Frequently asked questions: Cluster's Salsa reentry

What is happening? 

ESA’s Cluster mission, which has spent 24 years revealing the secrets of Earth’s magnetic environment, is coming to an end. 

The first of the four satellites in the Cluster quartet will reenter Earth’s atmosphere in a ‘targeted’ reentry’ over a remote part of the South Pacific Ocean on 8 September 2024.   

The end of the Cluster mission with its four identical spacecraft offers a rare chance to target and study safe atmospheric reentries under different conditions.  

Why is ESA ending the Cluster mission? 

Our mission operators kept Cluster flying for 24 years, despite an original expected mission lifetime of just two years. The mission’s long life has helped scientists spot longer-term trends over two solar activity cycles, leading to the publication of more than 3600 scientific papers already.  

But as the fuel starts to run out, time has come to decide how to wrap it up. The decision to plan the reentry at this time made it possible for the Cluster spacecraft to contribute to reentry science as a final farewell.  

The scientific torch will be passed on to the ESA/Chinese Academy of Sciences Solar wind Magnetosphere Ionosphere Link Explorer (Smile) mission, which is set to launch in 2025.  

What is reentering?

On 8 September, Cluster spacecraft ‘Salsa’ (Cluster 2) is reentering Earth’s atmosphere. Salsa is one of four Cluster satellites together with Rumba (Cluster 1), Samba (Cluster 3) and Tango (Cluster 4).  

Salsa, like the others, weighed 1186 kg when it was launched in 2000. By using up its fuel, Salsa’s weight has been reduced to about 550 kg.  

Rumba is set to reenter in 2025, and Samba and Tango in 2026. 

Why is ESA deorbiting it and in this way? 

At the end of a mission, satellites should be removed from Earth orbits as quickly and safely as possible to prevent the creation of more space debris. At ESA, we are committed to the goal of Zero Debris by 2030 and advancing the science and technology development necessary to get there.

The end of the Cluster mission offers a rare chance to study the safe and ‘targeted’ atmospheric reentry of four identical satellites under different conditions. Its data can be used for future reentries as well as help the design of future ‘zero-debris’ satellites.

Cluster’s reentry follows those of ESA’s Aeolus and ERS-2 Earth observation missions. ESA is setting a precedent for a responsible approach to reducing the every-increasing problem of space debris and uncontrolled reentries.

When and where will Salsa reenter? 

On 8 September 2024, Salsa will come down above a very remote area in the South Pacific Ocean, far away from any populated areas.  

What is a ‘targeted reentry’? 

A targeted reentry aims for a specific and limited geographical region where the satellite will reenter the atmosphere at a specific time. It does not require the spacecraft to be controlled during the reentry itself. 

Targeted reentries involve manoeuvring satellites months to years in advance to line them up to reenter in a specific location. In January, spacecraft operators carried out a series of manoeuvres to ensure that Salsa's reentry will take place over a sparsely populated region in the South Pacific. 

The Cluster reentry is the first time that anyone has targeted the reentry of a satellite with an eccentric orbit like Salsa’s in this way.  

How does ESA make Cluster’s targeted reentry work? 

Cluster is in a highly eccentric orbit around Earth that takes 2.5 days to complete. It goes as far out as ~130 000 km (apogee) and at its closest is at an altitude of just a few hundred kilometres (perigee). It is this special kind of orbit that makes a targeted reentry possible. 

Because its large orbit is sensitive to the gravitational pull of the Sun and Moon, the altitude at perigee can sometimes change by more than 30 km from one orbit to the next depending on the where the satellites are exactly. For the reentry, the team targeted a particularly large drop in altitude on purpose during its very last orbit around Earth. Coming in at a steep angle, Salsa’s last approach will see a drop in altitude of over 30 km at perigee compared to the previous orbit.  

The big difference in altitude from one perigee to the next ensures that this exact turn around Earth is the one where the satellite comes down low enough, to the point where Cluster is expected to burn up without escaping fragments, which is at around 80 km altitude.  

The opportunities for such a targeted reentry are few and far between, as Cluster’s orbit naturally moves through 12-year cycles of coming closer to Earth, farther away and then closer again.  

Salsa’s trajectory naturally takes it so low that it will burn up this year in the southern hemisphere, instead of continuing with a new cycle.   

In January 2024, operators manoeuvred Salsa to target a specific region. The satellite will now reenter on 8 September without any further assistance. It will burn up above a very sparsely populated and remote area in the South Pacific Ocean to the west of Chile.  

When and where will ESA let the other three Cluster satellites reenter? 

Following Salsa’s reentry, the three remaining Cluster satellites will enter a special ‘caretaker’ mode. They can still be controlled but will not carry out new science until they reenter Earth’s atmosphere and burn up as well. 

Operators will manoeuvre ‘Rumba’ (Cluster 1) in August 2024 and then ‘Samba’ (Cluster 3) and ‘Tango’ (Cluster 4) in November 2024 to line them up for reentries in similarly remote locations over the South Pacific Ocean in November 2025 (Rumba) and August 2026 (Samba and Tango).  

Where can I find updates on the reentry?

ESA’s Space Debris Office will share information, ground tracks, observations and other updates via the Rocket Science blog, starting around two weeks before the reentry. 

How is ESA monitoring the reentry?

ESA’s Space Debris Office, supported by the flight dynamics and flight control teams, as well as the planetary defence office, are keeping an eye on the trajectory of Cluster’s Salsa.   

Through ESA’s Estrack ground stations, the flight control team are in contact with the satellite on a regular basis. It is getting increasingly difficult for Salsa to keep power on and send telemetry down to Earth. As the reentry comes closer and the spacecraft dips deeper and deeper into the atmosphere, it might stop responding entirely. If still possible, Salsa’s final telemetry will come in during its last ground station pass with Kourou ground station, two hours before its reentry.   

Multiple telescopes are observing the spacecraft during its last weeks to determine its precise location and trajectory. Because of the highly eccentric orbit sending the satellite far away every orbit, Salsa behaves somewhat similar to an asteroid. Near-Earth Object telescopes that usually detect asteroids are now supporting Cluster observations, including ESA’s Optical Ground Station, located in Tenerife (Spain), ESA’s Test-Bed Telescopes and the Schmidt telescope at the Calar Alto Observatory.  

Cluster is also the subject of a monitoring campaign by the Inter-Agency Space Debris Coordination Committee (IADC),asking international partners to support the monitoring of the satellite reentry and share data.  

As of 26 August, an observation experiment from a plane close to the reentry location has been confirmed. 

Why is ESA sending a plane to observe the reentry?

In the nearly 70 years of spaceflight about 10 000 intact satellites and rocket bodies have reentered the atmosphere. Yet we still lack a clear view on what actually happens during a reentry.

There is testing we can perform on the ground, and we have virtual modelling, but we also need real-life observations right at the scene of a reentry to complete the picture. The observation experiment from a plane is an exciting new possibility to collect data and gain confidence in the modelling to support new missions.

The airborne observation campaign also lets us observe a satellite class and reentry conditions which have never been accessible before.

What does ESA hope to learn from this reentry?  

Salsa is the first ever satellite to experience a targeted reentry. Because Cluster consists of four identical satellites, valuable data can be collected by safely reentering the same satellite under slightly different circumstances. We can observe what difference it makes as they reenter at four different angles and speeds, and under four different sets of atmospheric conditions.  

The data will improve our understanding of reentries and help us define the standard for the safe and controlled disposal of satellites in similar orbits. The validated reentry technique will then be used for future ESA missions like Proba-3, Smile and others with highly eccentric orbits. 

Knowing how a satellite burns up and what parts survive will also help us build zero-debris satellites for more sustainable future spacecraft.  

Will any parts of the satellite survive reentry and end up on Earth?

Not much of the 550 kg satellite is expected to survive the reentry, with most fragments burning up around 80 km above Earth’s surface. Some parts might partially survive the high friction and fragmentation.  

This is why we are targeting such a remote part of the ocean for a safe reentry and splashdown of any remaining fragments. None of Salsa’s fragments contain any toxic or radioactive substances. 

What if anything breaks off earlier? 

Cluster’s Salsa is repeatedly dipping low into the atmosphere before it comes down to the altitude of 80 km. Simulations show that the satellite is expected to remain intact through the last few perigees, even the last one at just 112 km altitude.  

In any case, if an exposed element like an antenna would break off early after all, the fragments would remain close to the main body of the spacecraft. Everything would continue to travel at the same high velocity and complete another orbit before reentering together, at almost the same place and time.  

What are the risks associated with the reentry? 

The risk of any satellite reentry causing injury is extremely remote. The annual risk of an individual human being injured by space debris is under 1 in 100 billion. In comparison, a person is about 65 000 times more likely to be struck by lightning.  

The limited risk is further reduced to a large degree because the reentry is targeted far away from densely populated areas.  

Do reentries release dangerous chemicals into the atmosphere? 

Understanding the direct and indirect impact of space industry activities on Earth’s climate is of utmost importance for the sustainable future utilisation of space for the betterment of humankind. ESA and other institutions are investigating the potential atmospheric pollution caused by the rapidly increasing volume of launch and reentry traffic. 

ESA organised a dedicated event in January 2024 to address this topic. The Agency also carried out two studies on the atmospheric impact of spacecraft reentries in 2019. They concluded that the short-term impact on the atmosphere due to the burn up of a single spacecraft is modest, primarily because the particles created during a reentry are generally too large to react chemically with the atmosphere. 

More details on the topic can be found here. 

Will observations from the plane detect the environmental effects of the reentry on the atmosphere?

The scientific instruments on board the plane will not detect them directly, but we can still derive important information about the environmental effects.

Direct measurements of the atmospheric impact of this event are not possible from the plane, because any small-scale particles potentially created during the breakup are released too high up, at 70–90 km in altitude. From there it can take months or even years before they reach the altitude at which the instruments on board the plane will operate.

However, observing where the break-up occurs and which large scale fragments are created in the process (possible at the plane's altitude), allows us to develop models that predict how spacecraft fall apart. This in turn lets us estimate how many by-products are created and gives us information on the environmental impact of a satellite reentry.

What action is ESA taking to minimise the creation of space debris? 

ESA is committed to ensuring the long-term sustainability of space activities through mitigating the creation of space debris wherever possible and ensuring the safest possible reentry of satellites at the end of their lives. 

ESA is taking proactive steps to protect the space environment through programmes, activities and initiatives including: 

• Space Debris Office – The SDO manages ESA’s activities related to measuring and modelling the space debris environment and mitigating the risk of in-orbit collisions and reentries. It coordinates such activities with national space agencies such as ASI, CNES and DLR and other external partners. 

• Clean Space initiative – ESA’s Clean Space initiative coordinates the development of new technologies for more sustainable space missions, including design for demise activities to reduce the risk on-ground caused by reentering objects. 

• Zero Debris approach – The Zero Debris approach is ESA’s new effort to significantly limit the production of debris in Earth and lunar orbits by 2030 for all future ESA missions, programmes, and activities. It has so far involved updating the documents that govern how ESA designs, builds, operates, and disposes its missions and the co-development of the Zero Debris Charter (see below). 

• Zero Debris Charter – The Zero Debris Charter is a community-driven and community-building document and initiative for the global space community. Facilitated by ESA and created and written by 40 space actors, the Charter contains both high-level guiding principles and specific, jointly defined targets to achieve Zero Debris by 2030. 

• Protection of space assets Accelerator – The Accelerator supports high-level efforts towards the safeguarding of space assets from hazards such as space debris and space weather. This work often concerns collaboration with national, EU or other European initiatives, and the European space sector, such as the development of the Zero Debris Charter.