As space becomes increasingly crowded with operational satellites and space debris, monitoring satellite re-entries is crucial for helping to ensure the safety of people and property on Earth.
The National Space Operations Centre (NSpOC) plays a key role in this effort through our re-entry tracking and early warning capability, which monitors re-entry incidents and where relevant provides warnings to response agencies to minimise any associated risk to the UK or the UK Overseas Territories.
On 8 September 2024, The European Space Agency (ESA) undertook the safe de-orbit of their Cluster 2 satellite named 'Salsa'. The spacecraft re-entered at 6.47pm GMT into a designated region in the South Pacific as planned. NSpOC analysts monitored the event, as it does with every re-entry globally, to ensure potential risks to the UK were assessed and addressed. In this article, we'll explain how our re-entry tracking and early warning capability works, using ESA's Salsa as a case study, and highlight the key contributions of the satellite to both government operations and academic research.
NSpOC's Re-entry Tracking and Warning Capability: Monitoring Re-entries Like Salsa
NSpOC's re-entry tracking and warning capability operates 365 days a year and is dedicated to monitoring the re-entry of satellites and other space objects that could pose a risk to the UK and UK Overseas Territories, as well as the re-entry of objects for which the UK holds liability. On average, NSpOC monitors around 40 uncontrolled re-entry incidents per month, warning UK response agencies when there is a risk to the UK or our Overseas Territories.
How does the capability work?
As part of our space hazards warning and protection services, NSpOC continuously monitors space objects in orbit, providing early warnings when satellites begin their descent. In the case of uncontrolled re-entries, and semi-controlled re-entries such as ESA's Salsa, orbital analysts assess the object's trajectory and predict when and where it will re-enter the atmosphere. This data is shared with response agencies and government departments to ensure preparedness.
Real-Time Monitoring and Modelling
36 hours before a re-entry event, NSpOC's team of orbital analysts start to monitor the object in more detail, using a global network of sensors and data streams which provide information on its path. In uncontrolled re-entry events, data on the last known position of the object as well as its predicted Centre of Impact Window (COIW) is provided by the US in the form of Tracking and Impact Predictions (TIPs). This TIP is a single point which can come with high levels of uncertainty. To better understand the probability of where, within that window, the event may occur, UK Space Agency analysts run an extra re-entry assessment model using Monte Carlo simulations (see below for an explanation).
Monte Carlo simulations incorporate additional data sets, for example forecasted atmospheric density over the re-entry period, to allow for more accurate predictions of where surviving objects might land. The model runs 30,000 times with slight adjustments to values for each variable.
Rather than a single point of re-entry, this results in a probabilistic output of potential re-entry locations; shown visually on a map by red dots, with each red dot representing a potential re-entry location. The denser the collection of red-dots, the greater the likelihood of the object re-entering in that location.
In the case of ESA's Salsa satellite, ephemeris (positional data) was provided rather than TIPs and our analysts converted this to a Two-Line Element (TLE) to extract the orbit, and ran the Monte Carlo simulation to produce a visual prediction of re-entry.
For Salsa, our analysts closely monitored the descent, predicting that the majority of the satellite would burn up in the atmosphere, with any surviving fragments expected to land in a remote region of the South Pacific - the likely re-entry path is shown via the red dots in the images below, starting 36 hours in advance on Friday 6 September.
Re-entry map generated by NSpOC analysts on Friday 6 September
As the time to the re-entry gets closer, more observations on the object are received by global networks. This results in new data which can be input into the Monte Carlo simulation, producing a more accurate assessment of re-entry locations.
Re-entry map generated by NSpOC analysts on Saturday 7 September
Final re-entry map generated by NSpOC analysts on Sunday 8 September before the event
Post-Re-entry Assessment
After the satellite re-enters, NSpOC conducts a post-event analysis to confirm the re-entry location and assess any potential impact. It is not always possible to receive tracking data confirming the location of a re-entry, but in the case of Salsa, ESA were tracking the re-entry event closely, even with sensors on an aeroplane, to confirm that it re-entered in the South Pacific.
The re-entry location is shared with relevant stakeholders to ensure transparency and public safety. In the case of Salsa, the risk to human life and infrastructure on Earth was extremely low because of the semi-controlled nature of the re-entry bringing it down in an unpopulated location.
Why Monitoring Re-entries is Important
Whilst risk from re-entering satellites is very low, with the majority of satellites burning up upon re-entry, large objects or those with dense components will survive the re-entry process, posing a risk to wherever the object makes landfall. Since most re-entries are uncontrolled, there is a risk to populated areas and so warning authorities when there is a risk is important for both safety and to ensure we take responsibility for any space debris for which the UK is liable. Each re-entry incident provides valuable data that helps improve our ability to track and monitor future re-entering objects.
By responsibly managing satellite re-entries, such as ESA's Salsa, operators can reduce the amount of debris left in orbit, making space safer for future operations. Later this year, NSpOC will launch a new digital service on our Monitor Space Hazards platform, called 'Track Re-entry Events', which will provide government users with real-time updates and enhanced analytics for monitoring satellite re-entries. You can find out more about upcoming features on our website.
Angus Stewart, Joint Head of NSpOC said: "By managing satellite re-entries responsibly, as is the case with this event, operators reduce space debris and make space safer for future missions. Tracking re-entering space objects is a critical mission for the UK National Space Operations Centre"
The ESA Cluster Salsa Mission: why was it significant?
Launched as part of the ESA Cluster mission in July 2000, the Salsa satellite provided critical data that advanced our understanding of Earth's magnetic environment and space weather over its 24-year operational life.
Salsa's contributions to space weather research were invaluable. The satellite's data helped improve predictive models that assist in safeguarding critical infrastructure such as electrical power grids, pipelines, and satellite communications systems. By studying how solar winds and geomagnetic storms interact with Earth's magnetic field, Salsa provided insights that helped operational teams understand and mitigate the impacts of space weather on essential services.
Salsa was part of a constellation of four satellites which was crucial because it allowed scientists to gather data from multiple points in space simultaneously, giving researchers a 3D view of Earth's magnetic environment.
Met Office Space Weather Manager Simon Machin said: "Cluster has been a key contributor to advancing space weather science in recent decades. By advancing what we know about the near-Earth environment at multiple scales, this mission pushed forward research understanding, which underpins global space weather operations. Cluster's observations are a unique resource and will continue to provide considerable value in the years to come."
Conclusion
The ESA Cluster 2 Salsa re-entry provides a clear example of how NSpOC monitors re-entries, ensuring the safety of the UK and its Overseas Territories. As satellites continue to play an increasingly important role in both government operations and academic research, NSpOC's re-entry capability will remain vital to ensuring the safe and responsible use of space for future missions.
