The First of Its Kind: PROBA-3

7 min readDec 8, 2024

Why is the Sun’s corona significantly hotter than the solar surface?

Launched on December 5, 2024, the PROBA-3 mission which hopes to answer this question marks a significant milestone in international space collaboration. A partnership between the Indian Space Research Organisation (ISRO) and the European Space Agency (ESA), this mission aims to study the corona, the outermost layer of the Sun’s atmosphere.

The PSLV-C59 mission will deploy the PROBA-3 satellites into a highly elliptical orbit, marking the world’s first-ever demonstration of precise formation flying! This groundbreaking mission involves two spacecraft: the Occulter Spacecraft (OSC) and the Coronagraph Spacecraft (CSC). Once in orbit, the two will separate to a distance of 150 meters, maintaining this formation to create an artificial solar eclipse. This will enable continuous and detailed observation of the Sun’s corona for up to six hours.

Proba-3’s coronagraph, called ASPIICS is the mission’s primary scientific instrument and stands for Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun. The instrument is made of a large 1.4m diameter occulting disk mounted on the Occulter spacecraft, and a Lyot-style solar coronagraph system carried by the Coronagraph spacecraft.

When the two satellites (CSC and OSC) are co-aligned with the Sun, approximately 150 meters apart during formation flying, the occulting disk will produce an artificial eclipse by blocking the blinding body of the Sun.

Technical Details:

Launch Vehicle: PSLV-C59, a variant of the Polar Satellite Launch Vehicle (PSLV), will carry the PROBA-3 satellites into orbit. The PSLV-XL configuration, known for its increased payload capacity, will be used for this mission.

Satellites: The combined weight of the OSC and CSC is approximately 550 kg. The OSC will block the Sun’s bright light, while the CSC will capture views of the solar corona.
Orbit: The satellites will be placed in a highly elliptical orbit with an apogee (farthest point from Earth) of 60,530 km and a perigee (closest point to Earth) of 600 km.
Scientific Instruments: The ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), aims to address the long-standing solar mystery: why is the Sun’s corona significantly hotter than the solar surface?

This may remind you of NASA’s Parker Solar Probe which was launched on August 12, 2018, whose objective was to study the outer corona of the Sun, trace the flow of energy, and explore what accelerates the solar wind, a phenomenon that was proposed by the mission’s namesake — Dr. Eugene Parker.

Recently, it completed a final Venus flyby on November 6, 2024, to position itself for its closest-ever approach to the Sun, coming within just 3.86 million miles (or 6212067.84 km) of the solar surface on December 24, 2024. This will allow the probe to gather unprecedented data on solar phenomena, particularly during the Sun’s most active phase of its solar cycle.

So what’s different with PSLV-C59/PROBA-3?

While NASA’s Parker Solar Probe (PSP) dives deep into the Sun’s outer corona, tracing solar wind acceleration and reaching record speeds of 430,000 mph (692.01792 km/hr) during close approaches of just 3.9 million miles (6276441.6 km), the PSLV-C59/PROBA-3 mission focuses on creating artificial solar eclipses via precise formation flying.

Unlike PSP’s single-craft design with four specialized instruments operating in extreme proximity to the Sun, PROBA-3 prioritizes extended, high-precision coronal imaging in a safer elliptical orbit, highlighting complementary approaches in solar research.

This innovative setup is the world’s first demonstration of precision formation flying, highlighting advanced navigation and coordination capabilities.

So how precise formation flying works is that it involves two spacecraft flying in a controlled configuration to achieve a specific scientific goal.

Proba-3’s Occulter spacecraft stacked on top of its Coronagraph in the SHAR cleanroom at ISRO’s Satish Dhawan Space Centre in Sriharikota, India, in November 2024. The blue protective cover around the Occulter’s 1.4-meter occulting disc was removed ahead of launch.
Source: **ESA**

In the case of PROBA-3, the two spacecraft, the Occulter Spacecraft (OSC) and the Coronagraph Spacecraft (CSC), are launched together in a stacked configuration as shown above. Once in orbit, they separate to a distance of 150 meters, maintaining this separation to create an artificial solar eclipse. This precise positioning allows the Coronagraph to observe the Sun’s corona without the bright light of the Sun’s surface interfering.

To maintain their precise 150-meter separation, the PROBA-3 satellites employ advanced navigation and positioning systems.

First, inter-satellite communication is established using an S-band RF link. Around perigee (the closest point to Earth), GNSS receivers use GPS signals to determine their relative positions, with a specialized algorithm ensuring accuracy. As the satellites rise beyond GPS coverage, a visual-based sensor takes over, where cameras on the Occulter Spacecraft track LEDs on the Coronagraph Spacecraft to calculate relative positions.

For even finer adjustments, a Fine Lateral and Longitudinal Sensor (FLLS) uses a laser system to measure relative positions down to millimetre precision. Additionally, the Shadow Positioning Sensor monitors the alignment of the spacecraft by analyzing light distribution on the Coronagraph’s lens.

Star trackers onboard both satellites provide absolute attitude determination, and precision cold-gas thrusters on the Occulter Spacecraft maintain their relative positions. Meanwhile, the Coronagraph Spacecraft features a monopropellant propulsion system for orbit adjustments and collision avoidance.

True to its name — PRoject for OnBoard Autonomy — PROBA-3 relies heavily on autonomous systems, allowing most formation-flying manoeuvres and experiments to be carried out without ground intervention. This level of automation ensures the mission’s precision and success.

Creating an artificial solar eclipse offers several benefits for solar observation:

Extended Observation Time: Unlike natural solar eclipses, which are brief, artificial eclipses can last for up to six hours, allowing for extended and detailed observations.
Improved Image Quality: By blocking the Sun’s bright light, the Coronagraph can capture clearer and more detailed images of the corona.
Continuous Monitoring: Artificial eclipses enable continuous monitoring of solar phenomena, providing valuable data on solar dynamics and space weather events.

The primary scientific goal of the PROBA-3 mission is to study the Sun’s corona in unprecedented detail.

By creating artificial solar eclipses, the mission aims to:

Understand Solar Dynamics: Investigate the structure and dynamics of the corona, which is essential for understanding solar activity and its impact on space weather.
Monitor Solar Phenomena: Observe solar phenomena such as solar flares and coronal mass ejections (CMEs) in greater detail.
Contribute to Space Weather Forecasting: Provide data that can improve the forecasting of space weather events, which can affect satellite operations, power grids, and communication systems on Earth.

The PROBA-3 mission has the potential to revolutionize our understanding of the Sun and its impact on Earth.

By providing unprecedented insights into the Sun’s corona, this mission can significantly contribute to:

Improved Space Weather Forecasting: Accurate space weather forecasting is crucial for protecting satellites, power grids, and communication systems. PROBA-3’s detailed observations of solar phenomena like solar flares and coronal mass ejections can enhance our ability to predict these events and mitigate their potential consequences.
Understanding Solar Physics: By studying the intricate dynamics of the Sun’s corona, PROBA-3 can help scientists unravel fundamental questions about solar physics, such as the mechanisms behind solar heating and particle acceleration.
Space Exploration: A deeper understanding of the Sun’s environment is essential for planning future space missions, especially those venturing into deep space. PROBA-3’s data can provide valuable insights into the challenges and opportunities associated with long-duration space travel.

The PSLV-C59/PROBA-3 mission is not just a technological and scientific milestone but also a celebration of international collaboration. By pioneering precision formation flying to study the Sun’s corona in unprecedented detail, ESA and ISRO are pushing the boundaries of solar observation. This mission not only aims to unravel the mysteries of solar dynamics and space weather but also strengthens India’s role as a key player in global space exploration. Together, ESA and ISRO are paving the way for groundbreaking discoveries that could genuinely shape our understanding of the universe for decades to come!

The double-satellite Proba-3 stack and its upper stage were encapsulated within their launcher fairing on 29 November. Proba-3 is due to launch on a PSLV-XL launcher at the SHAR base of the Indian Space Research Organisation, ISRO, on 4 December.
ESA’s twin Proba-3 platforms will perform precise formation flying down to a single millimetre, as if they were one single giant spacecraft.
Source: **ESA**

References:

ISRO. (2024). PSLV-C59 / PROBA-3 Mission.
Retrieved from https://www.isro.gov.in/PSLV_C59_PROBA-3_Mission.html
Hindustan Times. (2024). ISRO’s PSLV-XL to launch Europe’s PROBA-3 mission for solar studies from Sriharikota.
Retrieved from https://www.hindustantimes.com/india-news/isros-pslv-xl-to-launch-europes-proba-3-mission-for-solar-studies-from-sriharikota-december-4-101733198638529.html
ESA. (2024). Proba-3 Technologies.
Retrieved from https://www.esa.int/Enabling_Support/Space_Engineering_Technology/Proba_Missions/Proba-3_Technologies
NASA Science. (2024). Parker Solar Probe.
Retrieved from https://science.nasa.gov/mission/parker-solar-probe/
ESA Multimedia. (2024). Search results for Proba-3 images.
Retrieved from https://www.esa.int/ESA_Multimedia/Search?SearchText=Proba-3&result_type=images
ISRO. (2024). PSLV-C59 Brochure.
Retrieved from https://www.isro.gov.in/media_isro/pdf/PSLVC59/PSLV_C59_Brochure.pdf
India Today. (2024). PSLV-XL Proba-3 launch: First pictures of ISRO’s workhorse ready for lift-off. Retrieved from https://www.indiatoday.in/science/story/pslv-xl-proba-3-launch-first-pictures-of-isros-workhorse-ready-for-lift-off-2643368-2024-12-02
ESA. (2024). Search results for Proba-3 videos.
Retrieved from https://www.esa.int/esearch?q=Proba+3+videos
ESA. (2024). Proba-3 Mission.
Retrieved from https://www.esa.int/Enabling_Support/Space_Engineering_Technology/Proba_Missions/Proba-3_Mission3