As part of our observational activities, we do not only observe natural objects, but occasionally also artificial satellites. Sometimes we use them as proxies for a potential threating asteroid (as we did in 2020 with BepiColombo). Other times we use them as training subjects or for calibration purposes. In this occasion, it was for a collaboration with ESA's Space Debris Office to support the orbit determination of a target of their interest.  

This artificial satellite is "Salsa", one of the four identical satellites of ESA's Cluster mission (the other three are similarly nicknamed Rumba, Samba and Tango). These satellites were designed to study the Earth's magnetosphere and its interaction with the solar wind. In order to achieve their mission goals, they are placed in an uncommon orbit (HEO – high-eccentricity orbit), which allows for a neat disposal at the end of its lifecycle . Moreover, this orbit positions the satellite, for a significant time of its orbit, well beyond the LEO and MEO regimes, resulting in sky conditions closer to those commonly seen for NEOs (farther and slower than most of Earth satellites). This regime enables a synergy between PDO and SDO, with the use of the PDO assets and techniques for NEOs applied to satellites.   

Observing an object like Salsa is therefore an excellent opportunity for us to train in the observation of very fast objects. Only a few regular NEOs become as fast as Salsa could get, and they are typically of special relevance when they become 'impactors', because they help learning about the nature of the asteroids and their interaction with the atmosphere, both fundamental for the mitigation activities in case of a threat. Until now, only nine asteroids have been observed before impact, and they were all tiny, intrinsically faint and therefore challenging because of the need of fast response times and short visibility times.  

However, with Salsa we had a large visibility window and excellent predictability of its motion, allowing us to plan our observing strategy in advance. Thus every 2 days and a quarter, the object provided us with a very low perigee, with an observational geometry resembling an impact. We thus took the opportunity to observe Salsa for a few months, with different telescopes from which we routinely observe NEOs (ESA's Optical Ground Station – OGS, CAHA's Schmidt), in order to test and validate the different formats and interfaces between the two teams, PDO and SDO, who usually work in different units and with different definitions.  

All of this preliminary work was part of the preparation for the final campaign, to be performed during the very last orbits of Salsa, when there was a risk of losing telemetry (and therefore trajectory information) at any time, because the already battered batteries were suffering more and more from the longer eclipses as Salsa was getting closer to Earth. Moreover, simulations showed that the last perigee passage of Salsa could be low enough to damage the spacecraft, leaving it with no communication and active tracking channels during its last orbit before the final plunge. From the moment telemetry would cease, any orbital determination would rely mainly on optical observations, bringing us to the same situation we have when we track a natural and thus unresponsive asteroid. And that was when the asteroid experience of the PDO team could really help: the observation recovery and following orbital determination after the last perigee was essential to determine the exact actual reentry point, where an airplane was going to monitor the reentry process.  

There was some positional uncertainty after the last perigee passage because of the difficulty of modelling the atmospheric interaction. We had to observe Salsa as soon as possible after perigee, because the uncertainty would keep growing. We decided to use our telescope network to observe it as soon as it became visible from somewhere around the world. We recovered it from Australia, less than an hour after perigee, but when the spacecraft was already 12 500 km above the planet surface. The target was very close to the Flight Dynamics and SDO predictions. We observed it from the Canary Islands, mainland Spain and from South Africa, to add parallax and improve the orbit determination. 

The following night Salsa was close to apogee and adding more observations would not help much to improve the orbital knowledge. However, there was still a chance to improve the orbit if we could observe it just before impact, when the object was briefly visible during twilight from Namibia. The object could be observable down to a range of less than 12 000 km, while moving in the sky at an angular speed of almost 90"/s. The conditions were challenging, but we could detect the object and, with proper calibration, the accuracy of the impact point location was improved. Flight dynamics got a last contact from Kourou tracking station and therefore we have a ground truth to validate our calculations. Finally the reentry took place as expected and it was observed from the air by the airborne observation mission ‘ROSIE-Salsa‘. 

This adventure just starts here, with all the experience to be used in the upcoming reentry of the three remaining Cluster spacecraft. With aging hardware, the potential need for optical observations is significant, and we are ready to execute them and to apply our expertise again to support the reentry of decommissioned spacecraft.  

Composite image showing Salsa satellite crossing a small portion of the field of view of Optical Ground Station, OGS, 1m telescope located at Izaña, Canary Islands, Spain. Credits: ESA / PDO.

Animated gif showing Salsa satellite crossing a small portion of the field of view of Optical Ground Station, OGS, 1m telescope located at Izaña, Canary Islands, Spain. Credits: ESA / PDO.