Fraunhofer Institute for High Frequency Physics and Radar Techniques FHRRemoving Debris from Orbit
Numerous pieces of debris orbit the Earth – such as rocket upper stages. If they are to be actively removed, numerous issues must be clarified beforehand. Is the upper stage rotating? Or is its attitude towards Earth? Is it intact? Researchers at Fraunhofer FHR were able to successfully answer such questions for the Japanese space agency JAXA using the Tracking and Imaging Radar TIRA.
Whether for communication, navigation, or weather forecasting: we rely on satellites for numerous everyday applications. But with most satellites launched into space in the past and sometimes still today, a rocket upper stage is left behind as debris in space. How can this debris be actively removed? In 2021, the Japanese space agency JAXA (Japan Aerospace Exploration Agency) brought Fraunhofer FHR on board to investigate the attitude and movement of various upper stages of the H-IIA family using the TIRA space observation radar. The focus of the "Commercial Removal of Debris Demonstration (CRD2)" project was on the upper stage with the NORAD ID "33500": What is its attitude? Does it rotate in some way? Is there any visible damage?
TIRA looks at the "33500" upper stage
Previous simulations by JAXA suggested that rocket stages in circular low Earth orbits initially rotate slowly and randomly, but after a certain time in orbit, they reach a stable state, for example, with the nose or thrust nozzle always pointing towards the Earth. "We were able to confirm this for the upper stage 33500 – it remained consistently oriented with its nose towards the Earth throughout the entire observation period from 2021 to 2025," summarizes Frank Schlichthaber, project manager at Fraunhofer FHR.
The researchers at Fraunhofer FHR carried out calculations based on measured radar data and simplified 3D models of several H-IIA rocket upper stages: How would the rocket look like in the radar images if it was always pointing with its nose towards the Earth? Rotations around the longitudinal axis were a particularly big challenge, as upper stages are rotationally symmetrical. In the case of upper stage 33500, however, lateral support structures protrude from the nose, to which the satellite was attached via an adapter. These structures were oriented in the same way on all radar images over the entire observation period. “Not only were we able to determine a stable position of the longitudinal axis in the zenith direction, we were also able to rule out rotation around this axis,” says Schlichthaber.
Satellite investigations support the result
This result is supported by studies carried out by JAXA and Astroscale Japan. Astroscale Japan was contracted by JAXA to build and operate the ADRAS-J satellite for Phase I of CRD2. In 2024, the satellite approached the upper stage to within 15 meters, orbited it, and analyzed its condition and movement using film and photographic recordings. The result matched that of TIRA – it also showed that the upper stage performs a slight libration of up to two degrees, which was not visible in the radar data. The researchers at FHR also kept an eye on the upper stage before and after the ADRAS-J maneuvers in order to detect a possible change in orientation. The next step is to build another satellite that will dock the upper stage and lower its orbit into denser atmospheric layers, where it will be slowed down more and burn up after a relatively short time.
In addition to upper stage 33500, TIRA also analyzed other objects from the H-IIA family – with varying results. “The upper stage 27601 was also stable in 2022, but 39771, for example, showed a noticeable offset from the vertical axis, though the cause remains unclear." says Schlichthaber.
The collaboration between Fraunhofer FHR and JAXA was also successful for both sides from a scientific perspective. As the first author, JAXA published and presented these results together with their own simulations and the analyses of Astroscale Japan at the 35th AAS/AIAA Space Flight Mechanics Meeting on Hawaii and at the 9th European Conference on Space Debris of the ESA in Bonn.