ESA's Solar Orbiter Should Solve Mystery Of Sun's Outermost Atmosphere

Forbes

21-06-2025

ESA's Solar Orbiter mission will face the Sun from within the orbit of Mercury at its closest ... More approach.
The European Space Agency's Solar Orbiter mission recently stunned the world with the first-ever full images of our Sun's South pole, proving that this was going to be a mission like no other. Using an orbital gravity assist from the planet Venus, the Solar Orbiter mission spacecraft was able to maneuver into an orbit that has taken it to an angle 17 degrees below the Sun's equator.
Over the coming years, the spacecraft will tilt its orbit even further, so the best views are yet to come, says ESA.
The 1.2-billion-euro Solar Orbiter mission, with NASA participation, should finally help us understand the origin of the Sun's solar winds as well as our understanding of the Sun's poles. And arguably most importantly, it should solve the puzzle of why our star's outermost atmosphere, or corona, is heated to millions of degrees Kelvin and is thus so much hotter than the Sun's own surface. By contrast, our Sun's visible photosphere, or surface, averages only 5,500 degrees K.
With Solar Orbiter, we are clearly seeing energy releases on the nano-flare scale, Daniel Mueller, a solar physicist and ESA project scientist for both ESA's SOHO and Solar Orbiter missions to the Sun, tells me in his office in The Netherlands. But the question is, would these nano-flares continue like that infinitely, or is there a certain lower limit to the production of these nano-flares, Mueller wonders.
The puzzle is whether these nano-flares are enough to heat up the Sun's corona to the temperatures with which it is routinely measured.
A Unique View
Launched in 2020, from its highly elliptical orbit just inside Mercury's perihelion, the closest point in our innermost planet's solar orbit, the ESA spacecraft offers the best views yet of our own yellow dwarf star.
We can see on scales down to about 200 kilometers on the Sun, which shows us a lot of dynamics of our star, says Mueller.
And thanks to its newly tilted orbit around the Sun, the European Space Agency-led Solar Orbiter spacecraft is the first to image the Sun's poles from outside the ecliptic plane (the imaginary geometric plane in which our Earth orbits the Sun), says ESA.
We observed the Sun's North pole at the end of this past April, says Mueller. But we passed the Southern pole first and then the Northern pole six weeks later, he says.
At the moment, as seen from Earth, the Solar Orbiter is almost behind the Sun, so the data downlink has slowed to a trickle. But by early October, Mueller expects to have downloaded all the data from Solar Orbiter's Spring polar observations of the Sun.
And within a matter of two to three months after the data is on the ground, the first scientific results will have been written up and submitted to journals for publication, says Mueller.
These observations are also key to understanding the Sun's magnetic field and why it flips roughly every 11 years, coinciding with a peak in solar activity, says ESA. The spacecraft's instruments show that the Sun's South pole is a bit of a magnetic mess now, with both North and South polarity magnetic fields present, ESA notes.
Ready To Flip
Right now, there is not a clear dominant magnetic polarity, but a mix of the two, says Mueller. And that is exactly what you would expect to find during the maximum of the Sun's activity cycle, when the magnetic field is about to flip, he says.
The real applications are for space weather predictions. Case in point, better space weather forecasting may have saved many of Elon Musk's 523 Starlink satellites that reentered Earth's atmosphere between 2020 and 2024.
This period coincides with the rising phase of solar cycle 25, which has shown itself to be more intense than the previous solar cycle, the authors of a 2025 paper appearing in the journal Frontiers in Astronomy and Space Sciences write. Our results indisputably show that satellites reenter faster with higher geomagnetic activity, the authors note.
There was a big solar storm that caused the earth's upper Earth atmosphere to expand, so, the satellites experienced more drag, and therefore didn't make it to orbit, says Mueller.
One option may have been simply to hold off on launches until this increased period of solar activity enabled a less risky geomagnetic environment in Earth's upper atmosphere. The hope is that the Solar Orbiter mission and other missions like it will lead to better and more reliable space weather predictions that could potentially save hundreds of millions of dollars in the commercial satellite industry.
Solar Orbiter should do its share in solving both pure solar physics conundrums as well as in more practical applications like space weather. The good news is that the spacecraft still has plenty of fuel left.
Our current funding goes until the end of 2026, but because we had a picture-perfect launch provided by United Launch Alliance and NASA, we saved a lot of fuel, says Mueller. So, the onboard fuel reserves are so large that we can keep going for a long time, he says.