Europe's Proba-3 satellites to soon be mated with India's PSLV rocket

Hectic activity is underway at the Indian Spaceport in Sriharikota, Andhra Pradesh, where the European Proba-3 twin satellites and the Indian PSLV rocket are being prepared for launch on December 4. According to the European Space Agency (ESA), the disc-bearing Occulter Spacecraft has been placed atop the Coronagraph spacecraft. This is the final configuration of Proba-3, where the two spacecraft will be placed in the payload fairing (nose cone) of the rocket and mated with the rocket systems. Because this is a foreign-origin spacecraft, all work related to it is being carried out by teams from European private firms Redwire, Sener, Airbus (all of whom contributed hardware towards the mission), and the fueling team from ArianeGroup.

Of the two – Occulter spacecraft and Coronagraph spacecraft, the former has been fuelled up, while the latter is yet to be fuelled. After the fuelling procedure is completed, the integrated spacecraft will be moved to the rocket for mating with the launch vehicle.

Last week, these satellites were put through a fit check, where they are mated with the payload adapter – a mechanical component that attaches the satellites to the topmost portion of the rocket. This is a crucial step to ensure that the satellite is perfectly interfaced with the rocket, and the satellite can flawlessly separate from the rocket at the desired time. In the case of the Proba-3 satellite, the craft is made by European firms, while the payload adapter for the PSLV rocket is made in India. Therefore, a 'fit check' has to be done to confirm that the spacecraft fits onto the adapter as designed. Only after this step can the spacecraft be fueled up.

What makes Proba-3 unique?

Proba stands for ‘PRoject for OnBoard Autonomy’.

Proba-3 is the world’s first precision formation flying mission. It consists of two small satellites launched together that will separate apart to fly in tandem, to prepare for future multi-satellite missions flying as one virtual structure.

As a world first, its two satellites – the Coronagraph spacecraft and the Occulter spacecraft – will maintain formation to a few millimetres and arc second precision at distances of 144 m or more for six hours at a time. In effect, the pair will be forming a virtual giant satellite. This will be achieved autonomously, without relying on guidance from the ground.

What will Proba-3 study?

The solar corona (outermost part of the sun's atmosphere) has many mysteries associated with it. The solar corona is a million degrees warmer than the surface of the sun, and it is the origin of coronal mass ejections (highly charged particles) which can affect the functioning of satellites, or communication and power networks back on earth. There is immense scientific interest in better understanding the solar corona and related phenomena.

A Coronagraph is a device that can artificially block out the bright light of the sun, thereby enhancing the visibility of the faint corona. A solar eclipse, when the moon blocks out the sun, is a great natural opportunity to study the solar corona. But eclipses are rare and occur only for a few minutes.

The two Proba-3 satellites will be precisely aligned so that the Occulter spacecraft casts a shadow across the Coronagraph spacecraft, thereby enabling the visibility of the faint solar corona. If the two are not perfectly aligned, then the bright disc of the Sun will not be hidden from the instrument and the corona will be obscured by its bright light.

Why is Proba-3 being launched from India?

India's PSLV-XL launcher was chosen to ferry Proba-3 satellites to space, as the ESAdoes not possess a medium-lift rocket that can launch the 550kg Proba-3 satellites into the desired orbit. The mass of the satellites is above the capability of ESA's Vega-C small rocket, while the large Ariane-6 rocket would be too costly for a mission of this kind. PSLV offers the perfect balance in terms of lift capability and cost. ESA will be paying around €30 million (approx Rs 271cr) to avail the launch services from ISRO's commercial arm NewSpace India Limited (NSIL).