Cluster’s last dance
What makes our planet habitable?
Beyond the presence of liquid water on its surface, a protective atmosphere or the stability provided by the Moon, there is a hero that often goes unnoticed: the magnetosphere. This powerful and invisible shield protects us from the constant bombardment of charged particles from the Sun and deep space. Without it, life on Earth would be, well... rather complicated.
But to study the magnetosphere effectively, it is necessary to venture beyond our atmosphere. This is where a unique space mission comes into play: Cluster.
Dancing in space
The European Space Agency (ESA) devised an unprecedented mission: four identical satellites placed at the vertices of a tetrahedron whose edges could vary from a few kilometers to ten thousand. This arrangement would make it possible to study magnetospheric phenomena at different scales, ranging from electric currents, plasma gradients, and the divergence of the electron pressure tensor to three-dimensional structures such as boundaries, shock waves, and magnetic vortices.
The idea of a precise choreography around our planet had some influence on the names chosen for the satellites: Rumba, Tango, Salsa, and Samba.
Mission analysis showed that Cluster’s orbit would be polar and highly eccentric, with an apogee of 4 Earth radii and a perigee of 20, inertially locked to the Sun. This would allow the mission, expected to last two years, to study the entire magnetosphere twice.
An inauspicious debut
In June 1996, the four dancers sat in the cargo bay of the brand-new Ariane 5 rocket on its maiden flight. I witnessed this from a prime position: the control room of the ISO infrared telescope at the Villafranca del Castillo Satellite Tracking Station (VILSPA) in Villanueva de la Cañada, about 30 km from Madrid, now the European Space Astronomy Center (ESAC).
Well, the rocket exploded shortly after liftoff.
An investigation revealed that the cause was a software failure, at the time the most expensive “bug” in history. It turned out that software had been reused from the inertial system of Ariane 4, the previous generation of rockets. However, Ariane 4 had significantly lower horizontal velocities in the first few moments of flight, leading to a 16-bit integer overflow that tilted the rocket more than 20 degrees and caused it to begin disintegrating under aerodynamic forces. Ultimately, the flight termination system destroyed the rocket and its precious cargo.
Years of work by hundreds of engineers, the plans of ISO operators ready to handle Cluster, and the scientific aspirations of researchers were scattered in the swamps near ESA’s spaceport in French Guiana.
Rebirth of the dance crew
But the expected scientific return from the Cluster mission was so significant that ESA decided to resurrect it from the ashes. The technical parts of the previous satellites were integrated into a new one, and industrial partners were contracted to build three more.
In 2000, Cluster was launched “for the second time,” this time on two Russian Soyuz rockets.
Tireless, the four dancers exceeded their two-year design life and, thanks to the skill of the control teams in conserving fuel and overcoming challenges such as battery degeneration (the last one stopped working after nine years in space), they reached an incredible 24 years of operation!
Instead of the two planned orbits around our star, Cluster has studied the magnetosphere and its interaction with the Sun over two full solar cycles, working in tandem with other space missions from agencies such as ESA, NASA, JAXA, and CAS. The mission has produced a wealth of scientific knowledge, including 3,750 research papers, over 30 in Nature and Science, and 122 PhD theses.
The end of an era
Unfortunately, all things must come to an end.
On 8 September at 18:47 UTC, ESA “splashed Salsa,” making a controlled re-entry into the uninhabited area of the South Pacific Ocean, marking the end of operations for the first Cluster satellite.
Interestingly, there were no deorbiting guidelines when Cluster was launched, meaning the satellites were not built with re-entry in mind. But Salsa’s end goes far beyond current guidelines, which don’t require controlled re-entries. In fact, Salsa’s re-entry was the first for a satellite in a highly elliptical orbit.
In January, the necessary maneuver was carried out so that, a few months later, Salsa would pass below the 80 km threshold at which atmospheric drag would begin to decay its orbit, leading to its destruction at the designated point.
The precision of this operation was such that aircraft were sent to observe the re-entry from below. Indeed, this is another milestone of this operation: the rare opportunity to study the re-entry of four identical satellites under slightly different conditions, which will provide valuable data for ESA’s goal of building “zero debris” satellites in the future.
The other three satellites have stopped collecting data and are in so-called maintenance mode: Rumba will re-enter in 2025, and Samba and Tango will follow in 2026.
This will mark the end of the long-lived and extraordinary Cluster mission. However, the data collected by the four dancers will continue to provide scientific discoveries for many, many years to come, contributing to our understanding of the magnetic defenses of our home.
Author: Juan Carlos Gil