The Proba-3 mission is a $210 million experimental double spacecraft that will, for the first time ever, perform a precision dance to get in formation for its observations. And doing so, the two spacecraft will recreate the rare phenomenon of a total solar eclipse to help reveal a region around the sun that is invisible to telescopes on Earth, and to probes like the European Space Agency’s (ESA) Solar Orbiter or NASA’s Parker Solar Probe.

“We live in a golden age of solar physics, but we still have this gap,” Tim Horbury, a Professor of Solar Physics at Imperial College London, who is not part of the Proba-3 mission team, told Supercluster. “We can observe the surface of the sun in great detail with instruments like the Extreme Ultraviolet Imager on Solar Orbiter and we can study the corona with occulting coronagraphs, but those coronagraphs don’t see the region just above the surface.”

The corona is the sun’s atmosphere, which is about a million times dimmer than the sun’s visible disc. To observe it, scientists must obscure the sun with a round instrument known as the occulter that is inserted inside the coronagraph. Because light bends around objects and scatters from them — a phenomenon known as diffraction — this occulter must be much larger than the visible solar disc to prevent stray light from spoiling the image. Hence the blind spot, or rather a ring, in our view of the sun. 

This blind spot is not negligible. It extends to 1.5 solar radii from the sun’s surface, covering a dynamic region where physical processes, key to our understanding of the effects of the sun on the planets in the solar system, take place. Just how much must be happening in this in-between space is obvious from the comparison of images from ultraviolet cameras monitoring the surface and those obtained by coronagraphs. While the surface images reveal a jumble of magnetized plasma ribbons looping above the star’s surface, coronagraph images show an orderly assortment of even rays spreading away from the sun in straight lines.

“Proba 3 is going to look at that transition as you go from an environment dominated by plasma to an environment dominated by the magnetic field,” said Horbury.

Sometimes, the twisted ribbons burst, blasting into space vast clouds of charged gas known as coronal mass ejections (CME). These clouds spread across the solar system at enormous speeds up to a 1,000 kilometers per second. When they hit Earth, they cause geomagnetic storms that can damage satellites, knock out power grids and supercharge auroras. How strong such a solar storm would be depends to a great degree on what happens to this magnetized cloud in that invisible region.

“Tracing the way those magnetic fields move around as they come off the [solar] disc and out into space is incredibly important,” Horbury says. “It helps us understand the fundamental science that is going on that links the dynamics that happen on the sun’s surface with what happens in interplanetary space.”

We are not completely in the dark about this mysterious region. About every 18 months, we get a fleeting glimpse of it somewhere in the world when the sun’s disc gets obscured by a natural occulter — the moon — during a total solar eclipse. It’s a sheer cosmic coincidence that the moon has just the right size to perfectly cover the solar disc. Thanks to the moon’s vast distance from Earth-bound observers, the diffraction effect it causes is nearly non-existent, allowing a clear view of the corona’s white light immediately as it rises from the sun’s surface, weaving a wispy spider web along the sun’s magnetic lines.

“The farther the occulter from the observer, the smaller the diffraction effect,” Miloslav Druckmüller, a professor of mathematics at the Brno University of Technology in the Czech Republic and the world’s best-known solar eclipse photographer, told Supercluster. “During the solar eclipse we can capture the corona immediately after the photosphere [the bright disc] disappears.”

Druckmüller, an expert in imaging processing, became the world’s leading authority on solar eclipse photography completely unwittingly. In 1999, he photographed the first total solar eclipse visible from central Europe in 150 years, taking it as a personal challenge to visualize the corona better than anyone else. After months of careful post processing, he published the images online. They were quickly discovered by the solar physics community. The high-resolution photographs show a whirl of magnetized ribbons, exploding plasma bubbles and filaments emerging from the sun’s surface as it gradually smoothens into bright rays that stretch into space. Since 1999 Druckmüller and his growing team of collaborators have photographed every single total solar eclipse to have taken place on the planet.

It was indeed one of Druckmüller’s photographs that ESA’s Proba-3 mission scientist Joe Zander displayed during the pre-launch briefing as the aspirational outcome of the Proba-3 mission.

Druckmüller says that Proba-3 represents “a major leap forward.” The two satellites comprising the mission will fly 150 meters apart, carefully aligned, one serving as an artificial moon, the other being the observer. The long distance between the 340-kg observer and the 200-kg occulter will reduce the diffraction effect and with it the invisible region, making Proba-3 the most efficient coronagraph ever built by humankind.

Instead of the at-best 7-minute-long observation windows provided by natural eclipses, the Proba-3 formation will block out the sun artificially for periods of up to six hours at a time. Where Druckmüller’s photographs provide a chance glimpse into the processes in the blind region around the sun’s surface, Proba-3 will create endless movies. Not relying on serendipitous timing, the satellite and its occulter will be there to capture the most powerful solar flares and coronal mass ejection as they happen and track how they stir the little explored region close to the sun’s surface.

“It will provide us with a lot of information on the details and complexity of the solar corona and the events that are happening there,” Zander said during the pre-launch briefing. “There are many processes of interest that are happening in this region close to the sun. And we hope that with a better understanding of the physics, we can also improve our modelling of these processes and our predictions of their impacts on Earth.”

One of the big questions scientists want Proba-3 to answer is the mechanism of the acceleration of solar wind. Solar wind is a constant stream of ionized gas that emanates and sometimes bursts from the sun’s surface. The solar wind emerges in two forms — the slow solar wind that streams through the solar system at speeds around 500 kilometers per second, and the fast variety that blows through interplanetary space at a mind-boggling two million kilometers per hour. Scientists know these two types of solar wind differ in composition and emerge from different structures in the sun’s corona. What exactly accelerates the solar wind, however, is a mystery. Models, however, show that this acceleration takes place somewhere within the invisible ring.

“With Proba-3, we will be able to see this acceleration as we’ll be able to trace the solar wind much closer to the sun,” said Horbury.

Horbury hopes Proba-3 will be able to observe odd phenomena known as magnetic switchbacks, first spotted in images taken by ESA’s Solar Orbiter a few years ago. Thanks to its ability to see the otherwise invisible region, Proba-3 should be able to trace those switchbacks, sudden changes in the direction of the magnetic field carried by the solar wind, to their origin close to the sun’s surface. Scientists think that these magnetic kinks might be behind the solar wind’s mysterious acceleration.

“It’s about finding a connectivity map all the way through,” Horbury added. “Proba-3 should help us do that.”

The mission’s two spacecraft follow an elliptical path around Earth with the nearest point to Earth at the altitude of 600 km and the highest point 60,530 km above the planet’s surface. When the duo draws the distant arc of that ellipse, far above Earth’s residual atmosphere, they assume their position with millimeter accuracy, guided by a novel optical navigation system that is being tested in space for the first time.

The observer spacecraft’s main instrument is the Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun (ASPIICS), which will observe the sun’s corona as it emerges behind the 1.4m-wide shield of the occulter spacecraft. Taking one image every minute, the ASPIICS camera will capture detailed movies of emerging coronal mass ejections in visible light, helping scientists understand what triggers them.

“Coronal mass ejections are initiated in this region, and we hope to get a very close insight into the process of their initiation," said Zander. "Right now, we can see a coronal mass ejection erupt but we can’t predict when that happens. With enough observations we hope our models would be able to make those predictions.”

Improving predictions of phenomena such as coronal mass ejections will help forecast space weather events on and around Earth, providing satellite and power grid operators with advanced warnings to prepare their systems ahead of time. 

Additional observations through special filters will reveal ions of iron, which will help scientists detect temperature differences within the corona and perhaps shed some light on another mystery of the solar atmosphere — its extreme temperature, which is about 200 times higher than that of the solar surface. 

The mission, which launched on December 5th on India’s Polar Satellite Launch Vehicle, will take until March to test out its systems before attempting its first eclipse-creating formation flights. It will study the sun for two years, after which it will spiral into Earth’s atmosphere to burn up. 

ESA hopes the navigation technology enabling the precise formation flight could be used in the future for other applications, for example to create fleets of antennas detecting radio waves from space, similarly to radio telescope arrays on Earth. 

Although Proba-3 reduces the invisible region around the sun, the occulter is still not far enough to erase the diffraction effect completely. The closest region some 70,000 km above the sun’s surface will remain invisible. 

Solar eclipse photography is therefore not going to become obsolete anytime soon. The team of Miloslav Druckmüller in collaboration with scientists from the University of Hawaii is already preparing equipment to photograph the upcoming total solar eclipses in August 2026 and 2027. The researchers will ship ton of photographic equipment to capture the events and will also photograph them from aboard a NASA high-altitude plane.