Proba-3 stargazing to warm up for corona observations

To take images of the solar corona, Proba-3 carries ASPIICS, short for Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun. This instrument, developed for ESA by Centre Spatial de Liège, Belgium, is made up of a large occulting disk mounted on the Occulter spacecraft and a solar coronagraph system carried by the Coronagraph spacecraft.

In their most precise formation, the Coronagraph and the Occulter will be flying 150 m apart, maintaining their relative position down to a single millimetre. The occulting disk will cover the bright body of the Sun, casting a shadow onto the optical instrument on the Coronagraph spacecraft and allowing it to study the solar corona.

Following their launch on 5 December 2024, the two satellites remained attached until their separation on 14 January 2025. The mission control team at ESA’s European Space Security and Education Centre in Belgium managed the initial in-orbit commissioning.

While the spacecraft were still attached to each other, the team tested the accuracy of the mission’s pointing. Essentially, the operators rotated the spacecraft by telling it which stars it should be facing. ASPIICS images of the star field were then used to verify if the pointing was correct.

Andrei Zhukov, Principal Investigator for the ASPIICS coronagraph at the Royal Observatory of Belgium, explains: “We looked at the star maps to see which stars would be visible for Proba-3 on that specific date. Choosing a single star would not be enough, because then you cannot be sure which star you are looking at. Ideally, you want at least three stars – a triangle like that can give you full orientation information.

“We chose two bright stars from the constellation of Ophiuchus – meaning ‘serpent-bearer’ in Ancient Greek – which are marked with the Greek letters δ (delta) and ε (epsilon). They are situated close enough to fit into an ASPIICS image, together with a few weaker stars. The Ophiuchus constellation is actually visible to the naked eye from anywhere on Earth.”

Andrei then asked the control team to point the spacecraft towards those stars and to capture an image of the star field using the coronagraph’s optical instrument.

“With excellent precision, the spacecraft pointed exactly where we asked it to. When we got the images, we saw the two stars straight away. They are very sharp – this is great news, because it means that during the ten seconds it took to capture the image, the spacecraft were very stable.”

At least eight stars are visible in this ASPIICS image, which is enough to confirm the pointing of the telescope. Positioning both spacecraft precisely will be crucial for observing the corona. If their alignment is off even by a few millimetres, the Sun will not be fully covered by the occulter, leading to unwanted light interrupting the observations.

In the precise alignment, the 1.4-m large disc on the Occulter spacecraft – the external occulter – will fully cover the Sun. Even then, however, so-called ‘stray light’ will spill over the occulter’s edges, creating a haze that would interfere with the corona observations.

To block the stray light, the coronagraph is equipped with another, internal occulter. In the star field image, this internal occulter is visible as a black ring, corresponding to a blackened section of one of the coronagraph’s lenses.

Andrei adds: “You can also see cosmic rays in the image, marked in purple. This is normal – in coronagraph images, cosmic ray hits often look like stars. There was a second image taken some time apart – in this one, the stars stay, and the cosmic rays appear in different locations.

“Overall, we were very pleased with the accuracy of the spacecraft pointing and the quality of the image. It made us even more excited to see the corona images, expected as early as March.”

The Royal Observatory of Belgium is hosting the ASPIICS Science Operations Centre (SOC) – a dedicated team responsible for creating operational commands for the coronagraph based on requests from the scientific community and sharing the resulting observations.