Proba-3: seeing through shadow to view Sun’s corona

Every 18 months or so, scientists and sensation-seekers gather at set points on Earth’s surface, to await awe-inspiring solar eclipses. The Moon briefly blocks the Sun, revealing its mysterious outer atmosphere, the corona. Though what if researchers could induce such eclipses at will?

That’s the scientific vision behind ESA’s double-satellite Proba-3, the world’s first precision formation-flying mission, planned for launch in 2019.

PROBA-3: DANCING WITH THE STARS ufospace

When the Proba-3 double-satellite close formation flying mission launches in late 2018, the Occulter spacecraft will cast a precise shadow across the Coronagraph spacecraft to open up previously inaccessible views of the Sun’s inner corona.

ESA’s double-satellite Proba-3 mission will be flying where no previous member of the Proba minisatellite family has gone before – up to 60 000 km away, a seventh of the way to the Moon.

Set for launch in 2019, the two satellites will be launched together into a highly elliptical or elongated orbit, ranging from an perigee (low point) of 600 km up to an apogee (high point) of 60 000 km.

“This long 19.7 hour orbit will allow us to maintain sustained contact with the two satellites using  a single ground station,” explains Agnes Mestreau-Garreau, Proba-3 project manager.

“And around the high point of the orbit we will be able to spend around six hours on solar observation or devoted to experimental formation flying manoeuvres.”

The latest member of ESA’s experimental Proba minisatellite family, Proba-3’s paired satellites will manoeuvre relative to each other with millimetre and fraction-of-a-degree precision, intended to serve as the virtual equivalent of a giant structure in space and so open up a whole new way of running space missions.

As has become traditional with Proba missions, the success of Proba-3’s technology will be proven through acquiring high-quality scientific data. In this case, the smaller ‘occulter’ satellite will blot out the Sun’s fiery disc as viewed by the larger ‘coronagraph’ satellite, revealing mysterious regions of our parent star’s ghostly ‘corona’, or outer atmosphere.

When in Sun-observing mode, the two satellites will maintain formation exactly 150 m apart, lined up with the Sun so the occulter casts a shadow across the face of the coronagraph, blocking out solar glare to come closer to the Sun’s fiery surface than ever before, other than during frustratingly brief terrestrial solar eclipses.

The challenge is in keeping the satellites safely controlled and correctly positioned relative to each other. This will be accomplished using various new technologies, including bespoke formation-flying software, relative GPS information, intersatellite radio links, startrackers, and optical visual sensors and optical metrologies for close-up manoeuvring.

Fifteen ESA Member States are participating in the Proba-3 consortium, with SENER in Spain as prime contractor for the satellite platforms and Centre Spatial de Liège in Belgium as prime contractor for the coronagraph.

Coronagraph across two satellites

Coronagraph on single satellite

“We have two scientific instruments aboard,” explains Damien Galano, Proba-3 Payload Manager. “The primary payload is ASPIICS, a coronagraph to observe the corona in visible light while the DARA radiometer on the occulter measures the total solar irradiance coming from the Sun – a scientific parameter about which there is still some uncertainty. 

“The corona is a million times fainter than the Sun itself, so the light from the solar disk needs to be blocked in order to see it. The coronagraph idea was conceived by astronomer Bernard Lyot in the 1930s – and since then has been developed and has been incorporated into both Earth-based and space telescopes.

“But because of the wave nature of light, even within the cone of shadow cast by the occulter, some light still spills around the occulter edges, a phenomenon called ‘diffraction’.

“To minimise this unwanted light, the coronagraph can be positioned closer to the occulter – and therefore deeper into the shadow cone. However the deeper it is, the more the solar corona will also be occulted by the occulter. 

“Hence the advantage of a larger occulter and the maximum possible distance between the occulter and the coronagraph. Obviously a 150-m-long satellite is not a practical proposition, but our formation flying approach should provide us with equivalent performance.

Diffraction of light

“Many of these companies are new to ESA, and they’ve proved to be very motivated and eager to show their capabilities,” remarks Damien. “We’ve produced various prototypes of instrument elements, and our first complete ‘structural and thermal model’ should be complete in the autumn, ahead of our end-of-year Critical Design Review.

“We’re also looking into various optical aspects, such as the best occulter edge shape to minimise diffraction.”

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