What a Sun-day
Astrophysics (IPA)
Last Sunday night, the Solar Orbiter satellite has been launched from NASA’s Kennedy Space Center in Cape Canaveral, Florida, with the mission of capturing unprecedented images of the Sun and providing novel insight into its behaviour. ETH physics professor Louise Harra has been involved over two decades throughout all stages of the project.
When the Atlas-V rocket rose into the nightly sky above Cape Canaveral on Sunday at 23:03 h, a highly ambitious project got off the ground. Encapsulated in the nose section of the rocket was the Solar Orbiter satellite. With ten scientific instruments on board, Solar Orbiter — a mission of the European Space Agency (ESA), with strong participation from US space agency NASA — is on its way to provide fresh perspectives on the Sun.
From drawing board to launch pad
Among those present in Florida for the launch was Louise Harra, formerly Professor at University College London and since May 2019 Director of the Physical Meteorological Observatory Davos / World Radiation Center (PMOD/WRC) and Affiliated Professor of Solar Astrophysics at ETH Zurich. Harra is one of the world’s leading experts in the field of solar physics and is substantially involved in the Solar Orbiter mission. She has been a part of the endeavour since its very early days, when the mission was designed in the early 2000s. Later she contributed to securing funding for this project, whose total cost run to around 1,5 billion CHF, to the engineering phase and now, after the successful lift-off, to the operation and eventually to data analysis.
"A mix of emotions"
With the launch being the climax after such a long period of preparation, it also brought out a lot of emotions. This all the more as there were two delays to the launch time, due to technical and weather issues. "The actual launch itself went smoothly," say Harra. "Many people who had worked over two decades on the mission were there to see it go — from all across Europe and the US." The days in Florida were "a mix of emotions", she says, "of tension, happiness, and trepidation for the next steps forwards." So far, all went to plan: "The spacecraft is in the right orbit, the solar panels deployed, and communication was made."
And now, Solar Orbiter is set to become a milestone in our understanding of the Sun. Its mission is to observe the Sun from a relatively close distance and from new angles. Specifically, the satellite will approach the Sun up to 42 million kilometres, roughly a third of the average distance between Earth and Sun. Owing to this relative closeness, Solar Orbiter will be able to hover over a given region of the Sun and follow it, unlike other Sun-observing satellites, which watch the surface rotating underneath them. Moreover, the trajectory of the satellite will bring Solar Orbiter to relatively high latitudes as it orbits the Sun. That is, Solar Orbiter will view the poles from angles higher than 30 degrees, whereas from Earth, the viewing angle is 7 degrees, at best. This means that Solar Orbiter will be sending home the first images of the Sun’s poles. “At that stage, the mission is guaranteed to deliver entirely new results,” says Harra, “as we have never seen the poles before.”
Something new over the Sun
Studying the Sun from these new viewpoints is significant, in particular to understand the solar corona, the aura of plasma surrounding the star. The corona has a temperature of ~1 million °C, whereas the surface is ‘only’ at ~6000 °C. Why the corona is so tremendously hot is not fully understood. A key to understanding coronal heating is, however, the magnetic field of the Sun. And in this respect the polar regions, from where the field lines emerge, are particularly interesting. Related to the magnetic field are also eruptions on the surface of the Sun and resulting coronal mass ejections. These ejections are responsible for energy-rich ‘solar wind’, which gives rise to polar lights, but can also critically — and expensively — disrupt satellites (and thus global navigation systems), electric-power grids and aviation. Better forecasts of ‘space weather’ are not the primary focus of science missions such as Solar Orbiter, but the insights into the physical mechanisms underlying the behaviour of the Sun will be a crucial input to better understanding the effects we experience on Earth. “Currently, space-weather predictions are simply not sufficiently accurate to issue alerts that could help to mitigate damaging effects on critical infrastructure,” says Harra. “Today we are with space-weather forecasts at a stage where we were 50 years ago with forecasts for terrestrial weather.”
Important contributions from Switzerland
The ‘eyes and noses’ of Solar Orbiter, which enable it to image and analyse the solar corona and solar winds are ten scientific instruments. Harra is co-Principal Investigator of one of them, the EUI (Extreme Ultraviolet Imager). Moreover PMOD/WRC was involved with the SPICE (Spectral Imaging of the Coronal Environment) instrument. A further crucial contribution from the Swiss solar-physics community is the STIX (X-ray Spectrometer/Telescope) instrument, for which Säm Krucker from FHNW Windisch is the Principal Investigator.
From generation to generation
For Harra, Solar Orbiter is the second major mission she sees through. The first was the Hinode mission, a geocentric satellite that launched in 2006 and is likely to operate until the mid-2020s. Hinode carries three main instruments for investigating the Sun, among them the Extreme-Ultraviolet Imaging Spectrometer (EIS), for which Harra is the Principal Investigator. With Hinode still taking data — despite having been planned originally as a three-year mission — and with Solar Orbiter now ready for the exciting chapter of data taking, part of Harra’s focus is already on the next mission. Last year, the ESA Voyage 2050 mission reached its White Paper stage. Harra and her team contributed a paper on exploring the solar polar regions, to eventually provide further unique insight into magnetic phenomena on the Sun’s surface and on mass ejection. And while the fate of the Voyage 2050 mission is still written in the stars, as it were, that project serves already a very real purpose: “It inspires students,” says Harra, “and it is exciting to see how with each project — Hinode, Solar Orbiter and Voyage 2050 — a new generation of highly motivated scientists emerges, which in the course of these long-term projects grows together to become family.”