Flying too close to the Sun, the unwary Icarus melted the wax that held the feathers of his wings made by his father, Daedalus, in place. We know the rest. The myth of Icarus reminds us that the Sun is an obsessive and inaccessible goal.
As stubborn as Icarus, but better equipped, the European space probe Solar Orbiter will soon venture very close to the Sun. Astronomically “very close”: it will remain at a respectable distance of 42 million kilometers – out of the 150 million that separate us from the star – so as not to melt its ten instruments sheltered behind a heat shield.
–
A project of the European Space Agency (ESA), with strong support from NASA, Solar Orbiter is scheduled to take off from Cape Canaveral, Florida on February 10. Its mission: to help astrophysicists unravel the mysteries of our star. Because the Sun retains its share of shade. It has a hidden side which has only been observed on rare occasions. Similarly, its polar regions have been little studied, and for good reason: most of the probes remain in the plane of the ecliptic, that is to say that of the orbit of Earth. So they see it from the front, like we do.
Why is the surface of the star so much hotter than its atmosphere?
–
On a functional level, scientists still have a lot to learn from the Sun, and in particular from its eruptions, these gigantic expulsions of hot matter which release 100 times the annual global energy production in just a few minutes. They generate storms and solar winds, flows of energetic particles which spread throughout the solar system, including on Earth, where they can disturb electronic devices and electrical networks.
Understanding the mechanisms that govern them would allow them to lay the foundations for space weather. Or to solve an old puzzle: why is the surface of the star much hotter than its atmosphere (6000 ° C against 2 million)? These are some of the many challenges that await Solar Orbiter.
Two probes, one star
Notably, the Sun will not have one, but two satellites that will buzz in its ears. Put into orbit in 2018, the NASA Parker Solar Probe probe is already in the game and even delivered a first batch of results in December. Far from being competitors, the two missions are in fact rather complementary: certain phenomena detected by the Parker probe will be observed in a second step by his European cousin.
On our blogs:
Parker observes solar furnace from record distance
Smaller, the Parker probe is moving much closer to the Sun than the Solar Orbiter will do (it is currently at around 24 million km – a record – and will approach it at 6 million km). For its part, the European probe will carry out more numerous measurements, in particular by looking directly at the Sun, which Parker cannot accomplish. “The Parker probe is a bit of a prototype that breaks records, while Solar Orbiter is more like a control truck that harvests data,” compares Thierry Dudok de Wit, from the Laboratory of Environmental Physics and Chemistry and space at the University of Orleans, an astrophysicist involved in the two missions.
Storm hunting
The space management truck carries two types of instruments, some called remote sensing, others in situ. The first scan the Sun in the distance, the second analyze the immediate environment of the probe. In addition to allowing a larger data collection, the interest lies above all in their complementarity. Because to fully understand a physical phenomenon, it is necessary as much as possible to know its origin, the initial conditions which dictate its subsequent behavior. “It is as if, to understand the flow of a river, we went back to its source by observing the currents, the waterfalls and the whirlpools which explain what we observe downstream”, explains Miho Janvier, of the ‘Institute for Space Astrophysics at Paris-Sud University.
With Solar Orbiter, we will be able to study the same phenomenon at two different distances
–
As you can guess, this astrophysicist who presents herself as a “solar storm hunter” is more interested in the Sun than in rivers. But for her, the analogy holds: to understand a solar storm, we must first examine what happens during its genesis, near the surface of the star, using remote sensing instruments. Then, using in situ measurements, examine the consequences (variations in temperature, the intensity of the ambient magnetic field, etc.), once the radiation and particles are expelled into space and passing close to the probe. “With Solar Orbiter, we will be able to study the same phenomenon at two different distances”, she summarizes.
“By making the link between near and far, Solar Orbiter will give us an idea of the dynamics of the phenomena of particle production during solar flares” adds Nicole Vilmer, from the Laboratory of Space Studies and Instrumentation in Astrophysics at the ‘Paris Observatory. With these two separate looks, scientists will feed their models with new data. “It is fundamental for understanding the Sun,” she says.
Stone Age technology
This astronomer is involved in the design of a remote sensing instrument called STIX. Imagined at the Haute Ecole specialized in north-west Switzerland, in Windisch, this two-in-one device combining a telescope and spectrometer must observe solar flares in the X-ray field.
–
“It is important to observe the eruptions directly on the Sun and not only from Earth,” explains André Csillaghy, member of the STIX team and researcher in computer sciences at the HES. With Solar Orbiter, we will be able to follow the process from the Sun to the probe, an unprecedented and unique performance. “
At 42 million kilometers from the Sun, the light received will be around 13 times more intense than that which floods the Earth and the temperature will reach 520 ° C – enough to burn out the instruments. ESA engineers therefore protected them with a large heat shield made of layers of titanium. The properties of this shield, including its ability to absorb and reflect light, or to dissipate it in the form of heat, must remain stable for the seven years that the mission will last. To make sure of this, ESA used an ultra-stable black pigment, “a stone age technology” according to him: it is obtained from calcined bone charcoal, a method applied in the prehistoric cave Chauvet 30,000 years ago.
If the ship holds up, the science that will emerge looks exciting. “An instrument such as Solar Orbiter will answer many questions… while raising new mysteries. It’s exciting, ”says André Csillaghy.
–
