Proba-3 Satellites Achieve Perfect Alignment to Block the Sun in Deep Space First

The European Space Agency has successfully demonstrated a spaceflight milestone, as its Proba-3 mission achieved autonomous, millimetre-precision formation flying between two spacecraft. This marks the first time such precision has been accomplished. The breakthrough was reached when the Coronagraph and Occulter spacecraft maintained a 150-metre separation while orbiting more than 50,000 kilometres above Earth, simulating the behaviour of a single giant observatory.

A Pioneering Feat in Space Autonomy

Following earlier alignment tests this year, engineers at ESA’s European Space Security and Education Centre (ESEC) in Redu, Belgium, fine-tuned the system to allow the satellites to operate without ground intervention. Using onboard instruments, the spacecraft held position relative to each other for several hours, guided by a suite of visual and laser-based technologies.

“To do something that has never been done before, we needed to develop new technologies,” said Esther Bastida Pertegaz, systems engineer on the mission.

Precision Alignment Using Visual and Laser Sensors

The satellites use a Visual-based System that includes a wide-angle camera on the Occulter, which tracks flashing LED lights on the Coronagraph. As they draw closer, a narrow-angle camera enables tighter locking.

“Although we were previously able to perform formation flying using only the camera-based systems on board, we were still missing the desired precision,” explained systems engineer Raphael Rougeot.

That precision was achieved through two major upgrades. The first was the integration of the Fine Lateral and Longitudinal Sensor (FLLS), a laser-based system that allows positioning down to millimetre accuracy. “It consists of a laser beam fired from the Occulter spacecraft and reflected in the Coronagraph’s retroreflector back to the Occulter, where it is detected,” added Jorg Versluys, payloads manager.

The second innovation was the use of a shadow position sensor, which ensures the Coronagraph remains perfectly aligned in the artificial eclipse cast by the Occulter’s disc.

The satellites replicate a solar eclipse by holding formation for six hours during each 19.5-hour orbit. This alignment allows the Occulter’s disc to block sunlight, letting the Coronagraph capture faint details of the Sun’s corona. Achieving this from Earth is nearly impossible due to diffraction issues.