The number of residues has more than doubled in the last fifteen years; while, in parallel, the number of active satellites has gone from 540 in 2003 to 900 in 2013 and to 7,750 in April 2023.
The origin of these orbital objects is important: to date, 97.5% of the debris in low Earth orbit comes from the trio of the United States, Russia and China. That means that neither Europe, nor Japan, India, Korea, etc., can do much about such proliferation.
These orbital objects basically pose three families of problems.
• They generate an important disturbance for astronomical observations. They can spoil some images, producing interference in them, or even saturating the sensors with excessive light. They are predictable effects by consulting specialized sites that specify the ephemeris of the largest objects and their visibility from the ground. You also have to take into account the inconveniences related to active satellites, airplanes, clouds, meteorites, etc. Now, it is clear that the satellite trains of a well-known telecommunications mega-constellation that plans to deploy some 40,000 satellites, precursors of at least another 100,000 satellites by 2030, pose a serious problem with no easy solution to implement. Several working groups are working on this issue, and the first regulatory recommendations have already been identified.
• Uncontrolled orbital objects end up falling back to Earth. Indeed, in any low orbit (up to 2,000 kilometers in altitude), a residual atmosphere, tenuous but very present, slows down the objects and makes them descend towards the upper layers of the atmosphere. There, those objects burn up due to the double effect of mechanical and thermal restraints, but between 10% and 40% of their mass survive reentry; in particular, refractory materials: titanium, steel, carbon... The uncontrolled re-entry of potentially large masses endangers the population on the ground, and the circumstance is aggravated by the fact that there is no way to reliably predict a reentry zone. Every three days, a large object, a satellite or an entire stage falls out of control; This sword of Damocles to which we are subjected today may become intolerable in a few years if the international standards that require control of re-entries into uninhabited areas are not strictly respected.
• Finally, the effects of collisions in orbit are directly related to the kinetic energy of the impact given the relative speed of the collision, sometimes higher than 14 km/s:
_Very small debris, one millimeter or less in diameter (about 150 million), can disturb the proper functioning of an active satellite.
_The centimeter debris (about a million) have an impact energy of the order of one megajoule and render any satellite useless.
_Debris of about 10 centimeters (about 36,000) represent the current lower limit of detection with our means of observation; their impacts, similar to the explosion of 200 kilos of TNT, violently shatter the impacted satellites and generate large amounts of new debris.
_Finally, there are collisions between complete objects, stages or satellites, such as the one observed between Cosmos 2251 and Iridium 33 in 2009; they typically generate 5,000 new large debris and a myriad of smaller ones.
Current models show that there is an 8% probability of losing, throughout its useful life, a satellite in classical sun-synchronous orbit at 800 kilometers of altitude; orbital collisions have become the main cause of failure of satellites in low Earth orbit.
The evolution of the orbital population depends on two opposite phenomena: on the one hand, new orbital objects are generated, either through launches, explosions, or collisions; on the other, their number decreases, either due to voluntary withdrawal at the end of the mission, or due to atmospheric cleaning. The orbital population increases globally when more objects are generated than are cleaned; In 2022, for example, there were 3,244 objects injected into orbit and 2,457 withdrawn, so that at the end of the year a positive balance of some 787 new objects was obtained.
It is worth considering a particular case: imagine that all space activity ceased, that there were no more launches. In that case, the generation of new objects would only be due to collisions between objects already in orbit; and cleaning would be limited to the effect of the atmosphere. When the effect of collisions is greater than the effect of atmospheric cleaning, a chain reaction is produced that leads to the multiplication of orbital debris despite a total absence of space activity. It is the Kessler syndrome, named after the NASA engineer who theorized the phenomenon in 1992. It is observed today between 700 and 1,100 kilometers of altitude: even in the total absence of space activity, the number of orbiting objects increases exponentially, at often with less than 1% active satellites...
The first and most obvious action is to regulate space activity to prevent the generation of new debris. The corresponding regulation takes many different forms: standards, guidelines or even laws such as the French one on space operations. These texts, globally coherent at the international level, present five large families of recommendations.
• Above all, it is forbidden to knowingly generate orbital debris; that prohibits, in particular, anti-satellite tests.
• It is necessary to avoid any accidental explosion in orbit, adequately passivating satellites and stages at the end of their operational life.
• Two zones are defined: low orbits, up to 2,000 kilometers in altitude, and the vicinity of the geostationary orbit. In those two zones, it is prohibited to stay more than 25 years after the end of nominal operations.
• Collision avoidance is necessary when satellites are equipped with propulsion and when collision risks are clearly identified.
• Finally, it is necessary to minimize the risk they pose to populations on land; either by modifying the composition of orbital objects to favor their destruction during atmospheric re-entry, or by making them re-enter over a completely uninhabited area, such as the southern Pacific Ocean.
Unfortunately, these measures are not enough to counteract Kessler syndrome and today they are seriously questioned internationally. In addition, they respect each other very little.
It is therefore possible to reinforce the most critical satellites, in order to protect them against the impacts of orbital debris. Multi-layered shields can be placed over the surfaces to be protected from debris. Unfortunately, these shields are not very effective and only work on debris less than a centimeter. In practice, expensive, heavy, and complex shields are reserved for habitable modules, such as those on space stations.
The third family of potential actions is recycling in all its forms. A detailed analysis shows that this is unfortunately relatively utopian given the limitations of orbital rendezvous, the complex robotic operations required for recycling, and the degradation of materials over time.
To avoid Kessler syndrome, we must avoid collisions between large orbital objects:
• When these collisions involve active satellites with propulsion, it is possible to maneuver and avoid the collision. To do this, it is necessary to assess the risks by propagating the orbit of the satellite in time (calculating and predicting its trajectory) to be protected and determining the probability of individual collision with all cataloged objects, whether active or debris, that present an approach risk dangerous. This is a very complex and time consuming task. This service is provided today by the European Union through a program called EUSST, which brings together the activities of fifteen countries.
• When it comes to non-maneuverable wreckage, the task is clearly complex... However, it has been shown that it is enough to deflect one of the two objects very little to avoid a collision: an extremely low variation of speed (inferior at 4 mm/s) is enough to generate a margin of one kilometer after 24 hours. Currently, various technical solutions are being studied for this tactical action called “just-in-time collision avoidance”. An artificial cloud can be injected in front of one of the debris to generate drag and therefore a braking effect. Various laser-based solutions are also being studied, from the ground or from orbit. Finally, it is possible to revive a large piece of debris by equipping it with several cubesats called nanotugs, commissioned on demand from the ground and equipped with weak propulsion. Unfortunately, these solutions, which are very promising, are not given the priority they deserve.
Finally, to avoid the generation of new small waste, it is necessary to remove a certain number of large objects, which are the source of the small ones. This strategic action is called “active debris removal”; It would be necessary to remove a dozen large pieces of debris a year to stabilize the evolution of the environment. Numerous solutions exist or are being developed and are in different demonstration phases, either on the ground, in aircraft in weightless conditions or even in orbit on scales that are not yet representative. The main difficulties do not seem to be technical, but rather political, legal, related to insurance and, above all, economic: there is no obvious solution to make the investment profitable in this type of operation.
Several recommendations can be made at this stage to try to protect future orbital operations.
We must rethink international regulations by imposing much more drastic measures: all objects at the end of their operational life must be deorbited quickly and with a very high probability of success; orbital objects have to be routinely equipped with a collision avoidance capability to cope with the tens of thousands of new satellites announced by the end of the decade; a commercial service for failed satellites should also be created in order to achieve the “net zero waste” situation advocated by many agencies.
Collision avoidance between active and maneuvering objects must be the subject of international coordination. This space traffic management activity, which is essential, should serve as a highway code for all future activities.
Specifically, it is necessary to initiate just-in-time collision avoidance and active debris removal activities with the understanding that we have no choice: without these operations, Kessler syndrome will not be counteracted and orbital congestion will inevitably increase.
For this, it is essential to improve our knowledge of the orbital situation. This activity, called "surveillance and tracking of objects in space", consists of observing orbital objects using radars, telescopes, lasers, satellites... to determine their orbits with precision. This traditionally institutional, and often military sphere, today experiences strong private activity; in particular, through various startups.
On a less technical side, let us underline the importance of dialogue on these issues with our Russian, Chinese, Korean, Indian, etc. colleagues, to identify shared actions for the future.
The current situation of the sustainability of space operations is not good.
The regulations are outdated and, in any case, are applied very insufficiently. Only 20% of the satellites in low Earth orbit correctly respect the rules. In addition, each month there is significant fragmentation (by collision or explosion).
Space activity also increases; especially with regard to nanosatellites. Cube-sats, and even other much smaller satellites, are launched by the hundreds into orbits that are often not in compliance with regulations and pose major tracking problems due to their size. Most of the time lacking propulsion and therefore no collision avoidance capabilities, they often have far below standard reliability and are sometimes put out of service even before launch!
Finally, mega constellations are networks of hundreds or even thousands (or more) of satellites in low orbit; often very cheap and therefore unreliable, they look more like smartphones than satellites! Its proliferation in the coming years, associated with the spirit of the tragedy of the commons (absence of shared rules), represents one of the greatest challenges of the coming years.
In summary, if there was a good international awareness, with forceful, shared and financed actions, it would be possible to guarantee the sustainability of space operations in the medium and long term...
Christophe Bonnal is a senior expert in the Strategy Directorate of the National Center for Space Studies (CNES), Paris.
