Identifying the 50 statistically-most-concerning derelict objects in LEO
Introduction
While development of space debris mitigation guidelines began in earnest in the 1990s and space traffic management (STM) activities are accelerating, substantive operational debris remediation efforts have been slower to materialize. The reticence of the space community to execute debris remediation activities is primarily due to four issues: 1. the cost of debris remediation alternatives is uncertain, but probably expensive, and their performance is unproven; 2. ownership and access rights to physically manipulate and remove objects registered to other states are uncertain and potentially contentious; 3. the objects most needed to be remediated have not been identified clearly in a multi-national forum to enhance space safety and assure long-term space sustainability; and 4. the attempt to remove or nudge a massive derelict (i.e., remediate) does pose the potential for unanticipated debris generation.
This paper addresses the third issue, identifying a prioritized list of objects for debris remediation that was created by a multi-national group. This will hopefully motivate organizations to develop a range of remediation options that are both effective and reasonably-priced while encouraging governments to fund projects to jump start debris remediation. The aerospace community is actively discussing space traffic management (STM) and debris mitigation, providing a foundation to minimize the creation of space debris and collision risk. Nonetheless, there is a need to remove from orbit some of the millions of kilograms of intact derelict mass because of their debris-generating potential. This hardware has been deposited in orbit either due to non-compliance with established debris mitigation guidelines or abandoned before mitigation guidelines were established.
As a result, debris remediation concepts must be be developed, tested, and deployed quickly to remove some of this hardware. One means to motivate this process is to develop an internationally-derived “hit list” of the statistically-most-concerning (SMC) objects in low Earth orbit (LEO) with the greatest potential to disrupt space activity. While active debris removal (ADR) is often proposed, debris remediation alternatives, such as “just-in-time” collision avoidance (JCA), long-term debris management (LTDM), and nano-tugs, can also be applied to the list compiled in this paper [1]. A JCA system would simply nudge one of two derelicts that are on a collision course to prevent that imminent collision. The impulse could be delivered by a physical cloud (such as an upper stage rocket plume or a puff of talc from a ballistic trajectory) or by a laser (possibly either ground-based or space-based). LTDM is a system that whereby a space-based laser system will provide multiple small impulses to derelicts to maintain their spacing to avoid collisions or even close approaches. A nano-tug is a small satellite that could be attached to a massive derelict to provide it with attitude control, collision avoidance, and position determination capabilities. There may be a safety benefit to “remediate in orbit” objects that are very massive and, as such, will likely pose a significant ground and aviation impact hazard upon reentry.
As early as 2009, NASA identified the objects that would need to be removed to curtail space debris growth [2]. There are nearly 1300 massive derelict objects in LEO that comprise over 2,000,000 kg of mass distributed in clusters in altitude and inclination, creating pockets of high debris-generating potential [3]. These objects average over 1800 kg in mass, with one cluster of 36 objects centered around 850 km averaging over 6000 kg each. Most of these objects were abandoned before 2000, but due to poor mitigation compliance since then, derelict mass continues to accumulate as depicted in Fig. 1.
The international experts who contributed their “top 50” lists each (1) explain their process for selecting their SMC objects in LEO and (2) provide their list as of the time of the publication of this paper. All non-operational objects in the on-line Space-track.org catalog are eligible for consideration, and all contributed lists have focused on LEO objects, due to their larger collision velocities and higher spatial density.
These 11 methods generate different results based on diverse hypotheses and approaches. Some of the approaches focused on short-term space safety (e.g., McKnight, Witner, Nicolls, etc.) while others scrutinized long-term sustainability (e.g., Lewis, Letizia, Rossi, etc.). However, Baranov/Grishko is the only team that focuses largely on efficiency of removal. Space safety metrics are likely linked to immediate collision risk while sustainability may focus more on the accumulation of the debris population over time. Nevertheless, they are globally coherent in the identification of the subset of the statistically-most-concerning derelict objects in LEO. Indeed, this paper applies the diversity prediction theorem that states the combination of a series of reputable but different decision schema provide a superior aggregate model than trying to determine the “best” of the available models [4].
This paper, thus, describes and integrates a variety of methods to evaluate the criticality of derelict resident space objects (RSO). The consideration of these results, as an absolute listing, is limited by the completeness of the database provided by Space-track.org. While there is currently no better database of on-orbit objects, it is known that there are some sensitive military and intelligence community satellites excluded from Space-track.org.
Section snippets
Approaches for identifying objects to Be removed from LEO
A short description for each approach used to determine the top 50 SMC objects is provided below; please refer to identified references for full descriptions of these approaches.
McKnight, US, Centauri: The Massive Collision Monitoring Activity (MCMA) has monitored and characterized the encounter dynamics of massive (i.e., greater than 700 kg) intact derelict objects in LEO over the last five years [6,7]. This activity currently scrutinizes nearly 1300 objects that comprise over 2,300,000 kg in
Individual and composite results
Fig. 4 summarizes common features and differences of the algorithms. It highlights that all methods focused primarily on mass (as a surrogate for consequence), then some measure of probability of collision and, finally, the approaches diverged by looking at proximity to operational satellites, persistence of the debris, and ease of collective retrieval. The top 50 SMC objects for the 11 different methods are tallied by CSpOC Satellite Number in Table 3.
A special thanks is extended to the ESA
Closing comments
This paper describes and integrates a variety of methods to evaluate the criticality of derelict resident space objects (RSO). These 11 methods each generate different results based on diverse hypotheses and approaches. For example, several of the 11 methodologies were not geared towards ADR per se; they represent a broader view on space sustainability metrics. Other approaches, conversely, were geared more towards space safety concerns.
A state-of-the-art model consolidation approach was
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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