PROBA-3: the first ever “Precision formation flying” Mission

ESA’s PROBA-3 mission, which lifted off on ISRO’s PSLV-XL rocket on 5 December 2024, is a “solar eclipse-making” two satellite formation of occulter and the coronagraph spacecrafts. It will enable observation of solar corona by coronagraph spacecraft for 6 hours in every 19h hour 36 minute orbit through on-demand creation of total solar eclipse by the occulter spacecraft of the formation. As the demonstration of formation flying technologies, PROBA-3 mission paves the way for future interferometry-based LISA mission (which will be a formation of three spacecrafts in space) for detection of low frequency gravitational waves (GWs).

The Laser Interferometer Space Antenna (LISA) mission which received ESA’s go ahead early this year and is scheduled to be launched in 2035, will be the first space-based gravitational wave observatory dedicated to detection and study of low frequency gravitational waves (GWs) with frequency between 0.1 mHz and 1 Hz (or the millihertz ripples) caused by distortions in the fabric of space-time across the universe. It will be a constellation of three spacecrafts in accurate equilateral triangle formation in space. Each side of the triangle will be 2.5 million km long. The formation of the three spacecrafts will orbit Sun in an Earth-trailing heliocentric orbit between 50 and 65 million km from Earth while maintaining a mean inter-spacecraft separation distance of 2.5 million km.

LISA mission will build on two key capabilities – “low-frequency gravitational wave detection in space” and “precision formation flying in space”. While the technology needed for “low-frequency gravitational wave detection in space” was tested in flight and demonstrated by the LISA Pathfinder mission during 2015-2017, the technology for “precision formation flying in space” has recently been demonstrated on 5 December 2024 with the “total solar eclipse-making two satellite formation of PROBA-3 mission” entering highly elliptical orbit. It had lifted off on ISRO’s PSLV-XL rocket.

PROBA (Project for On-Board Autonomy) -3 mission of European Space Agency (ESA) has two key objectives: demonstration of formation flying technologies, and study of the inner solar corona.

PROBA-3 is the first ever “precision formation flying” mission. It is a formation of two spacecrafts: occulter and the coronagraph. The former (i.e., Occulter spacecraft) casts a precisely controlled shadow onto the Coronagraph spacecraft to produce total solar eclipses on demand for six hours at a time, enabling observation of much fainter solar corona. The two spacecrafts in the formation are separated by a distance of 150 metres with an accuracy of a couple of millimetres. Highly accurate formation flying of autonomous units in space is a significant advancement in space technology that opens possibility of new missions such as interferometry-based LISA mission for detection of low frequency gravitational waves (GWs) in space which cannot be detected using ground-based detectors such as LIGO, VIRGO, etc or Pulsar Timing Arrays (PTAs).

While formation of three spacecrafts in the LISA mission will enable measurement (through laser interferometry) of extremely small changes in the distances between test masses in spacecrafts caused by gravitational ripples, the formation of two spacecrafts in PROBA –3 mission is designed to enable observation of solar corona by coronagraph spacecraft for 6 hours in every 19h hour 36 minute orbit through on demand creation of total solar eclipse by the occulter spacecraft of the formation.

Natural total solar eclipse events do not provide enough opportunities to observe outer atmosphere of sun hence on demand opportunity provided by PROBA-3 will be very helpful in study of solar corona and associated phenomena for better prediction of space weather and solar storms which are known to disrupt satellites, communication infrastructures and power networks on Earth.

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