On the morning of December 5, a pair of satellites ascended into the skies from India’s eastern coast. Their mission? To cast shadows in the vacuum of space. These two spacecraft, part of the European Space Agency’s (ESA) Proba-3 mission, are set to do what no other satellites have done before: create solar eclipses on demand.

The stakes are high. Scientists believe these artificial eclipses could help unravel some of the Sun’s most perplexing mysteries, particularly those lurking in its outer atmosphere — the corona.

Tiny solar eclipses on demand

The corona, an ethereal halo of plasma that surrounds the Sun, remains surprisingly uncharted. Counterintuitively, it is orders of magnitude hotter than the Sun’s surface (around 5,500°C), reaching temperatures of millions of degrees. This is the only instance in the known universe where the thing doing the heating is actually cooler than the thing it’s heating.

This fiery aura is also the birthplace of space weather — powerful phenomena like solar flares and coronal mass ejections (CMEs) that can disrupt communications, damage satellites, and even threaten power grids on Earth.

But studying the corona is like trying to see a candle’s glow next to a floodlight. The Sun’s brightness, around 127,000 lumens per square meter, drowns out the corona’s faint light. For centuries, the best chance to glimpse this hidden region has been during fleeting total solar eclipses, when the Moon briefly blots out the Sun. Both those events are so rare that progress is stymied.

Now, Proba-3 is set to change that.

Millimeter-scale Precision Engineering in Space

The Proba-3 mission, as ESA puts it, “has the potential to change the nature of future space missions.” It consists of two synchronized satellites: the Occulter and the Coronagraph. The Occulter carries a large disk designed to block the Sun’s light. It will fly 144 meters ahead of the Coronagraph, which will observe the corona from behind the shadow.

For this to work, everything has to fall in place with unprecedented precision.

“Think about standing at one end of a football field and landing a pass on a penny at the other end; that would be easy compared to what they’re doing here,” said Daniel Seaton, a co-investigator on the mission and a solar physicist at the Southwest Research Institute. The satellites must align with millimeter precision to create the shadow.

Maintaining that perfect alignment is no small feat. The Occulter uses gyroscopes, star trackers, and cold-gas thrusters to stay on course. Meanwhile, the Coronagraph mirrors these movements with its own guidance systems.

A New Era of Solar Eclipses

These artificial eclipses won’t be visible from Earth; they’re created in the void between the two satellites, far above our planet. But ESA promises to share the images, offering a new window into the Sun’s corona.

Unlike natural eclipses, which last only a few minutes, these on-demand eclipses can stretch up to six hours. This extended view means scientists can observe solar phenomena without chasing eclipses around the globe.

“This shade provided by the first spacecraft will cover the fiery face of the Sun so that its faint surrounding ‘coronal’ atmosphere becomes visible,” ESA explained. The hope is to understand how space weather originates and to predict its impacts on Earth.

Looking Beyond the Sun

If Proba-3 succeeds, the technology could pave the way for even more ambitious missions. Seaton speculated that larger versions of these satellites could one day help scientists search for distant planets by blocking out the light of other stars.

For now, the world will have to wait a few months until March 2024, when the duo is scheduled to begin their synchronized shadow dances. These artificial eclipses may be invisible to our eyes, but the secrets they reveal could shed light on some of the Sun’s deepest enigmas — and perhaps beyond.