Watch eclipse-making Proba-3 launch

The launch is being covered by the Indian Space Research Organisation, ISRO, and can be watched via ESA Web TV and on ISRO’s YouTube channel. The ISRO broadcast will begin around 11:08 CET (10:08 GMT, 15:38 local time), half an hour before liftoff.

Satellite separation is due to take place about 18 minutes after liftoff, with the first acquisition of signal by the flight control team at ESA’s ESEC establishment in Redu, Belgium, anticipated around a quarter of an hour later.

The latest member of ESA’s family of in-orbit demonstration missions, Proba-3 is in fact two spacecraft being launched together, which in orbit will separate to begin performing precise formation flying, precise to a single millimetre, about the thickness of an average fingernail.

To prove their performance, Proba-3 has been devoted to an ambitious scientific goal. The pair will line up precisely with the Sun 150 m apart so that one casts a precisely controlled shadow onto the other.

By blocking out the fiery disc of the Sun, Proba-3’s ‘Occulter’ spacecraft will mimic a terrestrial total solar eclipse, to open up views of the Sun’s faint surrounding atmosphere, or ‘corona’, which is a million times fainter than its parent star. Proba-3’s second ‘Coronagraph’ spacecraft hosts the optical instrument that will observe the solar corona.

On Earth, total solar eclipses only occur every 18 months on average, and last just for a few minutes. Solar scientists have to travel all over the world to take advantage of them. Proba-3 will be able to create solar eclipses on demand, observing closer to the edge of the Sun than any previous Earth- or space-based instrument, down to just 1.1 solar radii. And it will do so for six hours per 19-hour 36-minute orbit.

Proba-3 will also perform general formation flying experiments including rendezvous, resizing the distance between the pair and joint retargeting. The aim is to achieve performance equivalent to a single virtual spacecraft measuring about 150 m across, demonstrating a novel method of operating missions in space, where instruments can be shared between multiple platforms.

India’s four-stage PSLV-XL launcher has been chosen because of its high performance combined with an appropriate price tag for a tightly budgeted technology demonstration mission.

Proba-3 requires an extremely elliptical (or elongated) orbit extending more than 60 000 km from Earth. The mission’s active formation flying will take place around the ‘top’ of this orbit, where perturbations from Earth’s gravity are minimised and less fuel is needed to shift position.

This marks the first time that an ESA mission has flown with an ISRO launcher since the Proba-1 Earth observation mission in 2001.

Probas are a series of experimental missions, their name coming from the Latin for ‘Let’s try,’ which commenced with Proba-1, continuing with the Sun-observing Proba-2 in 2009 and the wide-swath Earth-observing Proba-V for Vegetation in 2013.