• The PROBA-3 mission has passed its final test and is now ready to be sent to India, where it will be launched by the ISRO. PROBA-3 will be transported to the Satish Dhawan Space Centre later this month and the mission will be launched using ISRO’s PSLV-XL rocket in November.

• Proba-3 is the world’s first precision formation flying mission. A pair of satellites will fly together maintaining a fixed configuration as a ‘large rigid structure’ in space to prove formation flying technologies and rendezvous experiments.

• In its final test, ESA’s eclipse-making double-satellite Proba-3 mission has received commands from its science team and transmitted images back, exactly as it will operate in orbit.

• Now its testing is complete, Proba-3 will now be shipped to the Satish Dhawan Space Centre in India for a planned launch by PSLV-XL launcher on November 29.

Formation flying mission

• Proba-3 is devoted to the demonstration of technologies and techniques for highly-precise satellite formation flying. 

• 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.

• The 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. 

Why are there two satellites?  

• For Proba-3, there are two satellites to create the conditions for observation of the Sun corona, synthetising the equivalent of an extremely large instrument. One satellite features a telescope, kept in the centre of the shadow cast by the other satellite 144 m away, thanks to an occulter disk. Maintaining the correct position in the shadow requires a precise formation flying capability.  

• The two satellites will together form a 144-m long solar coronagraph to study the Sun’s faint corona closer to the solar rim than has ever before been achieved.

• In effect the pair will be forming a virtual giant satellite. And this will be achieved autonomously, without relying on guidance from the ground.

• The two small satellites will be launched together into a highly elliptical orbit in a stack configuration.

• After a short preparatory period, the two satellites will be separated and injected into a safe relative tandem orbit. Included in the commissioning period will be a demonstration of the mission’s Collision Avoidance Manoeuvre, ensuring they can be left safely in an orbit with no risk of collision or running away from each other. 

• The formation flying will take place on a fully autonomous basis, towards the top of each 60,000 km altitude orbit, where gravitational, atmospheric and magnetic perturbations are minimised. The rest of the time the pair will spend the rest of their orbit in passive safe drifting.

• Normal operations will include both formation flying manoeuvres and scientific observations using a giant solar coronagraph that will block out the blinding light from the solar disk in order to acquire continuous views of the Sun’s corona, or surrounding atmosphere.

• Successfully achieving precise formation flying would enable a whole new era for science and applications. Missions could be flown that are much larger than any single spacecraft – such as giant in-orbit radio and optical interferometry arrays – while precise orbital rendezvous would make in-orbit satellite servicing feasible, extending the lifetime of space infrastructure.

Why is the mission being launched from India? 

India’s PSLV-XL launcher was chosen since the lift required to place the combined Proba-3 satellites (550 kg) on their desired highly elliptical orbit is above the capability of ESA’s Vega-C launcher, while Ariane-6 would be too costly.

Studying the corona 

• The solar corona has many mysteries associated with it, starting with the counter intuitive fact that the corona is more than a million degrees warmer than the surface of the Sun underneath it. 

• It is a major focus of scientific research and study, not only to improve our understanding of the Sun, but also as the origin of solar weather, such as coronal mass ejections or solar storms which can affect the functioning of satellites, or communication and power networks down on Earth.    

• The inner corona is the corona region that is close to the Sun. The Sun is around a million times brighter than the brightest point within the corona. If the light from the Sun is not blocked, any observing telescope is blinded by this light and cannot see the corona, like trying to see a firefly next to a bonfire. 

• The concept of a Coronagraph instrument, invented in the 1930s, is to use one or more occulting disks to block the Sun’s light. However, when attempting to observe the inner corona, an optical phenomenon called diffraction causes lightwave to spill around the edges of the coronagraph, reducing its effectiveness.

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