ESA Proba 3: Millimeter-precision autonomous satellites shadow the Sun to reveal the corona

Overview

Astrum’s Alex McColgan explains how ESA’s Proba 3 uses two autonomous satellites to cast a precise shadow and observe the Sun’s corona for extended periods, enabling new solar physics insights.

Key insights

• Two tiny satellites fly millimetres apart to simulate a giant eclipsing telescope
• ASPiX coronagraph and DARA radiometer measure corona light and total solar irradiance
• First artificial solar eclipses in 2025 reveal a much busier inner corona and faster solar wind
• Autonomous formation flying demonstrates a new capability for space missions and future science

Intro: A new window on the Sun

Astrum’s host, Alex McColgan, introduces the corona, the outer atmosphere of the Sun, and the coronal heating problem. The video explains why the inner corona is difficult to observe from Earth or traditional spacecraft and how eclipses have historically provided glimpses into this region. The European Space Agency responded with Proba 3, a mission built around two autonomous minisatellites designed to reproduce a solar eclipse on demand, without the need to rely on natural eclipses.

Mission concept: two satellites, one shadowed view

The heart of Proba 3 is a pair of fully autonomous satellites that must fly in millimetre precision formation while in a highly elliptical orbit around Earth. They are separated by about 144 metres, a distance tiny in cosmic terms but enormous for coordinated imaging. One satellite carries the occulting disk that blocks the Sun, while the other hosts the coronagraph. By keeping a precise shadow on target, the system achieves an artificial eclipse that can last for hours, vastly longer than the few minutes of a natural total solar eclipse.

How formation flying works

The video details the multi-layered navigation and measurement approach behind the formation. The satellites use star trackers and GPS for attitude and position, inter-satellite links for ranging data, vision-based systems with LED targets for coarse distance measurements, and a sophisticated laser-based lateral and longitudinal sensor to converge to a 1 mm precision. Photodetectors at the shadow edges provide real-time feedback to correct any drift, ensuring the eclipse remains stable over long observation periods. Notably, the mission has no moving parts apart from a rotation in the coronagraph’s filter wheel.

Instruments and science

Proba 3 carries two instruments: the aspix coronagraph imaging the corona in multiple states and the dara absolute radiometer to measure total solar irradiance. Early data from the first artificial eclipse released in mid-2025 demonstrated the instrument’s capability to observe the inner corona much closer to the Sun than previous space-based coronagraphs could. A composite image from 16 July 2025 combined ultraviolet data from Proba 3, visible-light data from SOHO, and inner corona data from Proba 3 to show a CME forming and evolving through the corona in a single, continuous view.

Key results and ongoing work

By 2026, Proba 3 had produced thousands of artificial eclipses and hundreds of hours of high-resolution corona observations. Early science points revealed that inner corona structures travel much faster than previously thought, challenging existing solar wind acceleration models. The mission is also helping address how slow solar wind forms and how eruptions are triggered, topics central to space weather forecasting. The video notes that Proba 3 is still in early life and researchers are actively publishing new results as data streams in. The mission is repeatedly highlighted as an engineering milestone for autonomous, precise formation flying in space.

Challenges and outlook

The engineers recount a hiccup when Proba 3 briefly lost contact and orientation, delaying observations. The team recovered the coronagraph and returned to operation, showing the resilience and reliability of autonomous spacecraft that can survive extended autonomous operation and even weekends with minimal ground support. The video concludes with reflections on the broader implications for future autonomous spacecraft and potential new missions leveraging similar formation-flying concepts, as well as an invitation to a live solar eclipse event accompanying the channel's content.

Conclusion

Proba 3 marks a milestone in solar physics by enabling continuous, high-resolution views of the corona. The mission demonstrates the feasibility and value of autonomous formation flying for space astronomy and promises to enrich our understanding of how the Sun drives the heliospheric environment and space weather that can affect Earth and our technology.