Re-framing Orbital Debris: From a Statistical to a Dosage Approach

United States - Ekhbary News Agency

Re-framing Orbital Debris: From a Statistical to a Dosage Approach

Humanity gains invaluable insights into operating in the space environment with every satellite launched. True learning, however, stems from practical experience; otherwise, we risk becoming entrenched in lab-born biases derived from asking the wrong questions. This is a limitation the space sector appears to be encountering with micrometeoroid and orbital debris (MMOD) collisions. A significant, industry-wide shift in how MMOD risks are considered and mitigated for future missions is on the horizon.

The natural space environment, had it been uniformly and prohibitively dangerous, would have revealed its perils long ago. Early satellites like Sputnik would have been obliterated by natural meteoroids, comet tails, and asteroid fragments. Instead, we discovered space to be relatively stable, leading to the development of engineering norms focused on mitigating its extreme characteristics: temperature, vacuum, and electrical charge. What has fundamentally changed since the 1960s is the exponential increase in traffic and the slow but persistent "pollution" of space with millions of pieces of orbital debris, creating the modern MMOD environment.

Currently, most smallsats (small satellites) do not incorporate dedicated debris protection systems. The established Orbital Debris Assessment Report (ODAR) analysis, coupled with the observed success of commercial operators and a natural inclination to replicate past successes, has yielded modest results. Satellites are minuscule in comparison to the vastness of orbital volume, and onboard propulsion systems allow for avoidance maneuvers when potential conjunctions are identified. The high-impact events and spacecraft-on-spacecraft collisions are perceived as manageable because they are trackable and visible. This visibility is crucial, not just for belief, but for knowledge, fostering reliable instincts about the frequency of concern and the efficacy of various mitigation strategies.

However, micro-MMOD presents a distinct challenge, potentially holding the key to the industry's greatest mysteries. Micro-MMOD refers to the untracked population of particles smaller than 3 millimeters. This category constitutes the vast majority of objects in Low Earth Orbit (LEO) by count. The critical question arises: if we cannot track these particles, how can we accurately assess their presence and impact?

NASA's Long Duration Exposure Facility (LDEF), which orbited Earth from 1984 to 1990, was designed precisely to address such questions. Its mission was to expose standard materials and subsystems to the LEO environment for extended periods, then return them to Earth for detailed analysis of the resulting "scars." The findings were startling: a complex, directional, high-flux environment. NASA's summary revealed over 30,000 observable MMOD strikes on LDEF's exterior. Impacts were approximately 20 times more frequent on the forward-facing side compared to the aft, and 200 times more common on the forward side than the Earth-facing sides. The data indicated roughly 5,217 strikes per year at an altitude of 450 km over its 5.75-year mission. Considering LDEF's exterior area of approximately 151.975 square meters, this equates to about 34 hits per square meter annually at the debris levels of the 1990s. With the current debris population roughly three times larger than in 1990, this extrapolates crudely to an alarming neighborhood of 100 hits per square meter per year in busy LEO.

This raises a critical question: could modern ESPA-class satellites be experiencing one or two impacts daily? When this possibility is raised, constellation operators often express disbelief, yet simultaneously maintain that MMOD is not a significant mission driver and that their requirements do not extend beyond ODAR. This presents a significant paradox that needs reconciliation.

Part of the explanation lies in the fact that not all impacts are equal. While hypervelocity impact videos often depict dramatic scenarios turning aluminum into confetti, these represent extreme cases and not the average experience. LDEF's majority of impact holes were tiny, sub-millimeter punctures. MMOD strikes manifest in various ways; some are catastrophic, akin to the chain reactions depicted in the film "Gravity" (2014), while many others are micro-perforations through structures, fabrics, or tanks. These micro-perforations can generate secondary debris without necessarily causing immediate satellite failure. Furthermore, the infrequent in-situ observation of these events leads to misattribution of failures. The question lingers: how many satellites have quietly failed due to MMOD, and how many MMOD signatures are mistaken for proton hits or other radiation events? Such confusion is expected.

This underscores the author's repeated assertion that video documentation and sample return missions are prerequisites for an honest MMOD risk assessment. The perspective should shift towards that of an epidemiologist, analyzing particle populations and cumulative exposure rather than focusing solely on discrete conjunction events. The concept of "dosage" becomes paramount. This reframing positions MMOD risk analogously to alpha and beta radiation risks – both involve particle populations that are best avoided by sensitive systems. If dosage is the correct lens, the optimal protection strategy becomes clearer: enhance shielding around critical, irreplaceable systems and centralize these systems to maximize protection efficiency.

One of the enduring mysteries in the space industry is the precise cause of spacecraft failures. The inherent obscurity of the space environment means direct inspection is impossible; we rely on telemetry data, often received minutes late from hundreds of kilometers away. The author posits that on-orbit failures are consistently misattributed. A portion of failures currently categorized as radiation, software, or workmanship issues may actually be caused by micro-MMOD and its secondary effects. Both manufacturing defects and MMOD impacts can subtly perturb hardware on a millimeter scale, leading to failures that appear virtually identical when analyzed from the ground. This misattribution is particularly critical during the deployment phase, where even a millimeter-scale perturbation, whether from micro-MMOD or marginal workmanship, can cascade into a catastrophic failure that is indistinguishable in telemetry data.

Accurate failure attribution is vital for component manufacturers like Atomic-6, whose business relies on customer confidence that their products will not be the cause of mission failure. Recognizing that each component introduces a potential point of failure due to workmanship or uncertainty, Atomic-6 has focused on minimizing the parts count in its flagship Light Wing solar array product. This reduction aims to decrease the probability of such cascading failures.

The concept of secondary effects is particularly counterintuitive. Metallic spacecraft inherently generate debris upon impact, not just when they disintegrate. Even if an MMOD particle does not penetrate a metallic skin, the resulting spall and delamination can create fragments larger and more damaging than the original projectile. Some studies suggest that this secondary fragmentation accounts for orders of magnitude more impact marks on a spacecraft than the primary strike. To avoid generating such fragmentation, operators might need to consider alternatives to metallic-based MMOD solutions, opting for composite-based materials that absorb impact energy and self-preserve without ejecting hard micro-MMOD fragments.

A common question arises: if satellites are subjected to hundreds of impacts annually, why aren't they failing weekly? The answer likely lies in the fact that while MMOD may be less damaging on average than often assumed, its impact can be far more severe in specific, high-energy scenarios. This nuanced understanding necessitates a deepening of our mental models, rather than complacency. Atomic-6 is actively exploring these counterintuitive challenges to standard MMOD assumptions and developing products to address them.

Looking ahead, the risk of debris-generating events appears to be escalating. Several Defense Meteorological Satellite Program (DMSP) spacecraft have "broken up" years after decommissioning, with 16 still in orbit, their fragmentation timing unpredictable. Evolving conflict dynamics in space suggest an increase in satellite numbers, more aggressive maneuvers, and consequently, more opportunities for errors. The proliferation of interceptor concepts powered by solid rocket motors, alongside rendezvous and proximity operations (RPO), introduces both benefits and new debris-producing failure modes. While well-executed RPO can reduce debris by extending operational lifetimes, poorly managed approaches can lead to catastrophic satellite loss. The industry is at a critical juncture where a significant leap in capability could resolve the debris problem, while a marginal increase might exacerbate it.

Ekhbary News Agency