On 7 August 2024, a Long March 6A upper stage broke apart in Low Earth Orbit (LEO). The event generated more than 700 tracked fragments, adding to the already crowded orbital environment. Many of these objects could stay in orbit for decades, increasing the risk of collisions with operational satellites. The need to reduce such risks is why Post-Mission Disposal (PMD) is a necessary part of mission design.
In this post I am sharing key findings which resulted of the ASCenSIon project in an effort to better understand the practical options and challenges of PMD for rocket stages. This article highlights the main technical results.
Post-Mission Disposal (PMD)
PMD aims to remove spacecraft and rocket stages from orbit after their mission is complete, in order to prevent that they become a long-term collision hazard. While international guidelines recommend a re-entry within 25 years , this is not consistently met.
Additionally, ESA missions are subject to stricter requirements resulting from the Zero Debris Policy. Re-entry must happen within 5 years after the EOL, and the cumulative collision probability (CCP) with objects larger than 1 cm must be less than 1 in 1000.
The most common practices consist of lowering the perigee or orbit altitude to accelerate the decay thanks to the effect of atmospheric drag, design for short-lived orbits which already meet the required lifetime or perform a controlled re-entry.
The ASCenSIon project looked into how to improve orbital lifetime predictions and how better estimations of drag and ballistic properties can support PMD planning.
Accuracy of Orbital Lifetime Predictions (LEO)
A key part of PMD is predicting how long a rocket body will stay in orbit. Using data from the DISCOS database and Spacetrack, the orbital lifetimes of 340 rocket bodies in LEO were assessed. Their orbits were propagated with the OSCAR tool, the result was compared with the recorded re-entry date and the error distribution was analyzed. The analysis was performed using four different space weather scenarios. Two models for the drag coefficient (cD) were tested: a default value of 2.2 and an estimate based on each object's physical dimensions, assuming a cylindrical shape.
The analysis showed the impact of the solar and geomagnetic activity scenario chosen for the predictions. Moreover, the dimension-based model for the cD provided better results. Finally, it was shown that the prediction accuracy depends heavily on parameters like
- launch year,
- total time in orbit,
- the ballistic coefficient of the object
More accurate predictions help operators plan orbits that decay within acceptable timelines.
Figure 1. Distribution of the relative error for the orbital lifetime predictions of objects in LEO using the reference solar and geomagnetic activity scenario and both cD cases.
Figure 2. Distribution of the relative error for the orbital lifetime predictions of objects in LEO using the latest prediction solar and geomagnetic activity scenario and both cD cases.
Figure 3. Distribution of the relative error for the orbital lifetime predictions of objects in LEO with low and high B using the reference solar and geomagnetic activity scenario.
Estimation of the Ballistic Coefficient
Atmospheric drag is one of the main drivers of orbital decay. The ballistic coefficient B determines how strong the effect of drag is on an object. In this study, B values were estimated for several ORION 38 stages based on their Two-Line Elements (TLEs), using the RACER tool provided by ESA. It was found that the estimates varied with the solar cycle, suggesting that the estimated B was compensating for errors in the atmospheric model used.
To address this, a new method was introduced, which switches between different B values depending on solar activity levels.
This approach produced more accurate lifetime predictions than using a fixed theoretical value or even an averaged B from RACER. Better B estimations improve the reliability of decay forecasts, supporting more effective PMD planning.
Figure 4. Estimated B for the ORION 38 stages (5-months roll average) each stage in one color, solar activity in dotted blue line and theoretical B in black dotted lines.
Figure 5. Predicted (solid) and real (dotted) re-entry trajectories for the ORION 38 stages in the reference case.
Figure 6. Predicted (solid) and real (dotted) re-entry trajectories for the ORION 38 stages using a solar activity-dependent B based on RACER results.
PMD of Current Stages
To assess the PMD status of recent missions, data from DISCOS and Spacetrack covering rocket stages launched between 2016 and 2023 was reviewed. For stages still in orbit, their latest orbital elements were propagated using OSCAR. The cumulative collision probability (CCP) with respect to the 1 cm population was calculated for each object using spatial density data from MASTER. The analysis shows that many stages remain in orbit longer than 25 years, contributing significantly to long-term risk on orbit. These findings stress the importance of including reliable PMD mechanisms in mission planning.
Figure 7. Proportion of stages of each family within an orbital lifetime interval.
Figure 8. Proportion of stages of each family within a CCP(1cm) interval.
Summary
PMD is a crucial aspect of our commitment to keeping low Earth orbit usable for future generations. Improved modelling and better use of available tools can all raise the standard for responsible disposal of rocket stages. These efforts align closely with the goals of the Zero Debris Charter, a growing initiative to minimise the creation of space debris. As a committed member of this community, Terma continues to contribute through research, technology, and mission support.