The European Space Agency's Proba-3 mission launched from India on December 5, 2024, sending two satellites into orbit to test precision formation flying and study the Sun's outer atmosphere. The spacecraft rode an Indian Space Research Organisation rocket from the Satish Dhawan Space Centre on Sriharikota.
ISRO Chairman S. Somanath said after liftoff that the satellites had been placed in the correct orbit. The launch had been scheduled for the previous day but was postponed because of a technical problem. The approximately €200 million mission is the third in ESA's Project for On-Board Autonomy series, which uses orbital flights to demonstrate new technologies.
Proba-3 consists of two spacecraft called the Occulter and the Coronagraph. Once operating together, they are intended to fly about 150 meters apart with millimeter-level accuracy. The Occulter will block the bright disk of the Sun, casting a controlled shadow onto the Coronagraph. From inside that shadow, the second satellite can observe the much fainter solar corona.
The arrangement recreates the useful part of a total solar eclipse without waiting for the Moon to move between Earth and the Sun. Natural eclipses expose the corona only briefly and from limited locations. Proba-3 is designed to maintain its artificial eclipse for as long as six hours during an observing period, giving researchers much more time to examine the region close to the Sun's visible edge.
ESA said the mission would study the corona between the solar limb and a distance of about 1.4 million kilometers from the surface. That area is difficult for conventional instruments because scattered light from the Sun can overwhelm the faint structures scientists want to measure. Separating the occulting disk from the telescope by 150 meters reduces those optical limitations in a way that would be impractical inside a single spacecraft.
Researchers hope the observations will help address why the corona reaches temperatures far higher than the Sun's visible surface. The mission may also improve understanding of how the Sun's energy output changes and how activity in the corona contributes to space weather. Solar disturbances can affect satellites, radio communications and navigation systems around Earth.
The science depends on the engineering demonstration. The spacecraft must hold their relative position and alignment autonomously while orbiting Earth, then repeatedly move into and out of the eclipse configuration. Success would show that separate satellites can function as parts of one large virtual instrument.
Proba-3 therefore combines a solar observatory with a test of coordinated spacecraft control. Its data will be judged both by the quality of the coronal observations and by how accurately the two vehicles maintain the formation needed to create them.
