Proba-3: Why studying Sun, space weather have become imminent

Space weather is known to affect the health of satellites and communication networks

Proba-3 builds on the legacy of ESA’s earlier solar missions, the Proba-1 and the Proba-2, which were also launched using ISRO's launch vehicles. Image Credit: ISRO

On December 5, 2024, the Indian Space Research Organisation (ISRO) successfully launched the PSLV-C59 vehicle carrying the Proba-3 spacecraft into a highly elliptical orbit. It is a dedicated commercial mission undertaken for the European Space Agency (ESA) by NewSpace India Limited (NSIL). Owing to technical glitches, ISRO was required to postpone the launch by 24 hours.

The Proba-3 satellite is actually a set of two spacecraft operating as a single unit. The design of this system is such that it blocks the Sun’s glare, enabling continuous observation of the corona. This mission is meant for knowing more about solar dynamics. It is expected to increase our knowledge about the forces driving coronal mass ejections (CMEs) and the solar wind. This mission is also about demonstrating precise formation flying by the satellites. These crafts are known as the Coronagraph Spacecraft (CSC) and the Occulter Spacecraft (OSC). They were launched in a stacked configuration. There would be a four-month commissioning period, and after that, Proba-3 will become fully operational and start undertaking the observations. These satellites will be in a 600x60, 530 km orbit around Earth.

For many years, studying the solar corona—a faint, ultra-hot layer of the Sun—has been a significant challenge. This part of the Sun is responsible for generating solar winds and storms. However, studying it in detail is difficult because it is usually obscured by the Sun’s intense and powerful light. Satellites or instruments studying the Sun cannot avoid unwanted sunlight reaching their sensors, which limits the quality of observations.

The best observations are typically made during a solar eclipse, when the main disc of the Sun is temporarily obscured. However, during this period with the Sun’s corona lasting just a few minutes at a time during total solar eclipses, it was possible to collect limited sets of observations, and that too only rarely, since the frequency of occurrences of total solar eclipses is not very high.

Proba-3 consists of an occulter and a telescope (coronagraph), with each unit being a separate spacecraft. An occulter spacecraft weighs 240 kilograms, and a coronagraph spacecraft is about 310 kg in weight. These spacecraft are positioned 150 meters apart. This formation ensures that the occulter blocks the intense rays from the Sun, while the telescope can study the Sun’s edges more clearly.

ASPIICS (Advanced Solar Plane Instrument for Imaging and Coronagraphic Studies), an instrument onboard Proba-3, has been developed by various European groups, mainly the teams of scientists from Spain, Switzerland, Poland, Italy, and Belgium. It helps to study the Sun’s corona in unprecedented detail. The technique employed over here is a dual-disc system for effectively offsetting stray light. This system comprises a primary 1.4-metre occulting disc, made from carbon fibre-reinforced plastic. The private sector is involved in making these sensors, mainly Airbus in Spain. This disc works in tandem with a smaller central disc to block diffracted light waves and thus allows sharper observations of the Sun’s corona.

The instrument is hosted on the coronagraph spacecraft, which, along with the occulter spacecraft, forms a virtual telescope with a distance of 150 meters between the two. This formation allows for observations of the corona from just 1.1 solar radii, a significant advancement over conventional coronagraphs. 1.1 solar radii refer to a distance that is 1.1 times the radius of the Sun. Since the Sun’s radius is approximately 696,340 km, 1.1 solar radii would be around 765,000 km from the centre of the Sun. Proba-3 is designed to conduct six-hour observations twice a week. Rapid imaging at intervals as short as 30 seconds is expected to provide unique detail of solar events.

The other two instruments onboard of Proba-3 include the Digital Absolute Radiometer (DARA) to keep a nonstop measurement of the Sun’s total energy output (total solar irradiance) and the 3D Energetic Electron Spectrometer (3DEES) for measuring electron fluxes as it passes through Earth’s radiation belts, and this information would be important for space weather studies.

Proba-3 is the third satellite launched under the ESA’s program called the Project for On-Board Autonomy (Proba). Incidentally, Proba-1 was also launched by ISRO during 2001. This mission was expected to last for two years; however, it is still partially operational. This small satellite, less than 1 cubic meter in size, has been sending useful data, which has been used by space weather and space debris scientific communities. Proba-2, a small satellite (130 kg), was launched during 2009 and also has scientific instruments, which are designated to observe the Sun.

For India, the launch of Proba-3 has great commercial importance. Also, it has demonstrated the capability to undertake launches involving ‘satellite formation flying’. For a long time, ISRO’s commercial interests were catered to by the company called Antrix Corporation Limited (ACL), which was established in September 1992. In recent times (2019), ISRO has established another company called NewSpace India Limited (NSIL). This company is providing space-related products and services designed and developed by ISRO to global customers.

Within two years of its formation, NSIL undertook the first commercial launch during February 2021. The mission PSLV-C51 had successfully launched the Amazonia-1 satellite for Brazil along with 18 co-passenger satellites. Subsequently, during 2022 and 2023, in two separate missions, NSIL executed a contract with OneWeb (this UK company is establishing a 648-satellite constellation) and launched a total of 72 satellites (36 satellites per mission) to low Earth orbit (LEO). Earlier launching of 428 satellites, the Russian vehicle Soyuz was used. However, owing to the conflict in Ukraine, Russia has been put under sanctions, and the UK has suspended its launch services contract for Soyuz missions.

India grabbed this opportunity and, within a limited time, ensured the readiness of the vehicle and conducted both missions successfully. Today, NSIL already has several launch agreements in place for the next couple of years. They have also reached an agreement with the French agency called Arianespace for a long-term collaboration to enable satellite launch missions. India’s ministry of defence has also made an agreement with NSIL in connection with an advanced communication satellite, GSAT 7B (Indian Army).

It is expected that the Proba-3 satellite would contribute much towards increasing understanding about the Sun’s behaviour. Currently, there are around 30 space systems that are trying to study the Sun and space weather.

It is also important to note that India has a strong interest in studying the Sun, which is the centre of our solar system and whose gravity holds the entire system together. In September 2023, ISRO launched the Aditya-L1 satellite, a coronagraphy spacecraft designed to study the solar atmosphere. It is expected that data from the Proba-3 mission will be shared with India, and when combined with the data from Aditya-L1, it will significantly enhance our understanding of the Sun.

The recent example is during February 2022, when 49 Starlink satellites were launched, and the very next day, a geomagnetic storm increased the density of the atmosphere, unexpectedly increasing the drag force on the satellites, and owing to this, 40 of the 49 satellites became inoperative. Predicting space weather has become increasingly important, as it is known to affect the health of satellites and communication networks.

The author is Deputy Director General, MP-IDSA, New Delhi. The views expressed in the above piece are personal and solely those of the author. They do not necessarily reflect Firstpost’s views.