PROBA-3 ABOARD ISRO’S PSLV : India to Launch Europe’s Sun Mission in December

The Proba-3 Coronagraph spacecraft has been tested to ensure that it fits perfectly with the launcher payload adapter. This adapter is the part that will securely connect the spacecraft to the Indian PSLV-XL rocket, which is scheduled to launch it into space on December 4. This ‘fit check’ is roan essential step to confirm that everything aligns, and functions, properly before the final launch preparations begin. The mission is expected to last about two years.

The two satellites arrived at ISRO’s Satish Dhawan Space Centre earlier this month. They were tested in the Spacecraft Preparation Facility-1B (SP-1B facility) to ensure that they were in perfect working condition after their journey from Europe. These tests confirmed that the satellites were not damaged during transportation.

After the initial checks were carried out, the Proba-3 the satellites were transferred to the SP-2B facility to prepare for the next stage of testing. The Coronagraph was gently placed and securely attached to the adapter. A clamp band was then attached to securely connect the two and ensure a proper fit. Umbilical cables were fitted for keeping a check on the spacecraft during fuelling and launch. These cables were tested to make sure they worked correctly and could disconnect smoothly when the clamp band is released, allowing the satellite to separate from the adapter after launch.

Next-Gen Satellites: Safe & Synchronized

On December 4, 2024, the two small satellites will be launched together in a stacked set-up—one satellite placed on top of the other during launch—into a highly elliptical orbit (600 x 60,530 km at about 59° inclination).

§  600 km is the altitude at the lowest point of the orbit (perigee)

§  60,530 km is the altitude at the highest point of the orbit (apogee; in a highly elliptical orbit, the apogee is the point where the satellite is farthest from Earth. In this case, the apogee is 60,530 km above Earth)

After a short preparatory phase, the two satellites will be separately placed into a safe orbit where they move together in a controlled, coordinated manner, to ensure that there is no chance of a collision.

As part of the commissioning phase, the mission will demonstrate a Collision Avoidance Manoeuvre. This will ensure that the satellites can stay safely in their orbit without the risk of colliding with each other or drifting apart uncontrollably. During normal operations, the satellites will perform formation flying manoeuvres, which involve moving and maintaining specific positions relative to each other with precision. These manoeuvres allow them to work together as a single system.

Revealing the Sun's Corona with Proba-3

For the first time ever, two satellites—the Coronagraph Spacecraft (CSC) and the Occulter Spacecraft (OSC) will fly in formation with incredible accuracy. They will stay precisely aligned within a few millimetres and very small angles, even while being about 150 metres apart, for six hours at a stretch.

According to the European Space Agency (ESA), the two spacecraft will work together in a precisely controlled formation, maintaining a distance of about 150 metres. The OSC will block out the Sun’s direct light by casting a shadow onto the Coronagraph’s telescope. This arrangement will allow the Coronagraph to capture detailed images of the Sun’s faint outer atmosphere, called the corona, for extended periods of time.

This distance of 150 metres must be maintained with extreme precision—accurate to a few millimeters, both in distance and side-to-side alignment. The goal is to observe the corona as close as 1.1 times the Sun’s radius using visible light. Observing the corona is usually difficult because the Sun is intensely bright. However, with this setup, the Coronagraph will take clear pictures in different types of light.

These include:

§  Visible light, which we can see

§  Ultraviolet light, which is invisible to the eye, but crucial for studying the Sun, and

§  Polarized light, which reveals special details about the corona’s structure

These observations will help scientists learn more about the Sun’s behaviour and its impact on space and Earth.

Satellite Formation that’s Fuel-Efficient

To save fuel, the satellites do not stay in perfect formation for the entire time that they are orbiting Earth. Instead, they split their orbit into two parts:

o   For six hours when they are farthest from Earth, the satellites use fuel to stay in a precise formation

o   For the rest of the time, they drift naturally in space without using fuel, because they do not need to be perfectly in sync during this time

This way, they only use fuel when it is really necessary, which helps conserve energy.

Joint Precision Manoeuvres in Every Orbit

The Proba-3 satellites will repeatedly perform a series of important tasks during each orbit. These tasks include coming together, flying close to each other, maintaining precise formations, observing the Sun’s corona and then separating to fly in a convoy, moving together in a coordinated group. They will practise these manoeuvres again and again, showing how satellites can work together in space.

Proba-3: Testing Future Tools for Space

Proba-3 will act as a space laboratory, testing different strategies and systems for guiding, navigating and controlling satellites. It will also try out such techniques as using GPS to track the satellites’ positions relative to each other, which were previously tested on the ground in simulators.

The techniques and simulators developed for Proba-3 will later be shared more broadly and play a key role in preparing for future space missions. This means the tools tested during Proba-3 will help improve and guide future missions.

(The author of this article is a Defence, Aerospace & Political Analyst based in Bengaluru. He is also Director of ADD Engineering Components, India, Pvt. Ltd, a subsidiary of ADD Engineering GmbH, Germany. You can reach him at: girishlinganna@gmail.com)

(Disclaimer: The views expressed above are the author's own and do not reflect those of DNA)