Proba-3 carries ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), an advanced coronagraph developed by the Centre Spatial de Liege in Belgium for ESA. The system is comprised of two satellites: the Occulter spacecraft, which houses a large occulting disk, and the Coronagraph spacecraft, which carries the solar observation instrument.
During operations, these two satellites will fly in precise formation, maintaining a 150-meter separation with an accuracy of just one millimeter. The Occulter spacecraft will block direct sunlight, casting a shadow onto the Coronagraph spacecraft's instrument, thus allowing for unobstructed observations of the corona.
After launching on December 5, 2024, the two satellites remained connected until their scheduled separation on January 14, 2025. The European Space Security and Education Centre (ESEC) in Belgium oversaw the initial in-orbit commissioning phase.
While still attached, mission operators tested the spacecraft's pointing accuracy by directing it toward preselected stars. ASPIICS then imaged the star field to confirm the spacecraft's orientation.
"We examined star charts to determine which stars would be visible to Proba-3 on that specific date," explained Andrei Zhukov, Principal Investigator for the ASPIICS coronagraph at the Royal Observatory of Belgium. "A single star is not sufficient for orientation, so we selected at least three stars, forming a triangle for precise positioning."
For the test, the team targeted two bright stars from the Ophiuchus constellation-Delta (d) and Epsilon (e) Ophiuchi-along with additional weaker stars, all of which fit within the ASPIICS field of view. The spacecraft successfully oriented itself toward these celestial markers and captured a clear image of the star field.
"The spacecraft pointed exactly where we directed it, with excellent precision," Zhukov reported. "The stars appeared sharply in the image, confirming that the spacecraft remained extremely stable during the 10-second exposure."
The ASPIICS image revealed at least eight stars, sufficient to verify the telescope's alignment. This precise positioning is crucial for solar corona observations, as even minor misalignment could allow excess sunlight to interfere with the data.
When in full observational mode, the 1.4-meter external occulter aboard the Occulter spacecraft will completely block direct sunlight. Despite this, stray light will still scatter around the occulter's edges, producing a haze that could obscure corona features. To mitigate this, ASPIICS includes an internal occulter within its optical system, visible in the star field image as a black ring corresponding to a darkened lens section.
Zhukov added, "Cosmic rays also appear in the image, shown as purple marks. These artifacts are common in coronagraph images, often resembling stars. A second image taken later confirmed their transient nature, as the stars remained stationary while the cosmic rays appeared in different positions."
With this successful test, the Proba-3 team is now preparing for its primary mission-solar corona imaging-anticipated to begin as early as March. The Royal Observatory of Belgium hosts the ASPIICS Science Operations Centre, which will coordinate coronagraph observations based on requests from the scientific community and manage the resulting data.
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