The European Space Agency’s (ESA) Proba-3 satellite pair are in orbit, but commissioning so one can create solar eclipses for the other is still incomplete. However, three weeks after launch most of the planet is in the dark about how the mission is going.

When the JWST launched in 2021 it was in a blaze of publicity brighter than the images it has returned of any star. Every small step in its deployment was covered in great detail on a NASA blog, and picked up by media outlets worldwide. The situation is very different in the case of Proba-3, launched on December 5 2024, but maybe that is appropriate for a mission designed to learn in shadow.

Total solar eclipses have become enormous tourist draws and an opportunity to educate the public about science. Long before that, however, they were important for scientific research purposes, and that continues. The existence of the solar corona was first discovered during a total solar eclipse in 1724, and the General Theory of Relativity was confirmed in two events in 1919 and 1922.

However, despite centuries of observations, many important questions that solar eclipses should be able to help with remain unanswered. That’s because total eclipses only occur every 18 months, last at most 7 minutes (usually much less), and are subject to the vagaries of cloud and atmospheric distortion. 

Tired of these obstacles, the European Space Agency (ESA) decided to make their own eclipses. To create a total eclipse on Earth from a satellite would require an enormous disk – not Moon-sized if its location was closer, but still very large and fast-moving. Instead, the ESA decided to have two satellites 150 meters (500 feet) apart, a 1.4 meter-wide disk perfectly sized to block out the Sun’s light from the other. Proba-3 was born. Similar instruments called coronagraphs have created “eclipses” by blocking the Sun before but these have previously used one spacecraft to block and image. Proba-3 is groundbreaking in using two separate satellites to block the Sun and image the resulting shadow.

One satellite, the Occulter, will block the Sun’s light from the other (the Coronagraph Spacecraft) for almost a third of each orbit. This will allow the Coronagraph satellite to study the corona, which being a million times fainter than the Sun itself, is undetectable without such assistance.

Some existing satellites carry their own disks, which they use to block out the Sun itself. Unfortunately, diffraction around these objects means that to work they need to block out the lower corona, site of some of the most important mysteries. Increasing the distance between the disk and the instrument reduces the diffraction, and at 150 meters we’ll be able to see “the gap” region other satellites can’t.

Key questions Proba-3 aims to answer include:

• Why is the solar corona much hotter than the Sun itself, against all logic. Some explanations for this have been advanced, but aren’t considered complete. Extended opportunities for observing the corona without the Sun interfering may settle the question.
• What processes accelerate the solar wind to such high speeds – sometimes 2 million km/h (1.2 million mph)? Some sort of magnetic push is thought to be involved, but the details remain as unclear as the corona itself under imperfect viewing conditions.
• What are the forces driving coronal mass ejections (CMEs). This could be the most important question of all. Big CMEs can represent a threat to a technological civilization such as our own, but we don’t really understand why some solar flares lift them off the Sun, and others do not.
• How much is the Sun’s energy varying? Although we have many instruments already measuring this, Proba-3 will spend most of its time much further from the Earth than these, meaning less interference and a more accurate report.
• How do the electrons trapped in the Van Allen belts behave? Most satellites orbit below the Van Allen belts, while those in geostationary orbit are far above the inner belt. Proba-3 will pass through the belts twice every 19.7-hour orbit, giving it plenty to study when its eyes aren’t on the Sun. 

It will probably be years before we get answers to any of these. Indeed, the two satellites, which were launched together, will not separate until early in 2025. The JWST took 7 months before taking its first high quality images, and Proba-3 is scheduled to take at least half that long to test its systems before detailed observations of the corona begin. 

In the meantime, little information is available about how deployment is going, but the ESA has released two photographs showing its startrackers, which will keep the satellites orientated, are operating.