OrbitalHub

Credits: NASA

The International Organization for Standardization (ISO) has implemented the UN Space Debris Mitigation guidelines in a number of standards.

The standards prescribe requirements that are derived from already existing international guidelines, but they capture industry best practices and contain specific actions to be taken by hardware manufactures to achieve compliance.

The highest level debris mitigation requirements are contained in a Space Debris Mitigation standard. This standard defines the main space debris mitigation requirements applicable over the life cycle of a space system and provides links to lower-level implementation standards. It is also important to be able to assess, reduce, and control the potential risks that space vehicles that re-enter Earth’s atmosphere pose to people and the environment. The Re-entry Risk Management standard provides a framework that is useful in this regard.

The seven guidelines endorsed by the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS), also known as the Space Debris Mitigation Guidelines of COPUOS, are:

“limit debris released during normal operations;

minimize the potential for break-ups during operational phases;

limit the probability of accidental collision in orbit;

avoid intentional destruction and other harmful activities;

minimize potential for post-mission break-ups resulting from stored energy;

limit the long-term presence of spacecraft and launch vehicle orbital stages in LEO after the end of their mission;

limit the long-term interference of spacecraft and launch vehicle orbital stages with GEO region after the end of their mission;”

The good news is that as of the end of 2010, most of the space faring nations have implemented regulations on space debris mitigation at the national level.

Credits: CSA

Canada is actively involved in space debris mitigation research and development activities. At the international level, Canada hosted the International Conference on Protection of Materials and Structures from the Space Environment (ICPMSE) in May 2008, and contributed to the 37th Committee on Space Research (COSPAR) Scientific Assembly in July 2008.

At the national level, the space debris research and development activities are coordinated by the Canadian Space Agency (CSA), which formed the Orbital Debris Working Group (ODWG). The group was formed in order to address a number of objectives:

“to increase the Scientific and Technical (S&T) knowledge and awareness of orbital debris in the space community;

to identify and encourage targeted Research and Development (R&D) in orbital debris and mitigation measures;

to identify and encourage development of orbital debris detection and collision avoidance techniques and technologies;

to promote Scientific and Technical (S&T) collaboration across Canada and with our international partners;

to identify Scientific and Technical (S&T) opportunities in relation to future potential missions which can directly benefit from the results of targeted Research and Development (R&D) and novel operational techniques, and develop and coordinate technical solution in Canada and with international partners; and

to establish and maintain technical liaison with our international partners in order to foster a sustainable space environment.”

The Canadian space debris mitigation research and development activities are focused on three main areas: hypervelocity impact facilities, debris mitigation and self healing materials, and spacecraft demise technologies. Hypervelocity impact facilities are facilities that are capable of accelerating projectiles to velocities of more than 10 km/s. Canada is developing an implosion-driven hypervelocity launcher facility. Such a facility could accelerate projectiles having a mass of 10 g to speeds of 10 km/s, facilitating meaningful impact studies. Self healing materials have the capability to initiate a self healing process after an impact, being an in-situ mitigation of space debris damage on board spacecraft. The Canadian Space Agency has supported the efforts to develop and test a self healing concept demonstrator. The spacecraft demise technologies ensure intentional and integral disintegration during re-entry, so that no debris reaches Earth. In this direction, studies that investigate various technologies that could be used to de-orbit micro- and nanosatellites have been conducted.

In Canada, the space operators and manufacturers are adopting the space debris mitigation measures on a voluntary basis. The Inter-Agency Space Debris (IADC) guidelines are used for monitoring activities to prevent on-orbit collisions and conduct post-mission disposal. There are also strict requirements integrated in its policies and regulations that address the post-mission disposal of satellites. For example, as required by the Canadian Remote Sensing Space System Act, space system manufacturers have to provide information regarding the method of disposal for the satellite, the estimated duration of the satellite disposal operation, the probability of loss of human life, the amount of debris expected to reach the surface of the Earth upon re-entry, an estimate of the orbital debris expected to be released by the satellite during normal operations by explosion, etc. There are also interesting recommendations made for the operation and post-mission disposal of satellites in Geostationary Orbits. The Environmental Protection of the Geostationary Satellite Orbit recommends “that as little debris as possible should be released into the geostationary orbit during the placement of a satellite in orbit”, and also that “a geostationary satellite at the end of its life should be transferred, before complete exhaustion of its propellant, to a super synchronous graveyard orbit”, where the recommended minimum re-orbiting altitude is given as 300 km.

Credits: NASA

Let us see how the areas mentioned in the previous Sustainability in LEO post are covered at national level in the United States.

The United States has implemented a space traffic management program in the form of the Joint Space Operations Center (JSpOC) of the U.S. Strategic Command at Vandenberg Air Force Base in California.

JSpOC conducts periodic conjunction assessments for all NASA programs and projects that operate maneuverable spacecraft in low Earth orbits (LEO) or in geosynchronous orbits (GEO). Depending on the mission, the conjunction assessments can be performed up to three times daily. If JSpOC identifies an object that is expected to come in the proximity of a NASA spacecraft, and the collision risk is high enough (for manned missions the minimal value accepted is 1 in 10,000, while for robotic missions the threshold is 1 in 1,000), a conjunction assessment alert message is sent to the mission control in order to have collision avoidance maneuver commands sent to the spacecraft. The alert messages contain the predicted time and distance at closest approach, as well as the uncertainty associated with the prediction.

The control of the creation of space debris is addressed by orbital debris mitigation standard practices in four major areas: normal operations, accidental explosions, safe flight profile and operational configuration, and post-mission disposal of space structures. There are also NASA standards and processes that aim at limiting the generation of orbital debris.

The commonly-adopted mitigation methods, which focus on minimization of space debris creation, will not preserve the near-Earth environment for the future generations. As a matter of fact, the debris population increase will be worse than predicted by LEGEND-generated models due to ongoing launch activities and unexpected (but possible) major breakups. Here is where active space debris environment remediation comes into play.

The active space debris environment remediation is mainly concerned with the removal of large objects from orbit. Such large objects are defunct spacecraft (i.e. communication satellites that exceeded their operational life), upper stages of launch vehicles, and other mission-related objects. The removal of large objects from orbit is known as Active Debris Removal (ADR). Several innovative concepts are under study. Among them, tethers used for momentum exchange or electro-dynamic drag force, aerodynamic drag, solar sails, and auxiliary propulsion units. LEGEND studies have revealed that ADR is a viable control method as long as an effective removal selection criterion based on mass and collision probability is used, and there are at least five objects removed from orbit every year. The electrodynamic tethers seem to lead the competition so far, as they have a low mass requirement and can remove spent or dysfunctional spacecraft from low Earth orbit rapidly and safely.

Re-entry in the Earth’s atmosphere of space mission related objects is an important aspect to be considered in this context. Even though no casualties or injuries have been reported so far being caused by components of re-entering spacecraft, fragments from space hardware pose a risk to human life and property on the ground. One big concern is caused by the fact that the point of impact from uncontrolled re-entries cannot be calculated exactly. The uncertainties are due to a large number of parameters that affect the trajectory and the heat of ablation of objects re-entering the atmosphere.

Credits: NASA

Space debris mitigation measures address issues in two major areas: protection from space debris and reduction of the space debris population growth.

Protection methods vary depending on the size of the space debris. Space debris fragments smaller than 0.1 mm in diameter do not have enough energy to penetrate protection panels. Even spacecraft thermal blankets and structural panels can offer protection from such small kinetic impactors.

However, larger debris fragments pose a more serious threat and there are few active measures that can be adopted to minimize the possibility or the consequences of impacts.

First, avoiding space debris is the method that always proves successful, so flying at altitudes and inclinations where space debris density is low should always be considered. Second, orienting sensitive surfaces away from or mounting a bumper on the leading edge can offer more protection for the spacecraft. Multilayered bumpers cause the fragmentation of the space debris and also prevent ejected material from dispersing from the point of impact. Bumpers add mass and volume to the spacecraft and this is why few space missions can exercise this option.

A promising new method for protection from orbital debris impact is shielding with metallic foams. Extensive research and experimental impact campaigns have proven that metallic open-cell foams provide improved protection against hypervelocity impacts with almost no ejecta generated at impact, while offering comparable mechanical and thermal performance to honeycomb structures, which are currently used for shielding. The fact that no ejecta are generated is of great importance, as fragments generated at impact can add to the already existing debris population in Earth orbit.

At this point, it appears that the best protection method is avoiding the creation of space debris.

The long-term projections of the space debris environment generated by models like LEGEND or DELTA have proven that only active methods can maintain a stable environment of artificial objects in Earth orbit. It is quite obvious that unchanged operational practices or even an immediate stop to launch and release activities will not prevent the collisions between already existing space hardware. Scenarios like the one proposed by NASA scientist Donald J. Kessler in 1978 are very likely to occur. In the scenario proposed by Kessler, which is also known as the Kessler Syndrome, space exploration and the use of satellites in proximity of Earth will become unfeasible due to an exponential growth of the debris population caused by collisional cascading. The exponential growth is due to the fact that the material ejected during a hypervelocity impact becomes space debris itself. Laboratory experiments have shown that as a result of such an impact, 1 kg of aluminum can form several hundred thousand 1 mm sized particles.

Failure to address the potential uncontrollable growth of the space debris population will lead to major restrictions on the ability to exploit space. There is a debate in the scientific community over whether or not critical density has already been reached in certain orbital regions and if we are beyond the point where we can address the growth of the debris population.

There are various classifications of the space debris mitigation methods. For example, two broad categories might include, on one hand, measures that restrict the generation of space debris in the near future, such as limiting the production of mission-related objects and the avoidance of breakups, and, on the other hand, measures that restrict their generation for the long term, which include post-mission disposal methods and active measures to remove space debris from protected regions.

There are three major areas that could allow space faring nations to maintain a stable debris environment: space traffic management, control the creation of space debris, and active space debris environment remediation.

Credits: NASA

As mentioned in a previous post, only a small fraction of the existing space debris population is detectable and tracked by ground systems. A smaller fraction is catalogued by special programs and/or departments of national space agencies. This is where statistics comes into play. Numerous models have been created in order to assess present collision risks associated with certain orbits and to predict future evolution of the debris environment around Earth.

The National Aeronautics and Space Administration (NASA) has developed two categories of applications for modeling of space debris environment and risk analysis. The first category, based on evolutionary models such as NASA’s long term debris environment evolutionary model (LEO-to-GEO Environment Debris model or LEGEND), are designed to predict the evolution of the debris environment.

These models cover the near-Earth space between 200 km and 50,000 km, provide space debris characteristics for a debris population consisting of particles as small as 1 mm, and have a typical projection period of 100 years. The second category, which consists of engineering models like ORDEM2000, is used for debris impact risk assessment for spacecraft and satellites, and also as benchmarks for ground-based debris measurements and observations.

The European Space Agency (ESA) has a different set of tools used for modeling the space debris environment and assessing risk associated with collisions in Earth orbit. The DISCOS database (the Database and Information System Characterizing Objects in Space) consolidates the knowledge on all known objects tracked since Sputnik-1, and it is recognized as a reliable and dependable source of information on space objects in Earth orbit. MASTER (Meteoroid and Space Debris Terrestrial Environment Reference) is the agency’s most prominent debris risk assessment tool, which uses statistical methods to determine the impact flux information from all recorded historic debris generation events. ESA also uses DELTA (Debris Environment Long-Term Analysis) to conduct analysis of the effectiveness of debris mitigation measures on the stability of the debris population. Such analysis can cover 100 to 200 year time spans.

Credits: NASA

As the primary cause of concern from space debris is physical damage upon impact, extensive efforts have been made for their detection. There are several detection methods, and they are grouped into two classes: active and passive.

Radar sensors fall into the first class, and radio interferometers and optical sensors in the second. One important element that has to be considered is the accuracy of the method used for detection.

The motion of an object in Earth orbit is completely determined if the so-called orbital elements are known. In theory, the orbital elements of a satellite can be calculated from only one observation. In practice, due to inherent observation errors, there is more than one observation needed to attain the precision required for orbital surveillance and prediction. Some 100-200 observations are required during the first days of orbit, 20-50 observations per day to update established orbits, and 200-300 observations per day to confirm and locate reentry in the case of decaying orbits.

In addition, the size of the debris is an important factor that affects the accuracy of the detection methods, and this is why only a small fraction of the space debris population is detectable, and as a consequence, catalogued. For example, present equipment is capable of tracking only objects bigger than 5 cm in diameter in low Earth orbit (altitudes of 160-2,000 km), and bigger than 50 cm in diameter in geosynchronous orbit (altitudes of 35,000 km). Further, the characteristics of certain type of orbits can make detection very difficult. For example, the debris population generated on highly elliptical and high inclination orbits with perigees situated deep in the Southern Hemisphere, also known as Molniya orbits, is very difficult to track. The geographic location of the ground stations used for space debris tracking makes detection impossible.

For these reasons, out of an estimated debris population of 600,000 objects bigger than 1 cm in diameter, only 19,000 can be tracked as of today.

The measurement and detection methods mentioned above are all remote methods. In-situ measurements of the characteristics of the debris environment have been conducted as well. In April 1984, the Space Shuttle Challenger placed into low Earth orbit a NASA spacecraft carrying a number of experiments for the purpose of characterizing the low Earth orbit environment. The spacecraft, the Long Duration Exposure Facility (LDEF), was a twelve-sided cylindrical structure and three-axis stabilized in order to ensure an accurate environmental exposure, and was supposed to spend one full year in orbit. Before the planned retrieval, the Space Shuttle fleet was grounded as a result of the Challenger accident on January 28, 1986. Eventually, the exposed facility was returned to Earth by the Space Shuttle Columbia during a mission in January 1990. After the extended mission, the results of the onboard experiments facilitated to a greater extent the understanding of the interactions between artificial objects and the space debris environment in Earth orbit as numerous impact craters were found on the outer layers of the spacecraft and analyzed.

In-situ measurements of the characteristics of the space debris environment have also been conducted by the European Retrievable Carrier (EURECA) and the Space Flyer Unit (SFU).