Littering in space? Fine! Fine!

In October 2023, the Federal Communications Commission of the United States issued a fine for the first time for a space debris case. A US company was required to pay $150,000 simply because it did not move a satellite that had reached the end of its life into a graveyard orbit. With the vigorous development of human space activities, the safety issue of space debris has become increasingly prominent. International supervision has become increasingly strict, but it has also created new space market demand. To this end, space forces of various countries are exploring various tricks for space debris cleaning and mitigation.

Rendering of densely distributed spacecraft and space debris around the Earth

The threat of space debris cannot be ignored

The space debris mentioned in this article refers specifically to the so-called "space junk", which is mainly useless man-made objects and their debris left over from human space activities, including abandoned rocket stages, satellites and their various components. These space debris may come from many man-made and non-man-made space activities such as the end of life of spacecraft, accidents, and kinetic interception weapon tests.

Since the successful launch of the first artificial satellite in October 1957, according to incomplete statistics, nearly 10,000 tons of artificial space objects have accumulated in the Earth's orbit, of which only about 5% are functioning spacecraft, and the rest are generally various types of space debris. Monitoring shows that the number of space debris with a size of more than 1 cm has reached millions, and smaller space debris may be in the hundreds of millions.

In recent years, the number of space debris has increased rapidly. In addition to the more frequent human space activities, it is also due to the intensification of collisions between space debris and spacecraft. With the increase of orbital objects and debris generated by space launches each year, the probability of debris collision increases, which in turn generates more debris, triggering a vicious cycle.

Shocking renderings of the threat of space debris

In the 1970s, NASA scientist Donald Kessler even proposed the concept of the "Kessler effect", arguing that when the amount of space debris reaches a certain critical point, it will trigger a chain of on-orbit collisions, making it impossible for spacecraft to operate safely in outer space.

Generally speaking, space objects with orbital altitudes below 300 kilometers will re-enter the atmosphere and burn up in a relatively short period of time, but space debris with orbital altitudes above 600 kilometers may theoretically exist for decades or even hundreds of years. They are not evenly distributed, and are mainly concentrated in areas where satellites are densely deployed, namely low orbit areas below 2,000 kilometers, geosynchronous orbits, and medium earth orbit areas.

In 1993, the United States, Russia, Japan and other space agencies jointly initiated the establishment of the Inter-Agency Space Debris Coordination Committee, which aims to coordinate the actions of space forces of various countries and jointly solve the problem of space debris. At the end of 2012, the committee stated in its report "Future Low Orbit Environment Stability" that in some areas of low orbit, there are too many space debris and they are no longer stable. The number of debris generated by collisions will exceed the number of debris that disappears naturally.

With the accelerated deployment of large-scale low-Earth orbit commercial constellations, tens of thousands of satellites are expected to be put into orbit in the near future. Their widespread deployment in low-Earth orbit will inevitably significantly increase the difficulty of management and increase the risk of spacecraft collision.

A study by Swiss Re shows that the probability of a satellite with a cross-sectional area of ​​10 square meters colliding with space debris with a diameter greater than 1 centimeter is more than 1 in 10,000. The US Joint Space Operations Center issues dozens of on-orbit collision warnings on average every day, and spacecraft need to perform more than 100 collision avoidance operations each year.

Space debris can travel at speeds exceeding 7 km/s, with kinetic energy sufficient to pose a huge threat to spacecraft in orbit. According to the mainstream protection level of current spacecraft, a collision with space debris larger than 1 cm in diameter could result in the destruction of a spacecraft.

At the end of 1991, two failed Russian satellites collided, causing one to break into two and the other to become a large number of untrackable debris. On July 24, 1996, fragments of the European Space Agency's Ariane rocket hit the gravity gradient stabilizer of the French Cherry electronic reconnaissance satellite in orbit at a relative speed of 14.8 km/s, causing the latter to lose control of its attitude. On February 10, 2009, the Russian "Cosmos-2251" satellite collided with the US Iridium 33 satellite at a relative speed of 11.64 km/s, generating more than 2,200 space debris that can be monitored and catalogued, posing a huge threat to the other 66 Iridium satellites in the area.

It is reported that in order to avoid being "attacked" by space debris, Europe's SPOT series of optical imaging satellites have to change orbits at least four times a year. Large spacecraft such as the International Space Station need to perform multiple orbit change maneuvers every year to avoid larger space debris.

The international community is paying more and more attention to space debris cleanup and mitigation. In June 2021, in the revised version of the Space Debris Mitigation Guidelines, the Inter-Agency Space Debris Coordination Committee proposed four space debris mitigation measures:

First, limit the generation of debris during the spacecraft's in-orbit period. If this is not feasible, the number, volume and time of debris generated should be minimized. In principle, countries should not carry out any space activity plan that may generate space debris unless it is evaluated and proved that its long-term impact on the orbital environment and other spacecraft is at an acceptably low level.

Second, minimize the possibility of spacecraft disintegration in orbit, including during normal operation of the spacecraft and after the mission, and avoid deliberate sabotage and other harmful activities.

Third, the spacecraft at the end of its life span should be deorbited. For example, the spacecraft in geostationary orbit should be moved to the graveyard orbit; the spacecraft in low-Earth orbit should be made to fall into the atmosphere.

Fourth, try to avoid spacecraft collisions in orbit.

Fifth, the United Nations Committee on the Peaceful Uses of Space has formulated the Space Debris Mitigation Guidelines based on the Space Debris Mitigation Guidelines, which put forward seven guidelines that are generally consistent with the above recommendations.

Rocket Space Debris Mitigation Measures

Generally speaking, only the last stage of a launch vehicle will remain in Earth orbit and become space debris. In order to reduce the generation of space debris in this regard, aerospace units need to take a series of measures: first, in the process of separating the payload from the last stage of the rocket, the release of space debris should be reduced as much as possible; then, after the payload is separated, the last stage of the rocket should be passivated to eliminate the risk of its in-orbit disintegration and generation of debris; finally, the last stage of the rocket should leave the operating orbit as much as possible to control its orbital retention time.

After the rocket's final stage has completed its mission of releasing its payload, it is necessary to perform a passivation treatment.

The so-called "passivation" refers to the release of remaining propellant and high-pressure gas after the rocket's final stage completes its mission. In order to ensure the success of the mission, hundreds of kilograms of remaining propellant usually remain in the rocket's final stage after the space launch mission is completed. In order to reduce the generation of space debris, it is necessary to set up the function of timely discharging the remaining propellant in the tank and the remaining gas in the high-pressure gas cylinder after the separation of the rocket and the satellite, thereby eliminating the potential danger of disintegration in orbit.

After the rocket's final stage completes its mission, the passivation process mainly involves releasing the remaining propellant through a dedicated discharge pipeline. In its internal pressurization and delivery system, it is necessary to add a set of discharge electric explosion valves and discharge pipes for the fuel and oxidizer, respectively, through which the remaining propellant in the tank is discharged from the rocket body, while ensuring that the discharge plan does not have a negative impact on the rocket's launch payload into orbit.

In addition, when the rocket's last stage is passivated, the plume field generated by the propellant discharge must be analyzed in order to optimize the discharge plan and minimize the interference with the rocket's attitude, while avoiding interference and "pollution" to satellites and other payloads caused by the passivation process.

In order to ensure that the remaining propellant in the rocket tank is discharged as quickly as possible, it is necessary to take propellant management measures after the separation of the satellite (ship) and the rocket, so that the propellant sinks to the bottom during the discharge process. Specifically, propellant management generally uses an attitude control sinking engine to form an inertial force field to make the propellant sink to the bottom reliably. The force generated during the discharge of the remaining propellant can also significantly improve the deorbit effect of the rocket's final stage.

The so-called "deorbiting" refers to the method of artificially evacuating the orbit of the payload by the last stage of the rocket after completing the established flight mission. For this purpose, it performs maneuvering flight. It is a method of controlling the time that the last stage of the rocket stays in orbit. There are mainly two types of deorbiting: active deorbiting and passive deorbiting.

Active deorbiting means that after the mission is completed, the rocket's last stage uses a power unit to perform orbital maneuvers, gradually slow down and reduce the orbital perigee altitude, leave the payload orbit, or directly re-enter the atmosphere. The rocket's last stage power unit here includes liquid propellant engines, attitude control engines or solid rockets.

Passive deorbiting is to lower the orbit of the rocket's final stage with the help of external forces after the mission is completed. The main technical means currently include drag enhancement devices, solar sails and orbital cables. The rocket can be equipped with an engine with multiple ignition capabilities and corresponding supporting systems. After the separation of the rocket (ship), the engine will ignite again to implement the active deorbiting of the rocket's final stage.

According to the latest international trends, the basic requirements for the deorbiting of the rocket's final stage include: the deorbiting measures of the rocket's final stage should not affect the reliability and safety of the rocket's established flight mission, or the risks brought are deemed acceptable after evaluation; the deorbiting effect of the rocket's final stage should be as consistent as possible with the "Space Debris Mitigation Guidelines" compiled by the United Nations Space Debris Working Group, which stipulates that "a spacecraft shall not remain in orbit for more than 25 years after the completion of its operating mission." New aerospace regulations of some countries and organizations have proposed shorter on-orbit residence time indicators; and during the deorbiting process of the rocket's final stage, it should be ensured that no new space debris is generated.

Satellite Space Debris Mitigation Measures

After satellites and other payloads enter the Earth's orbit, space debris is generated mainly in four ways:

The first is the space debris generated during the orbit entry process. Some satellites use solid propellant apogee engines, perform orbit change maneuvers at apogee, and eventually enter quasi-synchronous orbits. After that, the engines separate from the satellites, becoming an important source of space debris.

The second is operational space debris generated during operation. Satellites will discard waste during operation, such as wire attachments, the clamping mechanism of the deployed antenna, the heat shield of the apogee booster engine, the nozzle cover of the solid propellant thruster, the protective cover of the payload, explosive bolts, springs, and straps.

The third is space debris generated due to the end of its working life, that is, satellites that cannot be deorbited and re-enter the Earth's atmosphere in time or enter a graveyard orbit after their service life expires.

Fourth, space debris generated by the space environment. In space, various environmental attenuation factors are complex and will inevitably have a negative impact on spacecraft. For example, if the paint coating on its surface falls off, it may become space debris.

It is generally believed that over time, space debris in low Earth orbit will eventually re-enter the atmosphere and burn up due to solar activity and atmospheric drag, but this may not be the case for space debris in high orbit. Therefore, if we want to slow down the generation of space debris for satellites and other payloads, we should start from the source, and the main measures include passivation, active deorbiting and active removal.

Passivation refers to taking timely measures to deplete propellant and battery power to prevent retired satellites and other payloads from exploding in the future. According to incomplete statistics, among the approximately 550 known disintegration accidents of satellites and other spacecraft in orbit, the largest number of space debris was generated due to the lack of passivation treatment.

Active deorbit refers to satellites and other spacecraft leaving their working orbits using thrusters, balloons, light sails and other means. Subsequently, satellites in low-Earth orbits are lowered into the atmosphere and burned up, while satellites in geosynchronous orbits are raised into graveyard orbits, thereby protecting spacecraft in orbit from being hit by space debris, or at least significantly reducing the risk of impact.

Traditionally, passivation and active deorbiting measures require satellites and other spacecraft to add corresponding hardware measures or carry more propellants to ensure that relevant operations can be performed at the end of their life. However, due to various factors, satellites that take these measures and meet international standards do not account for the absolute majority.

In fact, currently satellite design must take into account space debris mitigation requirements to ensure that the satellite has the disposal capability after completing its mission and reduce the release of operational space debris. The specific requirements are mainly divided into three aspects.

During the feasibility study phase of the satellite program, potential space debris risks should be confirmed in the early stages of geosynchronous orbit satellite development, and an operational space debris mitigation design should be adopted to meet the requirements of residual propellant discharge, battery passivation, gas discharge in high-pressure gas cylinders, and space debris disposal at the end of the mission and after the mission is completed.

During the design phase, the satellite should have passivation design requirements for the energy storage system or single machine, involving the battery pack of the power system, the flywheel system of the control system and the propulsion system; it should have requirements for mitigating the generation of operational space debris, such as changing the detachable radiation-cooled cover to a deployable one; it should have the means to measure the remaining amount of propellant and pressurized gas; it should consider the fuel reserve required for deorbit control, such as raising the satellite's orbital perigee by 200 kilometers, which requires a velocity increment of 11 meters per second.

In terms of satellite design materials, researchers must also optimize their choices and select materials and processes that are less likely to produce space debris. For example, composite materials are used to manufacture high-pressure gas cylinders and tanks, and through radiation, impact, temperature alternation and other tests, materials and processes that can prevent the large-scale generation of space debris are selected.

Active removal measures vary

Studies have shown that nearly one-third of the catalogued space debris in Earth orbit is produced by the 10 largest space disintegration events. Therefore, on the basis of actively preventing the generation of space debris, it is necessary to manage the space debris in orbit. If the larger space debris can be removed, it will significantly improve the space environment and effectively curb the occurrence of "chain collisions".

According to incomplete statistics, more than 10% of retired satellites in geostationary orbit are either still in place or their orbits are not raised high enough; in low-Earth orbit, a large number of satellites do not have the ability to change orbits after retirement and cannot actively leave orbit. Foreign aerospace companies and research institutions have proposed a variety of active removal methods and strategies for space debris of different sizes, hoping not only to improve the safety of spacecraft operations, but also to expand new commercial aerospace markets.

The US orbital service provider Vivisat has proposed the concept of Mission Extension Vehicle (MEV), hoping to launch space robots to relocate retired "stranded" satellites to different orbits. The US Tether Unlimited has launched a program called "Capture and Despin Small Satellites and Space Debris" (WRANGLER), hoping to use a more complex combination of on-orbit vehicles to capture a large number of retired small spacecraft and re-enter the atmosphere as soon as possible.

The MEV space robot developed abroad is designed to tow retired satellites to change their orbits.

Concept image of the WRANGLER scheme by Tether Unlimited

More American commercial aerospace companies have proposed a variety of space debris removal plans. The main technical equipment includes orbital tug plans, high maneuverability space vehicles, sun-synchronous orbit space-based ultraviolet laser transmitters, etc., hoping to send batches of abandoned satellites to graveyard orbits or melt them down.

As early as 2012, ESA set the goal of removing the out-of-control European Environmental Satellite and widely recruited EU aerospace companies to carry out mission demonstration and technology development. The preliminary active removal technical means include robotic arms, "tentacles", flying nets, ion beams, etc.

Concept of using a flying net to recover space debris

In 2016, Airbus Defence and Space, together with 10 European partners, implemented the "Spacecraft Self-Clearing Technology" project to conduct initial research on cost-effective and highly reliable removal module prototypes, hoping to ensure that the spacecraft automatically leaves orbit when it fails, loses control or ends its life.

In June 2018, the space debris removal mission test satellite developed by Airbus subsidiary SAR Satellite Technology was deployed from the International Space Station. From September of the same year to March 2019, the satellite successfully completed the in-orbit verification of technologies such as the use of flying nets and "harpoons" to capture cubic satellites, space target motion tracking, and off-orbit sail towing cubic satellites.

After the satellite is retired, it can be actively deorbited using the deorbit sail

On December 1, 2020, ESA signed a contract worth 86 million euros (about 680 million yuan) with a Swiss industrial team to purchase the unique "Clean Space-1" mission service: launching a space robot to rendezvous with the upper half of the secondary payload adapter left in space by the Vega rocket in 2013, capture it, and then drag it into the Earth's atmosphere to burn up. At that time, the space robot will use artificial intelligence to autonomously evaluate the target and match the motion state. The specific capture operation will be carried out by a robotic arm under the supervision of ESA. At present, the mission is carrying out propulsion subsystem manufacturing, satellite assembly integration and testing, and is expected to be launched in 2025.

Concept image of a space robot arm capturing space debris

Japan's aerospace forces are also active in this field. The Japan Aerospace Exploration Agency and Nitto Networks are working together to develop the so-called "space electromagnetic net", which will be carried on Kagawa University's ultra-small satellite to carry out technical tests for cleaning up space debris. In addition, the RIKEN Institute of Physical and Chemical Research in Tokyo, Japan, has proposed a plan to install a fiber laser on the International Space Station and use the space observatory's ultra-wide-angle telescope in the Japanese experimental module to clean up space debris with a diameter of 1 cm.