First of ESA's Cluster satellites prepares for fiery finale over South Pacific

Over 20 years into a 2-year mission, but all good things come to an end

The first of the European Space Agency's Cluster satellites is set to return to Earth next month after an extraordinarily long mission. The spacecraft is destined to burn up over the South Pacific.

There are four Cluster satellites: Rumba, Salsa, Samba, and Tango. Salsa will be the first to take a dive, with the rest following during 2025 and 2026. Unlike recent uncontrolled re-entries, controllers have carefully targeted Salsa so that any surviving fragments do not impact populated regions.

Salsa's orbit was tweaked in January to ensure that it would experience a steep drop from approximately 110 km to 80 km on September 8. Cluster Operations Manager Bruno Sousa said: "This gives us the greatest possible control over where the spacecraft will be captured by the atmosphere and begin to burn up."

The four identical Cluster spacecraft were designed to fly in formation to study the Earth's magnetosphere, which protects the Earth from particles emitted by the Sun. The science produced by the mission has proven invaluable in understanding "space weather" and the mission has received extension after extension as controllers devised ways of keeping the aging spacecraft operational.

We covered the Cluster mission during our Space Extenders series. Then, the team was celebrating being 20 years into a two-year mission. We spoke to Sousa and Cluster Mission Manager Philippe Escoubet, who explained that by careful maintenance of the batteries, engineers were able to eke out extra life from the spacecraft, but, at the ten-year point, "it became a mission where we did not have any more battery."

Some have referred to Cluster as the "zombie spacecraft" because when the satellites pass into eclipse, they must completely shut down and revive when the Sun shines on their solar arrays. Clever work by the engineering crew ensured the shutdown and recovery process were automated as much as possible, and the science return from the spacecraft is something to behold, particularly since they have been in orbit for multiple solar cycles.

However, as ESA observed, all good things must come to an end. Even in 2020, project leaders knew that 2024 would be the final hurrah for the mission. While the fuel consumption of the spacecraft had been reduced dramatically, controllers elected to ensure the inevitable re-entries would be carefully targeted.

Following Salsa's re-entry, the remaining Cluster satellites will enter "caretaker" mode. There'll be no more science, but the spacecraft will remain active. Controllers will keep an eye on them before they also make a re-entry.

ESA's next mission to study the Earth's magnetic environment, the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE), is due for launch at the end of 2025, and there is every chance that at least two of the Cluster spacecraft will still be in orbit.

SMILE's mission is set to last three years. Cluster was only supposed to endure for two, but has continued for nearly a quarter of a century.

As Sousa told us in 2020: "Never give up. Never surrender." ®