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ESA restored contact with its Proba-3 Coronagraph spacecraft on March 19, 2026, after more than a month without communication. The spacecraft had not vanished or left Earth orbit: a software failure had left it tumbling, short of power and unable to communicate normally. A brief period when its solar panels caught sunlight helped bring its essential systems back online. By June 4, Proba-3 had returned to formation flying and captured new images of the Sun’s corona.
What Proba-3 is—and why two spacecraft fly together
Proba-3 is a European Space Agency mission that uses two spacecraft to create an artificial solar eclipse. The Occulter spacecraft flies between the Sun and the Coronagraph spacecraft, blocking the bright solar disk so the Coronagraph’s ASPIICS instrument can study the much fainter outer atmosphere, or corona.
For observations, the spacecraft hold a separation of about 150 metres. That precise geometry is the mission’s scientific advantage, but it also makes Proba-3 a demanding formation-flying demonstration: both vehicles must maintain and coordinate their positions. The Coronagraph was the spacecraft that lost contact; the Occulter remained operational. ESA says the pair were about four kilometres apart when the anomaly occurred, rather than in their close observing formation. ESA’s account of the search explains the separation and recovery challenge.
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What failed aboard the Coronagraph
ESA’s investigation traced the incident to an unusual software failure, not a collision or a spacecraft breaking apart. The Coronagraph was preparing to desaturate its reaction wheels—an operation that restores their ability to control the spacecraft’s orientation. Thrusters needed for the maneuver were being warmed up.
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During this process, an equipment temperature crossed a permitted threshold while the software was managing semaphore locks. A semaphore is a software mechanism for controlling access to a shared resource. ESA says two nearly simultaneous events left one semaphore permanently locked, disrupting the planned wheel desaturation and also preventing the expected automatic response into safe mode.
- The reaction-wheel desaturation did not take place.
- The spacecraft progressively lost its correct orientation.
- Its solar panel stopped pointing reliably at the Sun, reducing power generation.
- The battery discharged, and the spacecraft entered survival mode, interrupting normal communications.
This chain matters: a spacecraft can remain physically intact in orbit yet become electronically silent when an attitude-control problem also cuts off power. ESA reported that this particular software vulnerability had not appeared in ground testing or earlier flight operations. The agency said it was examining the failure and reviewing similar software architectures. ESA’s investigation describes the semaphore failure and its consequences.
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Why locating it was not the same as contacting it
Proba-3 travels in a highly elliptical Earth orbit, reaching roughly 60,000 kilometres from Earth. At that distance, the spacecraft cannot continuously rely on navigation satellites to establish its position. With the Coronagraph tumbling and separated from its partner, engineers had to estimate not just where it was, but how it was rotating and when it might be possible to communicate.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsESA combined observations from its Estrack ground-station network with optical observations from Neuraspace and Sybilla Technologies, and radar data from Fraunhofer FHR’s TIRA system. Changes in the spacecraft’s brightness as it rotated helped reveal its spin and tumble. That tracking work let the team model its motion and identify times when a solar-panel exposure or communications attempt might be worthwhile. It did not, by itself, restore a radio link.
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How sunlight helped restore contact
A tumbling spacecraft can face the Sun only intermittently. When the Coronagraph’s solar panels briefly caught sunlight, they could generate enough energy to power essential electronics and recharge the battery. ESA used a favourable power-and-communications opportunity to re-establish contact through its Villafranca ground station in Spain on March 19, 2026.
That was not a planetary alignment, nor a single act of switching the spacecraft back on. It was a timing and geometry problem: the spacecraft had to receive enough sunlight to operate, while also being reachable by a ground antenna. The phrase “rare alignment” can suggest an astronomical event that was not involved; the unusual sequence was the software failure, while recovery depended on a temporary solar-panel orientation and a usable communications window. ESA’s tracking update and its recovery investigation describe those distinct parts of the effort.
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Contact was the start of recovery, not the finish
After weeks in extreme cold and with limited power, the Coronagraph needed a cautious recovery. Restored communication did not establish that every instrument, thermal system or autonomous function was ready for normal work. ESA allowed systems to warm, checked spacecraft health and proceeded gradually. Survival mode during the blackout and safe-mode operations after contact returned are separate stages, not interchangeable labels for the whole incident.
Nor did the first successful communications link prove that the two spacecraft could immediately resume their precise formation. That required later operational tests. ESA reported that an initial formation-flying attempt on June 3–4 failed because of a configuration oversight; a second attempt succeeded. On June 4, the agency announced that the pair were flying in formation again and that ASPIICS had captured new corona images—the first since the anomaly. ESA said the mission was expected to return to routine autonomous formation flying and observations after processing and operational checks. Read ESA’s June update on the return to formation flying.
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What the incident shows about autonomous spacecraft
Proba-3 had entered autonomous routine operations in June 2025. ESA says the anomaly occurred overnight, when no personnel were at the mission-control centre. Autonomy enables spacecraft to carry out routine tasks without continuous human intervention, but a rare fault can progress before operators can respond—especially if the fault also blocks the system’s designed recovery path.
The episode also shows why “finding” a spacecraft is not one event. Teams first had to observe it, estimate its orbit and rotational state, predict power opportunities, establish a radio link, check its condition and then verify that formation flying worked. A companion spacecraft can help with tracking, but it is not automatically close enough to perform an immediate rendezvous. Likewise, brightness changes can reveal a tumble even when radio contact is absent.
ESA said the relevant timing combination had eluded earlier testing and flight operations, and that it was considering advanced software-analysis methods, including AI-based tools, while reviewing comparable software architectures on other missions. That is a targeted response to a rare interaction, not evidence that all autonomous spacecraft share the same vulnerability.
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ESA said that even if the Coronagraph had not been recovered, the two Proba-3 spacecraft were designed to re-enter Earth’s atmosphere naturally within about four years. That is a statement about this mission’s design, not a general assurance for every unresponsive spacecraft in orbit. In this case, the Coronagraph was recovered and the pair later resumed formation flying.
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