Decades of the Cluster Program: A Look at the European Space Agency's First Project with Hungarian Researchers

The Cluster program, developed over several design phases in the 1980s, aimed to map Earth's space environment within the magnetosphere and the nearby interplanetary region using a fleet of four space probes. It was the first space research program to deploy four identically equipped probes positioned at the vertices of an imaginary tetrahedron. This unique arrangement allowed simultaneous plasma physics measurements, effectively distinguishing spatial variations from temporal changes.

On June 4, 1996, tragedy struck when the Ariane 501 rocket, carrying the four probes, exploded shortly after launch from Kourou. Despite this setback, the significance and pioneering nature of the project enabled the participating researchers and their supporters to persuade the European Space Agency (ESA) to continue the Cluster mission, recalled Mariella Tátrallyay, who had been part of the program since 1991 and served as the head of the Cluster Hungarian Data Center until her retirement. She added that to minimize costs, which was an important prerequisite for continuing the program, the instruments were constructed according to the original plans. Three of the probes were rebuilt, and along with a fourth probe originally designated as a backup, they were launched into Earth orbit on July 16 and August 9, 2000, using two modified Russian Soyuz-Fregat rockets from Baikonur.

The research begins

Following the successful placement of the probes into polar orbit and the testing of the scientific instruments, the exploration of Earth's surrounding space began on February 1, 2001. The four probes, named Rumba, Salsa, Samba, and Tango, embarked on their mission to collect data. The data collection phase concluded on September 8, 2024, when the Salsa probe was directed into Earth's atmosphere, where it burned up upon reentry.

During the first six years of the program, the average distances between the Cluster probes were adjusted every six months, ranging from 100 km to 10,000 km, to study plasma physics phenomena of varying scales. However, after 2006, the probes were no longer maintained in their original polar orbits. According to the researcher, this decision was made to conserve fuel and to enable the exploration of previously uncharted regions of the magnetosphere that the probes had not been able to access earlier.

"Before the Cluster project, most magnetosphere research probes collected data primarily in the equatorial plane, so the data gathered by the Cluster probes at higher geomagnetic latitudes filled a critical gap," explained Mariella Tátrallyay. The Cluster mission was extended roughly every two years, a total of nine times. Tátrallyay emphasized that this was made possible largely because most of the scientific instruments remained highly reliable throughout the mission. For instance, all four probes were able to measure the magnetic field consistently until the very end of the project. "There was always great excitement when the two-year extension period came to an end, typically in late December," recalled the physicist. "This was because the decision on whether to approve the proposal for the next two-year extension was usually made by the ESA's scientific council in mid-November."

"Of course, there were malfunctions over the years," she noted. "The instruments detecting positive ions became partially or completely inoperable, and the device designed to neutralize the surface charge of the probes ran out of its indium ion source after eight years."

The program’s significance goes beyond its groundbreaking technology. Cluster was the first ESA mission to include members from a Hungarian research institute. Large-scale research often leads to substantial technological advancements on Earth as well. Mariella Tátrallyay recounted that, given the limitations of the internet at the time, the nearly 200 participating researchers aimed to establish a fast data exchange system. To achieve this, they envisioned a shared database for scientific datasets, stored across multiple servers. This was the first time in the history of space exploration that such a broad data-sharing network was planned, which brought a role for the Hungarian Data Center. Highlighting the state of Budapest’s telecommunications infrastructure in 1996, Tátrallyay noted that the ESA was unable to obtain a dedicated landline for the KFKI research institute in Csillebérc. As a solution, they rented a satellite connection.

Results and the legacy of Cluster

The Cluster mission significantly enriched our understanding of Earth's surrounding space and space weather, yielding many groundbreaking and surprising discoveries. Here are a few highlights of its achievements.

Based on measurements from the four Cluster probes, researchers were able to demonstrate that the magnetopause—a region previously thought to act as a solid protective shield—can behave more like a porous sieve, allowing high-energy particles from space to penetrate Earth's defenses more frequently than previously believed. Using simultaneous data from Cluster and other probes, scientists also proved that sudden shock waves can compress the magnetosphere, causing electrons in certain regions to accelerate rapidly, sometimes within just 15 minutes. This deformation of the magnetosphere can push these fast, "killer" electrons into orbital zones typically considered safe. Predicting such events is crucial for ensuring that vulnerable equipment can be powered down in time to prevent damage from radiation - a field where the Cluster program and its findings played a pioneering role. The program also led to the discovery and explanation of the phenomenon known as "black auroras." These dark patches between the bright rays of the aurora borealis were identified and analyzed using data collected by the Cluster probes.

To date, over 3,200 scientific publications have been produced using data from the Cluster mission. Researchers from the Space Physics Group at the HUN-REN Wigner Research Centre for Physics have contributed significantly, authoring 11 lead-author papers and collaborating as co-authors on numerous other studies.