioned that Solar Orbiter will also investigate the poles of the Sun. What is so special about these?

The poles are the white spot on our solar map, so to speak. We do not even know quite basic properties in this area – such as how fast the Sun rotates at the poles – because unlike the Earth, the Sun rotates at different speeds at different latitudes. We also know nothing more about the properties of the currents that transport the Sun’s magnetic fields from the equator to the poles on a sort of “solar conveyor belt”. Any complete theory of the Sun must include this area. After all, the poles play a special role in the production of solar magnetic fields. The field at the poles is responsible for the strength of the next solar cycle.

Your recent publications give the impression that you and your colleagues have already found out a great deal about solar magnetic fields. Are there still open questions?

A lot. For example, one of the very fundamental questions remains unanswered: Where do the magnetic fields of the Sun originate? Many theories identify the “tachocline”, the area between the radiation and convection zones, as the point of origin. In the radiation zone, the Sun releases its energy primarily through radiation to the outside. In the convection zone above, enormous upward and downward currents of solar plasma are responsible for the energy transport. Because the plasma consists of charged particles and is additionally swirled by the rotation of the Sun, magnetic fields are created in a sort of generator process. That’s the theory anyway. Other explanations suggest that near-surface turbulent flows are the source of the magnetic fields. Solar Orbiter will conduct helioseismic measurements and thus indirectly look into the crucial regions of the Sun – thus providing an answer to this question.

Solar Orbiter will approach the Sun to approx. 42 million km. This will require elaborate cooling measures. What do these entail? And why is the heat shield of the probe black – a colour that stores heat particularly well?

The heat is one of the major technical challenges of this mission. An ingenious heat shield protects the probe from overheating. At first, it might seem paradoxical that this shield is deep black and that a white surface would be more sensible. However, a light surface would darken over time, and the thermal conditions for the probe would deteriorate during operation. The European Space Agency (ESA) chose a different solution. Through a coating developed especially for the Solar Orbiter, the heat shield on its outer side absorbs as much heat as possible. This is then radiated laterally so that only a fraction of the heat reaches the actual body of the space probe. The probe is therefore designed to cope with the worst case scenario. Things cannot get any worse. That’s very clever. The remote sensing instruments look at the Sun through openings in the shield and partly protect themselves through special entrance windows. For example, the windows of our PHI telescopes allow only a tiny fraction of the red, visible light to pass through. The rest is reflected back into space – and thus cannot carry heat into the instrument. The wavelength range that is able to pass contains all the information we need for our measurements. Other measures have been taken to protect other instruments from the heat.

How long have you and your team been working on the project?

We have been working on Solar Orbiter for more than 20 years. The idea for the mission ultimately came from our Institute. My colleague Eckhard Marsch came up with it in the late 1990s. In 2011, ESA decided to implement the mission. We have been developing the hardware and instruments ever since.

When do you expect the first results?

Space missions demand a considerable amount of patience from everyone involved. Solar Orbiter is no exception. The instruments, including PHI, start operations in the second half of February. However, this will not yet produce the scientific data we are hoping for. We must wait until the Solar Orbiter reaches its intended orbit in the winter of next year. The instruments that measure the solar wind will start to collect data much earlier.

Many thanks for this interview!

This interview was conducted by Helmut Hornung