Le 4 octobre 2022 sera un jour propice alors que l'humanité célèbre le 65e anniversaire du début de l'ère spatiale. Tout a commencé en 1957 avec le lancement du satellite soviétique Spoutnik-1, le premier satellite artificiel jamais envoyé en orbite. Depuis lors, environ 8 900 satellites ont été lancés depuis plus de 40 pays dans le monde. Cela a conduit à des inquiétudes croissantes concernant les débris spatiaux et le danger qu'ils représentent pour les futures constellations, les engins spatiaux et même les habitats en orbite terrestre basse (LEO).

Cela a conduit à de nombreuses propositions de solutions pour nettoyer les «déchets spatiaux», ainsi qu'à des conceptions de satellites qui leur permettraient de se désorbiter et de brûler. Hélas, on se demande encore si une planète entourée de méga-constellations est durable à long terme. Une étude récente de James A. Blake, chercheur à l'Université de Warwick, a examiné l'évolution de l'environnement des débris dans LEO et évalué si les futures opérations spatiales peuvent être menées de manière durable.

Pour son doctorat. projet, Blake s'est concentré sur l'imagerie et le suivi des débris spatiaux sur des orbites terrestres géosynchrones (GEO) à environ 36 000 km (22 370 mi) au-dessus de l'équateur. Dans cette région de l'espace, les satellites suivent la rotation de la Terre et ont la même période orbitale que la Terre, ce qui la rend très recherchée pour les télécommunications. Cependant, l'espace dans cette région est très limité, ce qui pourrait entraîner de graves problèmes de surpeuplement et de débris.

En particulier, le principal travail de Blake était une enquête sur les débris géosynchrones faibles réalisée à l'aide du télescope Isaac Newton à l'observatoire Roque de los Muchachos sur l'île de La Palma. Son travail a été résumé dans une étude intitulée "DebrisWatch I:A survey of faible geosynchronous débris", parue en janvier 2021 dans la revue Advances in Space Research. Comme il l'a indiqué dans cette étude, la population de débris dans GEO n'est pas bien limitée mais représente un problème croissant.

Un problème historique

Selon le Space Debris Office (SDO) de l'ESA, au 3 mars 2022, environ 12 720 satellites ont été lancés en orbite terrestre depuis Spoutnik-1. Parmi ceux-ci, environ 7 810 restent en orbite, dont environ 5 200 sont toujours opérationnels. Au total, environ 29 860 débris dans LEO sont suivis régulièrement par les réseaux d'observation terrestres et sont conservés dans leur catalogue.

Auparavant, on pensait que la population de débris dans GEO serait assez négligeable en raison des règles d'espacement strictes qui visent à garantir que les satellites n'entrent pas en collision. Cependant, la récente destruction apparente de satellites de communication - AMC-9, propriété de la société de télécommunications luxembourgeoise SES S.A., et Telkom-1 de Lockheed Martin - a fourni des preuves claires de l'existence d'un champ de débris dans GEO. Cela présente de nouvelles implications pour les futures constellations dans GEO.

As Blake told Universe Today via email, charting the evolution of space debris is essential to the future of debris mitigation:

"Sputnik 1 was the first of thousands of satellites to be launched into Earth orbit over the past six decades, and that number continues to grow rapidly. Some have re-entered the Earth's atmosphere, while others are orbiting in an abandoned and uncontrolled state, posing a threat to the active satellites we rely on.

"Over time, the orbital debris population has grown due to accidental explosions and collisions, alongside intentional anti-satellite tests. The vast majority of debris produced by these events remains invisible to us, too small to be detected by our current generation of surveillance networks, yet still holding the potential to severely damage spacecraft."

According to Blake, there's a lesson to be learned from humanity's exploitation of the near-Earth environment. In keeping with the interconnected nature of space exploration and life on Earth, this same lesson applies equally to humanity's activities on the ground. In short, humanity needs to act sustainably so that future generations can enjoy and benefit from the freedoms we've enjoyed since the dawn of the Space Age. To do this, says Blake, collision avoidance is a must:

"Effective collision avoidance requires timely and accurate information. As satellite and debris catalogs grow ever larger, surveillance networks are being tasked with monitoring more and more objects to provide sufficient warning to operators, who can then opt to maneuver their craft out of harm's way."

Monitoring and mitigating

The current strategy for preventing an uncontrollable debris environment in orbit involves a two-pronged approach:tracking and "passivating." The task of tracking satellites and debris is handled by several space agencies and government offices worldwide. For instance, the Joint Space Operation Center at Vanderburg Air Force Base in California (JSpOC) uses the Space Surveillance Networks (SSNs), a combination of optical and radar sensors, to monitor satellites and debris in orbit.

The NASA Orbital Debris Program Office (ODPO), located at the Johnson Space Center, measures the orbital debris environment while developing measures to control debris growth. The Office of Safety and Mission Assurance (OSMA), located at NASA HQ in Washington D.C., is responsible for developing, implementing, and overseeing agency-wide policies and procedures to ensure safety, reliability, and space environment sustainability.

There's also the ESA's Space Debris Office (SDO)—located at the European Space Operations Center (ESOC) in Darmstadt, Germany—which is responsible for measuring and modeling the orbital debris environment and developing protection and mitigation strategies. It also coordinates activities and research efforts with the ESA's constituent agencies, which form the European Network of Competences on Space Debris (SD NoC).

At the international level, there's the Inter-Agency Space Debris Coordination Committee (IADC), a forum that includes thirteen national space agencies (including NASA, Roscosmos, the ESA, and the Indian and Chinese space agencies). This body developed guidelines in 2001 that have been revised multiple times (the most recent occurring in 2020) and have since been adopted by the United Nations Committee on the Peaceful Uses of Outer Space (UNCOPUOS).

On the other end of things, there's the famous "25-year rule," where operators are encouraged to dispose of satellites within 25 years of mission completion via atmospheric re-entry. Low-altitude satellites may already be naturally capable of doing this. In contrast, potentially non-compliant satellites can be outfitted with thrusters, drag sails, and other instruments to accelerate the deorbiting process. As Blake explained:

"Operators are encouraged to 'passivate' spacecraft at the end of their mission, by depleting or saving any remnant sources of internal energy onboard the satellite or rocket body, thus reducing the chances of explosion. Adherence to the '25-year rule' for deorbiting spacecraft in low Earth orbit is still concerningly low, and a boost to cooperation on an international scale will be paramount to tackling the debris problem."

A problem of policy

In the end, Blake indicates that one of the greatest hurdles to achieving sustainability in space is policy. For the past few decades, the IADC guidelines adopted by UNCOUPOUS have formed the basis for standard mitigation practices on the international stage. Unfortunately, these guidelines are voluntary (i.e., not legally binding), and some space-faring nations have chosen not to include them in their national regulatory frameworks.

In addition, adherence to the "25-years rule" remains very low in LEO, and the process of re-entry is not a viable option for objects in the high-altitude GSO region. As a result, operators will typically attempt to raise decommissioned satellites into so-called "graveyard" orbits well beyond GSO—or what is known as a supersynchronous orbit (SSO). This has the effect of clearing the operational zone in orbit for use by future satellites, but the debris can still pose a threat to spacecraft destined for the Moon or deep-space.

What is needed, says Blake, is a policy of active debris removal (ADR) that works in tandem with stricter adherence to regulations for debris mitigation:

"Ultimately, we'll want to conduct regular removal missions to actively dispose of dead spacecraft and debris, though a number of technological hurdles are yet to be cleared. As evidenced by the recent Russian ASAT test back in November 2021, there is also a need for internationally recognized, legally binding regulations, to sanction against reckless behavior."

In addition, NASA, the ESA, the China National Space Agency (CNSA), and other space agencies are currently testing ADR systems. Concepts include Earth-based directed-energy arrays (lasers), spacecraft equipped with plasma beams, harpoons and nets, and magnetic space tugs. In recent years, says Blake, there have also been efforts to formulate a "space sustainability rating" that would incentivize operators to adhere to safe practices and debris mitigation. However, several questions remain unanswered.

For instance, with access to space becoming more widespread, how does a regulatory framework compare University-led CubeSat experiments to commercial constellations of satellites (a la Starlink)? Also, how will lawmakers attribute liability in the event of a collision involving uncontrolled debris? And what mechanisms will be in place to ensure a level playing field between emerging space agencies and those with a decades-long presence in space?

The debate around these questions and attempts to find solutions are actively unfolding around us right now. It has also led to the rise of non-profit organizations like the Space Court Foundation (SCF) the Space Generation Advisory Council (SGAC). There are also the time-honored efforts to formulate and crystallize policy by the Institute of Air &Space Law (IASL) at McGill University and the United Nations Office for Outer Space Affairs (UNOOSA).

As our presence in space continues to grow, we can expect some spirited debate, resolutions, and impressive innovations in the coming years. As always, the driving force behind these developments will be a basic matter of necessity. Humanity's future in space depends upon accessibility and safety, something that cannot happen with huge debris fields in orbit.