The European Space Agency (ESA) has concluded an extremely intense year in which the Proba-3 mission evolved from an experimental precision formation flying project into a practical “solar eclipse on demand machine.” Two small satellites, which have been flying in a joint orbit around Earth since December 2024, have since achieved more than 50 artificial solar eclipses in space and collected about 250 hours of continuous observations of the Sun's atmosphere. This data fills a crucial gap in our understanding of the most mysterious part of the solar atmosphere – the inner corona.

Until the appearance of Proba-3, researchers had to rely on a combination of instruments that reliably capture the Sun's surface and the distant, outer corona, while the region in between was covered only fragmentarily and occasionally. A complete view of the corona was mostly possible only during short natural total solar eclipses, when the Moon obscures the solar disk for a few minutes. Proba-3 now reproduces this natural phenomenon in orbit – independent of the Moon's position, weather conditions, and geography, and from an orbit that allows for multi-hour, repeatable eclipses.

How the Proba-3 duo creates a solar eclipse in orbit

Proba-3 is the first ESA mission to demonstrate precision formation flying of two satellites with an accuracy of up to one millimeter. In an orbit with a peak altitude of about 60,500 kilometers, two spacecraft – Coronagraph and Occulter – fly separated by about 150 meters but behave like a single telescope stretched out in space. On the Occulter platform is a circular shield with a diameter of 1.4 meters that serves the system as an artificial Moon: it blocks the brilliant visible light of the solar disk and casts a narrow, precisely aimed cone of shadow onto the second satellite, the Coronagraph, which houses the main instrument ASPIICS.

When both satellites align with the Sun for several hours, a shadow only a few centimeters in diameter perfectly covers the instrument's aperture. In this way, ASPIICS obtains the key to any good coronagraphic image – a very dark “artificial night” in the middle of the day where the faint but physically extremely important glow of the corona comes to the fore. The whole trick succeeds thanks to a combination of GPS receivers, laser distance meters, star trackers, and radio links, which allow the computers on the satellites to autonomously maintain the specified formation without constant commands from Earth.

The orbital period of the mission is approximately 19 hours and 40 minutes, and in each orbit, the satellites can enter precise alignment and maintain an artificial total eclipse for up to six hours. Natural total solar eclipses occur on average once a year (and only from a narrow strip on the Earth's surface) and last only a few minutes. Proba-3, on the contrary, reproduces this condition every twenty hours – practically whenever the scientific community requests it – and offers a significantly longer period of totality.

Filling a critical gap in solar corona observation

The greatest scientific value of the mission arises from the fact that Proba-3 targets precisely the zone where traditional observations have their largest “blind spot.” Classic space coronagraphs, where the disk for obscuring the Sun and the telescope are on the same platform, are usually limited by the inner boundary of the field of view – too close to the Sun, too much stray light and scattering in the optics simply enters. On the other hand, extreme ultraviolet cameras on solar satellites provide excellent details of the surface and very low corona, but do not capture structures extending outward for several solar radii.

The result is that the area between approximately 1.1 and 2 solar radii above the surface has remained the most poorly covered by observations for decades. It is precisely there that the solar wind accelerates to speeds of several hundred kilometers per second, and many coronal mass ejections (CMEs) – giant clouds of plasma and magnetic fields – form and detach from the Sun. With its “stretched” coronagraph concept, Proba-3 separates the shutter and the telescope onto two platforms and thus drastically reduces the amount of stray light entering the instrument. ASPIICS can view the corona practically from the very edge of the solar disk to a distance of several solar radii in one continuous shot.

For scientists involved in solar physics, this means they can track how magnetic structures and plasma change shape and energy from the surface all the way to the outer corona. Particularly important is the long-standing mystery of why the corona is heated to more than a million degrees Celsius, while the visible surface of the Sun, the photosphere, is “only” at about 5800 K. At the same time, a detailed look at the region where the solar wind accelerates is crucial for understanding space weather forecasting, as the same processes that drive the wind are behind the strongest geomagnetic storms.

First images: a continuous view from the surface to the high corona

The first phases of ASPIICS's operation have already shown what Proba-3 can do. In June 2025, ESA published the first images of the inner corona taken during an artificial total eclipse, including a spectacular shot from May 23, 2025, in which the corona glows greenish – exactly as the human eye would see it during a total eclipse, observed through an appropriate filter. In that “frozen” frame, fine filaments of plasma, arcs of magnetic structures, and darker cavities above active regions on the surface are clearly discernible.

Even more impressive are the time-lapse displays created by combining several instruments: the SWAP extreme ultraviolet camera on the Proba-2 mission, which shows the solar disk and the very low corona; the classic LASCO C2 coronagraph on the SOHO mission, responsible for the higher corona; and ASPIICS on Proba-3, which “covers” the critical inter-space. In such composite animations, a coronal mass ejection becomes visible already at the edge of the solar disk, then spreads through the inner corona, which was previously almost impossible to film, and continues outward where instruments like LASCO take over.

For researchers like Andrei Zhukov from the Royal Observatory of Belgium, the principal investigator of ASPIICS, this “continuous story” is a key step forward. Instead of filling gaps with models or numerical simulations, they can now directly observe how a CME is born, accelerates, and changes shape through the entire range of heights in the corona. This not only helps in understanding the physics of the Sun but also in building more precise space weather forecasting models, which are the basis for protecting satellites, communication systems, and power grids on Earth.

More than 50 artificial eclipses and 250 hours of observation

From the beginning of the mission's operational phase until mid-December 2025, Proba-3 has already performed more than 50 successful artificial eclipses and achieved approximately 250 hours of corona observation in totality conditions. This is an amount of data that on Earth could only be collected during thousands of natural eclipses. ESA estimates that the collection of observations so far corresponds to what we would get if we organized about 6000 separate expeditions to observe total solar eclipses in different parts of the planet.

The operational rhythm of the mission is based on the fact that an artificial eclipse can be planned and repeated once in each orbit, thus approximately every 19.6 hours. In this way, an average of two total eclipses per week can be achieved, and in the two-year nominal duration of the mission, close to 200 eclipses and more than a thousand hours of totality are expected. Already in the first months of operation, the longest single eclipse lasted about five hours, and the goal is to standardly reach six hours of continuous observation in each orbit. Such long-lasting and frequently repeated “turning off of the Sun” in orbit is unprecedented in the history of heliospheric research.

Natural eclipses have not lost their scientific value in the process – they still allow for observations from different perspectives, with different instruments, and in different wavelengths. But Proba-3 has shown that key parts of the work can now be performed by a fully autonomous space laboratory, independent of the unpredictable schedule of eclipses in the sky. This means that the scientific community can plan observation campaigns related to announced increased activities on the Sun, for example, during periods of an increased number of spots or after the appearance of particularly active regions.

From launch to autonomous formation flight

The journey of Proba-3 from launch to a sophisticated operational facility unfolded in several stages. The two satellites took off on December 5, 2024, from India's Satish Dhawan Space Centre on a PSLV-XL rocket. After entering the target highly elliptical orbit, the platforms remained mechanically joined for another six weeks, during which engineers tested all systems in detail. Only after that did the separation and slow breaking-in of formation flight follow, first with larger gaps and shorter alignment periods.

In March 2025, the mission achieved its first autonomous formation flight with a 150-meter separation and multi-hour formation maintenance without active control from Earth. In the following weeks, precision was further improved to the millimeter level, which is a prerequisite for the shadow from the Occulter to stably “hit” the ASPIICS aperture. After that, the first real artificial total eclipses began, initially shorter, then increasingly longer, as the teams gained experience.

A key feature of the mission is the gradual transition to a higher level of autonomy. During the first formation flights, teams on Earth actively monitored every maneuver and were ready to intervene if the satellites strayed from the specified geometry. As navigation algorithms proved themselves in practice, the role of the controllers shifted to background supervision – the final goal is for the satellites to routinely perform alignments and eclipses with minimal supervision, which is an important step toward future fleets of autonomous space telescopes.

ASPIICS: a look into the very interior of the corona

The heart of the mission's scientific part is the ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) coronagraph, developed by a European consortium led by the Centre Spatial de Liège in Belgium. It is an instrument that uses the classic concept of an external shutter but adapts it to formation flight: since the disk that obscures the Sun is physically located on a separate satellite, the optical system on the Coronagraph is exposed to a significantly smaller amount of stray light.

Because of this, ASPIICS can “approach” the Sun more than any previous space coronagraph, observing corona structures from about 0.04 solar radii above the surface. The camera operates in visible light, in a narrow spectral region that is particularly sensitive to plasma and magnetic field structures. Each individual image is actually a combination of three exposures of different durations, from short ones that do not “overexpose” the brightest parts, to longer ones that capture the darkest parts of the outer corona. By combining these exposures, a dynamically rich image is obtained with details from the edge of the disk to the end of the field of view.

The data collected by ASPIICS is processed in a science operations center located at the Royal Observatory of Belgium. There, a team of experts plans new observation campaigns day by day, sends commands to the instrument, and retrieves images and distributes them to the international community. It has already been shown that the quality of the raw data is so good that many structures in the corona become visible even without aggressive numerical processing, which is an encouraging sign for future quantitative analyses.

DARA and 3DEES: a complete picture of solar energy and space particles

Proba-3 is not “just” a coronagraph. Along with ASPIICS, the mission carries two more scientific instruments that complement the picture of the Sun's influence on the surrounding space. The Digital Absolute Radiometer (DARA) measures total solar irradiance – specifically, how much energy per unit of time the Sun sends toward Earth. Long-term records of such measurements are key to understanding changes in solar radiation that may have an impact on Earth's climate and the upper layers of the atmosphere.

The third instrument, the 3D Energetic Electron Spectrometer (3DEES), is focused on high-energy electrons in Earth's radiation belts. While ASPIICS looks toward the Sun, 3DEES tracks how the solar wind and CMEs inject particles into the magnetosphere and how those particles move around the planet. This combination of a “look toward the source” and a “look at the consequences” allows researchers to better link solar events with changes in the space around Earth that directly affect the operation of satellites and other space systems.

Digital eclipses and new space weather models

The large amount of high-quality data that Proba-3 sends to Earth is already encouraging the development of advanced numerical models. Teams across Europe are comparing actual images with results from computer simulations to improve the description of plasma and magnetic field movements in the corona. Particularly important are models that create so-called “digital eclipses” – synthetic views of the corona as a coronagraph like ASPIICS would see it.

One such model, COCONUT, developed at the Catholic University of Leuven, has already been integrated into ESA's Virtual Space Weather Modeling Center. By comparing simulated and actual images from Proba-3, researchers can precisely calibrate the model and better connect active regions on the Sun's surface with phenomena in the corona and heliosphere. In the long run, this should lead to more reliable forecasts of when a specific CME will hit Earth, with what force, and what kind of response it will provoke in the magnetosphere and ionosphere.

Benefits on Earth: from aurora to overloaded grids

The seemingly abstract research of the corona has very tangible consequences. Strong CMEs and fast solar wind streams can trigger geomagnetic storms that create spectacular auroras but also cause problems in power grids, satellite navigation, radio links, and communication systems. During May 2024, a powerful wave of solar activity caused one of the strongest geomagnetic storms in decades, with visible consequences in many countries.

For transmission grid operators, airlines, and satellite service providers, reliable forecasting of such events is just as important as accurate weather forecasting on the Earth's surface. Proba-3 offers exactly what has been missing: the ability to track CMEs and other coronal structures from the moment of formation all the way to the moment they leave the corona. This reduces uncertainty in assessing the direction, speed, and potential impact energy on Earth's magnetic field.

As the Sun approaches the maximum of the 25th activity cycle, the number of strong events is expected to grow. This turns Proba-3 into an ideal tool for testing and improving space weather early warning systems. Every new artificial eclipse brings a new series of data, and every new wave of solar activity an opportunity to test how well-modeled forecasts match reality.

Technology for future missions

Besides the scientific, Proba-3 has an extremely strong technological component. Precision formation flying of two satellites will open the way to new concepts of space telescopes where optical elements are located on multiple separate platforms: for example, missions to hunt for exoplanets with an extremely strong “star-shade” or interferometric telescopes that use multiple satellites as segmented mirrors. The technologies developed for Proba-3 – from navigation algorithms to miniature sensors – have already demonstrated that such an approach is feasible in practice.

The mission is led by ESA with the support of a consortium managed by the Spanish company Sener, involving more than 29 industrial partners from 14 ESA member states and Canada. Key participants include the companies GMV and Airbus Defence and Space from Spain, and Redwire Space and Spacebel from Belgium. Proba-3 is thus a showcase of European industrial capability to realize very complex, highly integrated space systems in a relatively compact and financially accessible mission.

What Proba-3 means for the future of solar observation

Less than a year after launch, Proba-3 has already delivered what was promised – and more. Artificial total eclipses have become a tool used daily by solar physicists, and initial results show that the inner corona is no longer an inaccessible area between different types of instruments. With more than 50 eclipses and hundreds of hours of observation, the mission has confirmed that the “lack of natural eclipses” can be compensated for with precise technology, a well-designed orbit, and smart campaign planning.

As operations continue toward December 2025 and beyond, the amount of data is expected to increase in an avalanche. Every new series of corona images will add another piece to the puzzle in understanding the Sun's behavior, and every new successfully performed artificial eclipse will strengthen confidence in formation flying technologies. Proba-3 has thus become both a scientific instrument and a demonstrator of the future – a dual role that is highly sought after in the space industry.

For solar science, the mission marks a transition from the era of relying on rare and short natural eclipses to a period of “digital eclipses” that can be planned, repeated, and permanently analyzed. For the general public, it is perhaps even more important that this research contributes to better protection of the technology on which modern society rests. And for the space industry, Proba-3 proves that European teams can control two separate satellites tens of thousands of kilometers from Earth with millimeter precision – and, in the process, discover hidden layers of the Sun's shimmering “halo.”