Overview of Space Debris
Space debris is defined as all the inactive, manmade objects, including fragments, that are orbiting Earth or reentering the atmosphere. In near-Earth space, this debris is more significant than natural meteoroids, except around millimeter sizes, where meteoroids prevail in some orbital regions.
Routine ground-based radar and optical measurements track and catalog objects larger than 5–10 cm in LEO (Low Earth Orbit) and larger than 0.3–1.0 m at GEO altitudes. Some specialized sensors may also detect objects down to sub-cm sizes, but these cannot generally be maintained in catalogs or correlated with specific launch events. The presence of even smaller debris, of typically under 1 mm, can be deduced from impact craters on returned space hardware, or from dedicated in situ impact detectors.
At typical collision speeds of 10 km/s in low orbits, impacts by millimeter-sized objects could cause local damage or disable a subsystem of an operating satellite. Collisions with debris larger than 1 cm could disable an operational satellite or could cause the break-up of a satellite or rocket body. And impact by debris larger than about 10 cm can lead to a catastrophic break-up: the complete destruction of a spacecraft and generation of a debris cloud.
The fragments created by a collision can drive a cascading process, the ‘Kessler syndrome', in which each collision between objects generates more space debris, which increases the likelihood of further collisions.
Note: The Kessler Syndrome is a theory proposed by NASA scientist Donald J. Kessler in 1978, used to describe a self-sustaining cascading collision of space debris in LEO (Low Earth Orbit). It's the idea that two colliding objects in space generate more debris that then collides with other objects, creating even more shrapnel and litter until the entirety of LEO is an impassable array of super swift stuff. At that point, any entering satellite would face unprecedented risks of headfirst bombardment. 1)
Large debris objects (such as satellites, spent rocket bodies and large fragments) that reenter the atmosphere in an uncontrolled way can reach the ground and pose a risk to the population. The related risk for an individual is, however, several orders of magnitude smaller than commonly accepted risks in daily life (for example, the risk of serious injury from a motor vehicle accident is about 30 million times higher).
Spacefaring nations are now focusing efforts on controlling the space debris environment. Today, there is a consensus from long-term projections about the onset of a collisional cascading process in LEO. The ultimate goal is to limit this runaway situation, to safeguard future space operations. Mitigation actions have been identified and propagated into international and national standards by various spacefaring nations. Even with strict adherence to these mitigation requirements (which is not yet achieved), it is evident, however, that additional remediation measures will be required in order to limit the number of objects in LEO.
The ‘active removal' of a number of selected objects per year is needed, but this sets a global challenge that can only be achieved by the joint efforts of all spacefaring nations.
Since the mid-1980s, ESA has been active in every area of research, development, technology and operations related to space debris. Since 2006, the Space Debris Office at ESA/ESOC (European Space Operations Center) in Darmstadt, Germany, has operated as a standalone entity within the Ground Systems Engineering Department of ESA's Directorate of Operations. The office coordinates the Agency's research into space debris, as well as coordinating with national research programs, and provides operational services to ESA, its Member States and third parties.
As of April 2017, more than 290 break-ups in orbit have been recorded since 1961. Most were explosions of satellites and upper stages – fewer than 10 involved accidental and intentional collisions.
The Space Debris Environment
There have only been human-made objects in space since the start of the Space Age in 1957, and all resulted from the 5253 launches (as of January 2017) since then. The majority (about 58%) of the catalogued objects, however, originate from more than 290 break-ups in orbit, mainly caused by explosions, and from about 10 suspected collisions (of which four are confirmed between catalogued objects). 2)
Major contributions to the population of fragments came from a Chinese anti-satellite test targeting the Feng Yun-1C weather satellite on 11 January 2007, which created more than 3400 tracked fragments, and the approximately 2300 tracked fragments created from the first-ever accidental collision between two satellites, Iridium-33 and Cosmos-2251, on 10 February 2009.
About 24% of the catalogued objects are satellites (less than a third of which are operational), and about 18% are spent rocket bodies and other mission-related objects. Fragmentation debris dominates the smaller size regimes down to 1 mm. Below 1 mm, slag and dust residues from about 2440 solid-propellant motor firings prevail. Other debris sources can be associated with the release of liquid coolant from 16 Buk nuclear reactors on Russian radar ocean reconnaissance satellites in the 1980s, and with the release of surface materials from old satellites and rocket bodies due to impacts and/or surface degradation.
The main source of information on large space debris is the US SSN (Space Surveillance Network). As of January 2017, SSN is tracking, correlating and cataloguing around 23 000 space objects larger than 5–10 cm in Earth orbit (around 5000 of which are unpublished for various reasons). Debris environment models can be used to estimate total numbers, indicating that there are 29 000 objects larger than 10 cm, 750 000 from 1 to 10 cm, and more than 166 million from 1 mm to 1 cm.
Figure 1: Evolution of the tracked and published space object population and its composition by object class -status January 2017 (image credit: ESA)
Figure 2: Modelled space debris population for objects >1 m (a), > 10 cm (b), >1 cm (c), and >1 mm (d). The sizes of debris are exaggerated in relation to Earth. Figure 1statistics translated into Figure 2. Red: intact satellites (inactive or active), yellow: upper stages, green: mission-related objects, blue: fragments (ESA MASTER model)
Figure 3: This chart displays a summary of all objects in Earth orbit officially cataloged by the U.S. SSN (Space Surveillance Network). "Fragmentation debris" includes satellite breakup debris and anomalous event debris, while "mission-related debris" includes all objects dispensed, separated, or released as part of the planned mission (image credit: U.S. SSN) 3)
Debris is generated during normal operations by the injection of stages into orbit, the release of mission-related objects and the eventual retirement of a satellite. Subsequent break-ups and other release events may occur and contribute further debris. These combined debris sources are counteracted by natural cleaning mechanisms, such as perturbations to the orbital motion due to the Sun and Moon, and forces induced by air drag. LEO satellites are continuously exposed to aerodynamic forces from the rarefied upper reaches of the atmosphere. Depending on the altitude, after a few weeks, years or even centuries, this drag will decelerate the satellite sufficiently that it reenters Earth's atmosphere. At higher altitudes, above 700–800 km, the air drag is less and objects generally remain in orbit for at least several decades.
The result of the balancing effects of debris creation and orbital decay leads to maximum debris concentrations at altitudes of 800–1000 km and near to 1400 km. Secondary peaks of spatial densities in GEO (Geostationary Orbit), at 35 786 km altitude and near the orbits of navigation satellite constellations between 19 000 and 23 000 km altitude in MEO (Medium Earth Orbit) are smaller by two to three orders of magnitude.
Recognizing sustainable behavior
May 6, 2019: Solving the growing problem of space debris will require everyone who flies rockets and satellites to adhere to sustainable practices, which doesn't always happen. Now there will be a way to recognize those who do. 4)
Figure 4: Distribution of space debris in orbit around Earth. This animation shows different types of space debris objects and different debris sizes in orbit around Earth. For debris objects bigger than 10 cm the data come from the US Space Surveillance Catalogue The information about debris objects smaller than 10 cm is based on a statistical model by ESA (video credit: ESA)
Number of space debris objects in orbit:
We increasingly rely on satellites for every-day activities like navigation, weather forecasting and telecommunications, and any loss of these space-based services could have a serious effect on our modern economies.
Yet vital orbital pathways around Earth are becoming more congested with trash, such as abandoned satellites and rocket upper stages or debris fragments from old satellites that have exploded.
"There are numerous debris reduction and mitigation guidelines that can be applied at the design, manufacturing, launching, operating or disposal stage of any mission, but the challenge has been getting the global community to apply these in a consistent way," says Holger Krag, Head of ESA's Space Debris Office. "Applying these guidelines generally adds cost or reduces the useful life of a satellite, even if only slightly, so it's always been a tough sell," says Holger.
In a bid to address this issue, and to foster global standards in debris mitigation, the World Economic Forum will work with the Massachusetts Institute of Technology Media Lab and ESA to launch a new ‘Space Sustainability Rating' (SSR), a concept initiated by the Forum's Global Future Council on Space Technologies.
"The global economy depends on our ability to operate satellites safely in order to fly in planes, prepare for severe weather, broadcast television and study our changing climate," says Danielle Wood, founder and director MIT Media Lab's Space Enabled research group. "In order to continue using satellites in orbit around Earth for years to come, we need to ensure that the environment around Earth is as free as possible from trash leftover from previous missions."
The new rating system will also be supported by Bryce Space and Technology, a firm providing services in strategy, market analytics and policy for the space industry with offices in the US and UK, and a team from the University of Texas at Austin, USA with expertise in orbital dynamics and space law.
Similar to rating systems such as the LEED certification used by the construction industry, the Space Sustainability Rating aims to ensure long-term sustainability by encouraging and rewarding responsible behavior amongst all space actors, including designers, manufacturers, launch providers, spacecraft operators and even government agencies.
"Together with our collaborators, we aim to put in place a system that has the flexibility to stimulate and drive innovative sustainable design solutions," says Stijn Lemmens, a senior space debris mitigation analyst at ESA. "We also aim to put in the spotlight those missions that contribute positively to the space environment."
Today, there are more than 22,000 debris objects regularly tracked in orbit using radars and other methods, and any one of these could damage or destroy a functioning satellite if a collision were to occur.
In 2018, ESA-operated satellites had to conduct 27 debris avoidance maneuvers, a number that is growing year by year.
Later this year, ESA Member States will consider a range of new proposals related to space debris at the Space19+ council meeting. These include developing and demonstrating an automated collision avoidance system, an urgent need in view of the enormous constellations of small satellites that will be deployed by commercial companies in the next few years, and developing a European industrial capacity to conduct in-orbit servicing by flying a first-of-its-kind debris-removal mission.
The new SSR initiative is to be announced today at the Satellite 2019 conference in Washington, D.C., an international forum for companies, academia and agencies working in space.
ESA's 2019 Space Debris Environment report is available here.
Rocket break-up provides rare chance to test debris formation
April 12, 2019: The discarded ‘upper stage' from a rocket launched almost ten years ago has recently crumbled to pieces. "Leaving a trail of debris in its wake, this fragmentation event provides space debris experts with a rare opportunity to test their understanding of such hugely important processes", explains Tim Flohrer, ESA's Senior Space Debris Monitoring Expert. 5)
Fragmentation events like this one – either break ups or collisions – are the primary source of debris objects in space in the range of a few millimeters to tens of centimeters in size. Travelling at vast speeds, these bits of technological trash pose a threat to crucial space infrastructure, such as satellites providing weather and navigation services, and even astronauts on the ISS.
A remarkable video captured by the Deimos Sky Survey in Spain shows the stream of newly-made debris objects as they rush across the sky. Deimos Sky Survey tracked the fragmentation and debris formation of the ATLAS V Rocket. The analyses show the expected evolution of the fragments cloud around the Earth and the Spatial density at different altitudes and timeframes. 6)
On March 26th 2019 at the occasion of the IAF Spring meeting Space Debris Committee, Vladimir Agapov of Keldysh Institute of Applied Mathematics (Russia) unveiled the fragmentation event of object 2009-047B, estimated to have taken place on 25 March 2019. The 2009-047B object is the second stage of the Atlas V launcher which put in orbit a US satellite on 8th September 2009. The rocket body was placed on a GEO Transfer Orbit of 6,673 x 34,700 km, 23.1º, where it could stay for centuries.
Following the announcement, Deimos Sky Survey observatory has tracked the object in the nights of 26th, 27th and 28th of March, providing detailed images of the central body and between 40 and 60 fragments larger than 30 cm size. Animations (typically covering 30 seconds of real data) based on acquisitions on these days show the objects as fixed points, while stars are shown as trails, since the sensor is moving following the objects' movement.
Figure 5: Rocket body fragments from the object 2009-047B. In the clip, a number of small point-like fragments can be seen spread horizontally across the frame. As the observatory moves with the debris objects, the background stars are seen as white streaks. The remnant piece is clearly visible as the largest and brightest point at the center of about 40-60 smaller pieces, many larger than 30 cm in size, and has been traced back to the upper stage of a rocket launched in September 2009 (image credit: Deimos Sky Survey)
For an as-yet-unknown reason, the rocket body fragmented some time between 23 to 25 March 2019.
Figure 6: On the fixed image of March 27th, individual objects are detected for tagging and identification. Based on the acquired images and simulated data from breakup models, Elecnor Deimos SSA team is making use of the in-house processing, and fragmentation evaluation tools, to carry out analysis and perform simulations so as to characterize the resulting objects and predict is orbital evolution. Results of these analyses, such as the expected evolution of the fragments cloud around the Earth or the Spatial density at different altitudes and timeframe, shown below, are being shared with the Space Surveillance and Tracking international community. (image credit: Deimos Sky Survey)
Figure 7: Sample image of the Atlas V Centaur upper stage. A Centaur upper stage is lifted onto the first stage booster of a United Launch Alliance Atlas V (image credit: NASA / Roy Allison)
An international effort
During a meeting of the International Academy of Astronautics (IAA) on 26 March 2019, ESA's space debris team met their counterparts from Russia, who informed the international community of fragments detected orbiting in the sky.
Just hours later, the Zimmerwald Observatory in Switzerland scheduled immediate observations of the cloud of fragments, and by 26 March had acquired the first views.
Figure 8: The first series of exposures of a fragmentation event in March 2019, taken by the 0.8 meter telescope ZimMAIN, which followed the debris cloud. The animation reveals several small dots, each a fragment larger than a few tens of centimeters, with background stars appearing as long streaks (image credit: Zimmerwald Observatory, AIUB)
Not long after, the Deimos Sky Survey followed up with observations of the event from 26-28 March (Figures 5 and 6), using the ‘Antsy' optical sensor in Spain, which is adapted for tracking objects in low-Earth orbit. While Zimmerwald continues to observe the cloud in close collaboration with Russian and ESA experts, ESA's own 1meter telescope at the Optical Ground Station at Tenerife, Spain, has joined the observation campaign, detecting a large number of fragments down to 10-20 cm in size.
Modelling the mess
ESA keeps an eye on events like this and continually updates the international community through its public database, enabling researchers to find patterns and come up with mitigation strategies for spacecraft in all variety of shapes, sizes and orbits. The database also allows operators of satellites and spacecraft to determine the changing risk to their missions from specific fragmentation events.
Once detected and observed, events like these are put into ‘space debris environment models', allowing teams to compare the fragmentation of real-life debris with predictions – a rare but crucial opportunity to validate or improve models as necessary.
Figure 9: Tough Love. This Valentine's Day, look to the skies at night and you'll see stars twinkling, a glistening Moon and perhaps even an orbiting science lab passing by, the ISS. What you can't see is the thousands of bits of space debris that circle our planet – remnants of past scientific and technical endeavors, evidence of five decades spent in space. ESA's Space Debris Office constantly monitors more than 20,000 debris objects in orbit, issuing warnings and guidance to the operators of ESA spacecraft. (image credit: ESA, CC BY-SA 3.0 IGO)
Developing models of the space debris environment allows ESA to design spacecraft that can withstand impacts from small objects, and design systems to avoid collisions. These models are the baseline for predicting not just the present, but our future space debris environment, which is essential to developing efficient space debris mitigation guidelines.
International collaboration is essential to exchanging data and models, which takes place via a technical body called the Inter-Agency Space Debris Coordination Committee, which comprises all major European and international space agencies.
"As this example shows, international collaboration is essential if we want to respond quickly to debris creating events", concludes Holger Krag, Head of ESA's Space Safety Office. "Incidents like this are rare, so to have such rich observations and data from across the globe is a unique opportunity to better understand the human-made environment around Earth, in which our satellites live out their lives".
Figure 10: Space Safety & Security at ESA. As we discover more about the brilliant scale and nature of the Universe, planet Earth's blue oceans, green forests and glistening city lights appear even more unique, and even more fragile. Many hazards have been identified originating in space, which although unlikely, continue to pose real dangers to our way of life, and in the worst cases to human health and safety. Only in the past decades have we had the opportunity to understand the potential perils of our position in our Solar System, and as technologies continue to advance we are entering a period in which we can actually act. However, as technologies advance, so too does our dependence on them, making us more vulnerable to both human-made and natural threats in space (image credit: ESA)
Space Debris Measurements
Existing space object catalogs are typically limited to objects larger than 5–10 cm at low altitudes (LEO) and larger than 0.3–1 m at high altitudes (GEO). In a compromise between system cost and performance, passive optical telescopes are suited mainly to observing high altitudes, whereas radars are advantageous below 2000 km.
Satellite laser-ranging to debris objects is an emerging technology. Several experiments have provided promising results for the detection and follow-up of intact objects and of fragments not carrying laser retro-reflectors, as well as for the determination of their attitude and the attitude motion.
Knowledge of the meteoroid and debris environment at subcatalog sizes is normally acquired in a statistical manner through experimental sensors with higher sensitivities. ESA collaborates primarily with the Fraunhofer Institute for High Frequency Physics and Radar Techniques, near Bonn, Germany, which operates the TIRA (Tracking and Imaging Radar) system. Apart from dedicated tracking campaigns, TIRA also regularly conducts surveys that detect debris and determine coarse orbit information for objects of diameters down to 2 cm at 1000 km distance.
ESA also gains information on the submillimeter meteoroid and debris environment through analyzing retrieved space hardware, and through active in situ impact detectors by analyzing impact fluxes (the number of impacts per surface area and time).
The knowledge of space debris in the millimeter to centimeter range is poor by comparison, and space debris environment models do not agree well for this class. Such objects are too small for ground-based sensors and rarely impact the sensitive face of the in situ impact detectors. Novel approaches for statistical characterization, such as through in situ detectors providing a large collecting area or with space-based optical sensors, are being studied.
Space Debris Modeling
ESA maintains a number of software models for characterizing the debris environment and its evolution. 7) The Agency's preeminent debris and meteoroid risk assessment tool is MASTER (Meteoroid and Space Debris Terrestrial Environment Reference). It was first issued in 1995 and has been continuously improved since then. Impact flux information is provided with high spatial resolution for an object population that is derived from all known historical debris generation events, including the estimation of flux uncertainties.
At the Teide Observatory on Tenerife in the Canary Islands, Spain, ESA operates the Optical Ground Station – dubbed ESA's Space Debris Telescope (Figure 11). Its 1 m diameter Zeiss telescope, equipped with highly efficient cameras, is used to survey and characterize objects at high altitudes, often in collaboration with other telescopes, such as those operated by the University of Bern, Switzerland. The ESA telescope can detect and track near-GEO objects up to magnitudes of +19 to +21 (equivalent to down to 15 cm in size). With this performance, the ESA telescope is among the world's top-ranking sensors.
ESA's latest data indicate that the number of fragmentation events in GEO is much higher than previously believed. It also appears that objects in GEO tend to release extremely lightweight objects with high area-to-mass ratios, such as pieces of thermal blankets, and with strongly perturbed orbits that require frequent reobservations.
The acquired data on space debris are essential for developing and validating environment models. To validate model predictions and to optimize observation times and sensor sensitivity in the planning of observation campaigns, ESA developed PROOF (Program for Radar and Optical Observation Forecasting).
Figure 11: ESA's 1 m diameter Zeiss space debris telescope at the Optical Ground Station on Tenerife (image credit: ESA)
With today's rate of 70–90 launches a year, an increasing number of launches injecting 30 or more small satellites into orbit at once, and assuming future break-ups will continue at mean historical rates of four to five per year, the number of objects in space is expected to increase steadily. As a consequence of the rising object count, the probability of catastrophic collisions will also grow in a progressive manner.
Collision fragments can trigger further collisions, leading to a self-sustaining cascading process known as the ‘Kessler syndrome'. This is particularly critical for LEO, and may seriously endanger spaceflight within a few decades in certain orbit altitudes.
Spacefaring nations are focusing their efforts on controlling the debris environment. The ultimate goal is to limit the collisional cascading process in the Earth environment. Initial steps aim at reducing the generation of hazardous debris by avoiding in-orbit explosions or collisions with operational satellites, and by removing satellites from densely populated altitudes at the end of their missions. To this end, the IADC (Inter-Agency Space Debris Coordination Committee), recognized internationally as the technical authority on space debris, released a set of space debris mitigation guidelines in 2002. These guidelines have been used as the model for national legislation and international standardization. ESA is taking a leading role in monitoring adherence to these guidelines, and reports its findings annually to the international community, including the United Nations.
Figure 12: Estimates (by mass) of the adherence of space missions in LEO to post-mission disposal guidelines at end of life (image credit: ESA)
ESA missions that were designed during the previous century can only implement best practices for space debris mitigation. For example, in 2011, ESA implemented dedicated end-of-mission operations for its ERS-2 (European Remote Sensing-2) satellite, which at that time had been operational for over 16 years. During these operations, the remaining orbital lifetime was significantly reduced, to well below 15 years, and all residual fuel was consumed. This effectively reduced the risks of collision and accidental break-up by orders of magnitude. In 2013, ESA's astronomy satellites Planck and Herschel were injected into orbits around the Sun after their missions were completed, in order to avoid creating a collision threat or reentry hazard. In 2015, two large orbit-change maneuvers were implemented for ESA's Integral and Cluster-1 satellites. These maneuvers ensured that both satellites will reenter Earth's atmosphere during the next decade in a safe way, and avoid long-term interference with the protected LEO and GEO regions.
‘Business as usual' space activities will lead to a progressive, uncontrolled increase in debris objects, with collisions becoming the primary debris source.
Figure 13: Projected evolution of the number of objects larger than 10 cm in LEO, depending on the adherence to PMD (Post Mission Disposal) guidelines (image credit: ESA)
All new ESA missions now include space debris mitigation as part of the standard mission design. To facilitate analyses by mission planners, spacecraft engineers and space system manufacturers, ESA has developed the DRAMA (Debris Risk Assessment and Mitigation Analysis) software tool. DRAMA allows the estimation of delta-V budgets for collision avoidance, optimization of disposal strategies along with estimation of orbital lifetimes, and analysis of ground casualty expectations. DRAMA is available for use by industry and academia worldwide and is distributed free of charge by ESA.
The consequences of impacts on satellites can range from small surface pits from micrometer-size objects, via clear-hole penetrations from mm-size objects, to mission-critical damage from projectiles larger than a centimeter. The destructive energy is a consequence of the high impact speeds, which can reach more than 15 km/s.
ESA's space projects use hypervelocity impact tests in association with damage-assessment tools to predict potential risks from hypervelocity impacts of debris and meteoroids, and to define effective protection measures through shielding and design.
Smaller, uncatalogued objects can only be defended against by passive protection techniques, such as those as used by the ISS (International Space Station). The ISS is equipped with debris shields around the inhabited modules, known as ‘stuffed Whipple shields'. These shields are composed of two metal sheets, separated by about 10 cm. Between the walls, fabric with the same purpose as in bulletproof vests is used. This design enables the shield to defeat debris objects of up to 1 cm. At more than 7 km/s, depending on the materials, an impact on the bumper wall will lead to a clear-hole penetration with a complete break-up and melting of the projectile, such that the dispersed fragment cloud can be withstood by the back wall.
Figure 14: The impact of a small projectile on an aluminum cube, showing the (often counter-intuitive) dynamics during such impacts. The time-series shows ejecta upon impact (first image), the projectile exiting (second image), and the shock wave causing rupture of the structure (last two images). This illustrates the huge amount of energy released by small debris objects involved in a hypervelocity collision (image credit: Fraunhofer/EMI)
Protection of an unmanned satellite can be improved efficiently by moving sensitive equipment away from the most probable impact direction, and/or by covering sensitive parts with protective fabric layers. Such measures can significantly increase the survival chances of a satellite against debris of up to 1 mm.
Recent research focuses on the vulnerability of membranes used for solar sails and drag augmentation devices. These thin foils need to survive micrometeoroid and debris impacts without losing too much of their effective area.
Furthermore, with the number of in-orbit collisions rising, a better understanding of the structural destruction and fragment generation is required as input to environment modelling. Modern methods of computational physics are applied for this purpose.
Figure 15: Whipple shielding after impact testing. The left image shows the damage to the outside, and the right shows the innermost layer (image credit: ESA-Laagland)
Figure 16: Close-up of DEBIE-2 sensors on the ISS (image credit: ESA)
Legend to Figure 16: On 18 February 2008, DEBIE-2 (Debris In-orbit Evaluator-2) was launched to the ISS on Columbus. This in situ impact detector has three 10 x 10 cm sensors looking in different directions, which measure the submillimeter-size populations of meteoroids and space debris particles in space. They are attached as external payloads to the Columbus module.
Collision and Reentry Risk Mitigation
The first accidental collision between two intact satellites occurred at 16:56 GMT on 10 February 2009. An operational US commercial communications satellite, Iridium-33, and a retired Russian military satellite, Cosmos-2251, collided at an altitude of 776 km above Siberia at a relative speed of 11.7 km/s. Both spacecraft were destroyed and more than 2300 trackable fragments were generated, some of which have since reentered.
Avoiding collisions is an important mitigation measure but this requires that the orbits of the approaching objects (‘chasers') are known with sufficient accuracy. Benefiting from a data-sharing agreement with US Strategic Command, ESA uses Conjunction Data Messages provided by the US JSpOC (Joint Space Operations Center) together with ESA's own orbit data to analyze all close approaches (‘potential conjunctions') of a given satellite (‘target') with any of the catalogued objects. The collision risk is determined as a function of the object sizes, the predicted miss distance, the flyby geometry, the orbit uncertainties and the time to conjunction. Today, ESA has a highly automated process in place to process several hundred messages every day, and to screen autonomously planned routine orbital maneuvers. A modern, web-based front-end with visualization capabilities supports communication with the flight control teams, flight dynamics teams, and mission managers.
ESA executes on average 12 collision avoidance maneuvers per year, in cases where the estimated collision risk is above the mission-defined ‘tolerable probability threshold'.
ESA's Space Debris Office provides conjunction predictions and collision risk estimation as an operational service to ESA and third-party missions.
Figure 17: Locations of debris objects that have been retrieved after reentry (image credit: ESA)
Only a few very large objects, such as heavy scientific satellites, reenter Earth's atmosphere in a year. In total, about 75% of all the larger objects ever launched have already reentered. Objects of moderate size, 1 m or above, reenter about once a week, while on average two small tracked debris objects reenter per day.
In general, reentering objects pose only a marginal risk to people or infrastructure on the ground or to aviation. From an altitude of 110 km, during the last 10 minutes before an object reaches the ground, the atmosphere is dense enough that the object heats up due to air resistance and decelerates, leading in the majority of cases to its demise. In the case of a large or a very compact and dense satellite, and especially when a large amount of high-melting-point material such as stainless steel or titanium is involved, fragments of the object may reach the ground. As these are rare events, and as about 75% of Earth's surface is covered by water and large portions of the land mass are uninhabited, the risk for any single individual is several orders of magnitude smaller than commonly accepted risks faced in daily life. Even being struck by lightning is 60,000 times more likely. In fact, to date there have been no known injuries resulting from reentering space debris. However, it is important to monitor the risk to the global population.
Skylab (74 tons, July 1979) and Salyut-7/Kosmos-1686 (40 tons, February 1991) are well-known examples of large-scale uncontrolled reentries. In such cases, 20–40% of the spacecraft mass may impact the ground. Examples of recent uncontrolled reentries that generated a lot of public interest were the Russian Phobos–Grunt Mars mission in 2012 and ESA's GOCE (Gravity field and steady-state Ocean Circulation Explorer) in November 2013.
Figure 18: Amateur image of GOCE reentering the atmosphere over the Falklands on 11 November 2013 (image credit: ESA)
To assess the risks associated with reentries, ESA has developed finite-element and simplified models that allow simulation of the break-up of a spacecraft in Earth's atmosphere. The models take into account the aerothermal, aerodynamic, atmospheric chemistry and thermomechanical effects, and are coupled with population forecast models. ESA's Space Debris Office provides information on upcoming and past reentries to a wide target audience, including national protection agencies, researchers and the general public, via a web-based portal. 8) ESA participates in and hosts a reentry data exchange platform for the IADC.
Regulations and Treaties
Space debris is a problem to which all spacefaring nations have contributed. Likewise, debris poses a risk to the missions of all spacefaring nations.
Since analysts first became aware of an emerging space debris problem in the early 1970s, the understanding of debris sources, the resulting debris environment and the associated risks have improved significantly. Today, the global dimension of the problem is internationally recognized, and space system designers, operators and policymakers share the view that active control of the space debris environment is necessary to sustain safe space activities in the future.
Research results are regularly discussed at the quadrennial series of ESA-organized European Conferences on Space Debris, and at dedicated sessions of the IAC (International Astronautical Congress) and COSPAR (COmmittee on SPAce Research) Scientific Assemblies and other conferences. The IADC (Inter-Agency Space Debris Coordination Committee), the most-recognized international entity on space debris, has produced a set of mitigation guidelines, which also served as input to a set of space debris mitigation guidelines adopted by the UNCOPUOS (UN Committee on the Peaceful Uses of Outer Space). The key recommendations are:
• limit debris release during normal operations
• minimize the potential for break-ups during operational phases
• limit the probability of accidental collisions
• refrain from intentional destruction and other harmful activities
• minimize the potential for post-mission break-ups resulting from stored energy
• limit the long-term presence of spacecraft and launch vehicle orbital stages in protected regions after the end of their missions.
Since its foundation in 1993, IADC has conducted annual meetings to discuss research results in debris measurements, modelling, protection and mitigation.
IADC is internationally recognized as a center of competence for space debris and it also influences mitigation activities at the UNCOPUOS Scientific and Technical Subcommittee and at the Subcommittee for Space Systems and Operations (ISO-TC20/SC14) of the International Organization for Standardization. In order to guarantee an effective and balanced implementation of debris mitigation practices, identified control measures need to be based on an international consensus. As an example, the 2011 ISO standard 24113 defines primary debris mitigation requirements. This standard was adopted by the European Cooperation for Space Standardization, whose standards, via a formal ESA ‘ADMIN/IPOL' instruction, are applicable to all ESA projects. Verification of compliance with the ISO standard can be supported by using ESA's DRAMA tool.
The next step after technical definition and international standardization is the transfer of guidelines into actual regulations. While some countries have already taken this step and reflected space debris mitigation in their national regulations, worldwide implementation is still pending. In this context, the Scientific and Technical Subcommittee of UNCOPUOS recently achieved consensus on a set of guidelines that address this important implementation of regulations in the UN's Member States.
Active Debris Removal
Mitigation measures proposed by IADC to control the growth in the number of space objects have been adopted by various countries and organizations. Over the past 15 years, between half and two thirds of all satellites initially operating in the GEO region have been reorbited to a graveyard disposal orbit, in line with IADC recommendations – an improving trend. For LEO, which is most sensitive to an onset of collisional cascading, the first comprehensive statistics on compliance show poor results regarding the clearing of the region within 25 years of mission completion. Only 25% of the rocket upper stages and 10% of the satellites in LEO perform an active maneuver in order to comply with the IADC recommendations. Fortunately, many satellites are inserted into orbits where they comply naturally.
Studies performed with long-term evolution models like DELTA have shown that a ‘business as usual' scenario will lead to a progressive, uncontrolled increase of object numbers in LEO, with collisions becoming the primary debris source. The IADC mitigation measures will reduce the growth, but long-term proliferation is still expected, even with full mitigation compliance, and even if all launch activities are halted. This is an indication that the population of large and massive objects has reached a critical concentration in LEO.
Mitigation alone is therefore not sufficient, and it is necessary to introduce a program of remediation measures as well, namely ADR (Active Debris Removal). Studies at NASA and ESA show that, with a removal strategy focusing on large target masses, the environment can be stabilized if about five to ten objects are removed from LEO per year with the following priorities:
• objects with a high mass (largest environmental impact in terms of critical-size fragments)
• objects with a high collision probability (orbiting in densely populated regions)
• objects at high altitudes (long orbital lifetime of the object and of fragments in the event of a collision).
High-ranking hotspot regions have been identified at around 1000 km altitude and 82° inclination, at 800 km and 98°, and at 850 km and 71°. The concentration of critical-size objects in these narrow orbital bands could allow multi-target removal missions.
Actions to counter the exponential growth of space debris, such as mitigation and active removal, are most effective when they are applied early. The further the number of critical-size, intact objects in the debris environment deviates from a sustainable level, the more objects will have to be removed to suppress the additional growth and the multiplying effects. ESA's internal studies show that continuous removal actions starting in 2060 would be 25% less effective in comparison to an immediate start.
ESA, as a space technology and operations agency, has identified active removal technologies as a strategic goal. ADR is necessary to stabilize the growth of space debris, but even more important is that any newly launched objects comply with post-mission disposal guidelines (namely orbital decay in less than 25 years). If this is not the case, most of the required ADR effort would go to compensate for the non-compliance of new objects.
Legal constraints associated with the ownership of space debris and related liability issues cannot be neglected and the responsibility for a coupled remover/target is shared between the object owners.
Some examples of planned robotic servicing missions
1) ESA's e.Deorbit mission:
ESA's CleanSpace Initiative is looking at the required technology developments, including advanced image processing, complex GNC (Guidance, Navigation and Control) and innovative robotics to capture debris. Technologies for a wide range of removal targets will be studied, including real applications. ‘e.Deorbit', to be launched in 2023, will be the first ADR mission conducted by ESA, with the objective of removing a large ESA-owned object from its current orbit and performing a controlled reentry into the atmosphere.
One high priority target is Envisat, an uncontrolled, ESA-owned satellite of approximately eight metric tons in an orbit of about 800 km altitude and 98° inclination. Recently ESA has performed a phase A study called e.Deorbit. It aimed at identifying the most suitable concept for the capture and active removal of such a large debris object. Envisat has been chosen as an exemplary target by the industrial partners. 9) 10)
The e.deorbit mission objective is to "Remove a single large ESA-owned space debris (Envisat) from the LEO (Low Earth Orbit) protected zone". With this mission, ESA wants to pave the way for future ADR (Active Debris Removal) missions by removing a single large ESA-owned space debris from the protected LEO zone in the 2021-2024 timeframe. 11) 12)
In December 2016, ESA's member state ministers strongly supported a ‘maturation phase' for e.Deorbit, to foster the various advanced technologies required to make the mission feasible, from autonomous guidance to advanced images processing, along with a suitable capture mechanism. 13)
Figure 19: Artist's rendition of ESA's proposed e.Deorbit mission, shown left, using a robotic arm to catch a derelict satellite (image credit: ESA, David Ducros, 2016)
Figure 20: The mission life cycle from launch by Vega to reentry of e.Deorbit, an ESA mission to remove a single large ESA-owned debris from orbit, which will be the first-ever active debris removal mission. It will place European industry at the forefront of the world's active removal efforts and space tug applications. The mission was presented at ESA's Council meeting at Ministerial level, Lucerne, Switzerland, on 1-2 December 2016 (image credit: ESA, David Ducros, 2016) 14)
2) NASA's Restore-L mission
NASA's Restore-L mission is an ambitious, technology-rich endeavor to launch a robotic spacecraft in 2020 to refuel a live satellite. The mission — the first of its kind in LEO (Low Earth Orbit) — will demonstrate a carefully curated suite of satellite-servicing technologies. These on-orbit solutions for autonomous, on-orbit satellite rendezvous and grasping, with telerobotics-enabled refueling and satellite repositioning, could dramatically reduce or eliminate the need for crewed servicing flights from Earth.
In May 2016, NASA officially moved forward with plans to execute the ambitious, technology-rich Restore-L mission, an endeavor to launch a robotic spacecraft in 2020 to refuel a live satellite. The mission will demonstrate that a carefully curated suite of satellite-servicing technologies are fully operational. The current candidate client for this venture is Landsat-7, a government-owned satellite in LEO. 15) 16) 17)
NASA awarded the contract to SSL (Space Systems Loral) on Dec. 5, 2016 tasking the company with supplying a chassis, hardware and services for the mission. The Palo Alto, California-based satellite builder is responsible for supporting integration, test, launch and operations. 18)
April 10, 2017: SSL (Space Systems Loral (SSL) has successfully completed the SRR (Systems Requirements Review) for the Restore-L project to demonstrate satellite servicing in LEO (Low Earth Orbit). 19)
Figure 21: Artist's rendering of the Restore-L servicer extending its robotic arm to grasp and refuel a client satellite on orbit (image credit: NASA)
3) DARPA's RSGS (Robotic Servicing of Geosynchronous Satellites) mission
In developing the RSGS program, the US DARPA (Defense Advanced Research Projects Agency) seeks to:
• Demonstrate in or near GEO that a robotic servicing vehicle can perform safe, reliable, useful and efficient operations, with the flexibility to adapt to a variety of on-orbit missions and conditions.
• Demonstrate satellite servicing mission operations on operational GEO satellites in collaboration with commercial and U.S. Government spacecraft operators.
• Support the development of a servicer spacecraft with sufficient propellant and payload robustness to enable dozens of missions over several years.
RSGS is a research and demonstration effort that aims to speed the arrival of capabilities such as high-resolution inspection; correction of otherwise mission-ending mechanical anomalies such as solar array and antenna deployment malfunctions; assistance with relocation and other orbital maneuvers; and installation of attachable payloads, enabling upgrades to existing assets. 20)
DARPA Press Release on February 9, 2017: 21)
In an important step toward a new era of advanced, cost-effective robotic capabilities in space, DARPA today announced that it has selected Space Systems Loral (SSL), based in Palo Alto, CA, as its commercial partner for the Agency's RSGS (Robotic Servicing of Geosynchronous Satellites) program. DARPA and SSL seek to develop technologies that would enable cooperative inspection and servicing of satellites in GEO (Geosynchronous Earth Orbit), about 36,000 km above the Earth, and demonstrate those technologies on orbit. If successful, this research and demonstration effort would open the door to radically lowering the risks and costs of operating in GEO, a harsh and difficult-to-access domain that is critically important for both military and civilian space assets.
Under an agreement drafted jointly by DARPA and SSL, the two entities would share costs and responsibilities for the program. While such public-private partnerships have become common in several domains of research and development — saving taxpayer dollars by requiring commercial partners to invest significantly in projects rather than simply receive government funding — the RSGS public-private effort would be a first for DARPA in the space-servicing domain. As such, the Agency's selection of SSL and the pending agreement have been submitted for review by the Defense Department's Under Secretary of Defense for Acquisition, Technology and Logistics.
With RSGS, DARPA plans to develop a robotic module, including hardware and software, and provide technical expertise and a Government-funded launch. SSL would provide a spacecraft and would be responsible for integrating the module onto it to create a robotic servicing vehicle (RSV) and the RSV onto the launch vehicle, as well as providing a mission operations center and staff.
After a successful on-orbit demonstration of the RSV, SSL would operate the vehicle and make cooperative servicing available to both military and commercial GEO satellite owners on a fee-for-service basis. In exchange for providing property to SSL, the Government would obtain reduced-priced servicing of its satellites and access to commercial satellite servicing data throughout the operational life of the RSV, again at great taxpayer savings. The capabilities that RSGS aims to make possible include:
- High-resolution inspection
- Correction of some types of mechanical anomalies, such as solar array and antenna deployment malfunctions
- Assistance with relocation and other orbital maneuvers
- Installation of attachable payloads, enabling upgrades or entirely new capabilities for existing assets
"Servicing on orbit could provide significant cost savings compared to current practices and a major advantage to the security of both commercial and Government space assets," said Gordon Roesler, DARPA's program manager for RSGS. "The engineering challenges that need to be overcome to achieve this degree of facility at GEO are considerable, entailing significant technical risks but also carrying the potential for significant rewards. In addition to inspection and repair, RSGS robotics promise a new era in which satellite upgrades and enhancements at GEO are no longer just a dream."
Figure 22: Illustration of the RSGS program RSV (Robotic Serving Vehicle) in GEO (image credit: DARPA, SSL)
Call for a sustainable future in space
The 7th European Conference on Space Debris took place at ESA/ESOC in Darmstadt, Germany from 18-21 April, 2017 — covering the most important topics in orbital space debris to explore the challenges for modern space flight. The conference provided a forum for presenting and discussing latest results, and for defining future directions of research. 22) 23)
The call for international action came on the final day of the European Conference on Space Debris, a gathering of over 350 participants from science, academia, industry and space agencies worldwide held at ESA's mission control center, where the ESA Space Debris Office and the SSA effort are based. 24)
Figure 23: Decision-makers on stage at the media briefing following the 7th European Conference on Space Debris at ESA/ESOC. From left to right: Head of ESA's Space Debris Office, Holger Krag; ESA Director General Jan Woerner; and German Federal Minister for Economic Affairs and Energy, Brigitte Zypries (image credit: ESA)
With more than 750 000 pieces of dangerous debris now orbiting Earth, the urgent need for coordinated international action to ensure the long-term sustainability of spaceflight is a major finding from Europe's largest-ever conference on space debris.
"We require a coordinated global solution to what is, after all, a global problem that affects critical satellites delivering services to all of us," said Brigitte Zypries, German Federal Minister for Economic Affairs and Energy, at a press briefing on the conference's closing day in Darmstadt, Germany.
ESA Director General Jan Woerner appealed to space stakeholders to keep Earth's orbital environment as clean as possible. Developing and implementing the ESA SSA (Space Situational Awareness) program as decided during ESA's last ministerial council in 2016 will be a key factor. "In order to enable innovative services for citizens and future developments in space, we must cooperate now to guarantee economically vital spaceflight. We must sustain the dream of future exploration," he said.
The call for international action came on the final day of the European Conference on Space Debris, a gathering of over 350 participants from science, academia, industry and space agencies worldwide held at ESA's mission control center, where the ESA Space Debris Office and the SSA effort are based.
Findings from the week-long meeting were presented to media in front of Minister Zypries, who is also the German national aerospace coordinator, and Director General Woerner by senior ESA managers and representatives from the national space agencies of Italy, France, Germany and the UK, as well as the Committee on Space Research and the International Academy of Astronautics.
Addressing the space debris threat: The latest results of debris research were featured, especially the safe disposal of retired satellites and rocket stages and the still uncertain challenges posed by satellite megaconstellations being considered by commercial operators. "Only about 60% of the satellites that should be disposed of at the end of their missions under current guidelines are, in fact, properly managed," noted Holger Krag.
Researchers also confirmed there is now a critical need to remove defunct satellites from orbit before they disintegrate and generate even more debris. "This means urgently developing the means for actively removing debris, targeting about 10 large defunct satellites from orbit each year, beginning as soon as possible – starting later will not be nearly as effective," said Holger Krag.
Table 1: Findings from the 7th European Conference on Space Debris 25)
1) Donald J. Kessler, Burton G. Cour-Palais, "Collision Frequency of Artificial Satellites: Th eC reation o f a Debris Belt, Journal of Geophysical Research, Vol. 83, No A6, June 1, 1978, URL: http://webpages.charter.net/dkessler/files/Collision%20Frequency.pdf
2) K. Fletcher, "Space Debris: The ESA Approach," ESA, BR-336, March 2017, http://esamultimedia.esa.int/multimedia/publications/BR-336/offline/download.pdf
3) Orbital Debris Quarterly News, Vol. 21, Issue 1, p. 12, Feb. 2017, NASA, URL: https://orbitaldebris.jsc.nasa.gov/quarterly-news/pdfs/odqnv21i1.pdf
4) "Recognizing sustainable behavior," ESA, 6 May 2019, URL: http://www.esa.int/Our_Activities/Space_Safety/Space_Debris/Recognising_sustainable_behaviour
5) "Rocket break-up provides rare chance to test debris formation," ESA, 12 April 2019, URL: http://m.esa.int/Our_Activities/Operations/Space_Safety_Security/Rocket_break-up
6) "Deimos Sky Survey Spots Debris Resulting from the Fragmentation of an Atlas V Rocket Body," Deimos Elecnor Group, 1 April 2019, URL: http://www.elecnor-deimos.com/fragmentation-event/
9) "Baseline Concepts of the Kayser-Threde Team," e.Deorbit Symposium, Noordwijkerhout, The Netherlands, May 6, 2014, URL: http://space-env.esa.int/indico/getFile.py/access?resId=2&materialId=slides&confId=46
10) Jan-Christian Meyer, Marc Scheper, Rolf Janosvsky, Javier Vazquez Mato, Gonzalo Taubmann, Jaime Cavia Eguen, Ronan Le Letty, "Clamping mechanism – a tentacles based capture mechanism for Active Debris Removal," Proceedings of the 65th International Astronautical Congress (IAC 2014), Toronto, Canada, Sept. 29-Oct. 3, 2014, paper: IAC-14-A6.5.1
11) Robin Biesbroek, Luisa Innocenti, Stéphane Estable, Michael Oswald, Richard Haarmann, Gerrit Hausmann, Carole Billot, Simona Ferraris, "The e.Deorbit mission: results of ESA's phase A studies for an Active Debris Removal mission," Proceedings of the 66th International Astronautical Congress (IAC 2015), Jerusalem, Israel, Oct.12-16, 2015, paper: IAC-15-A6.6.5
12) "e.Deorbit: it is time to make active debris removal a reality for the European space sector," ESA, Jan. 30, 2017, URL: http://blogs.esa.int/cleanspace/2017/01/30/e-deorbit-it-is-time-to-make-active-debris-removal-a-reality-for-the-european-space-sector/
13) "e.Deorbit grabbing debris," ESA, January 18, 2017, URL: http://m.esa.int/spaceinimages/Images/2016/12/e.Deorbit_grabbing_debris
14) "e.Deorbit mission lifecycle," ESA, 23 Nov. 2016, URL: http://m.esa.int/spaceinimages/Images/2016/12/e.Deorbit_mission_lifecycle
15) Adrienne Alessandro, "Restore-L Mission to Demonstrate Technology by Refueling Landsat-7," NASA, June 23, 2016, URL: http://landsat.gsfc.nasa.gov/nasas-restore-l-mission-to-demonstrate-crosscutting-technologies-by-refueling-landsat-7/
17) Bringing Restore-L to Life," NASA Restore-L Factsheet, URL: https://www.nasa.gov/sites/default/files/atoms/files/restore_l_factsheet_062816_01.pdf
18) Caleb Henry, "NASA's Restore-L contract nudges SSL closer to in-orbit servicing ," Space News, Dec. 6, 2016, URL: http://spacenews.com/nasas-restore-l-contract-nudges-ssl-closer-to-in-orbit-servicing/
20) Doug Messier, "DARPA Robotic Servicing of Geosynchronous Satellites (RSGS) Program Fact Sheet," Parabolic Arc, Feb. 9, 2017, URL: http://www.parabolicarc.com/2017/02/09/darpa-robotic-servicing-geosynchronous-satellites-rsgs-program-fact-sheet/
21) "DARPA Selects SSL as Commercial Partner for Revolutionary Goal of Servicing Satellites in GEO — PPP (Public Private Partnership) to develop on-orbit robotic servicer that would radically lower the risk and cost of space operations," Feb. 9, 2017, URL: http://www.darpa.mil/news-events/2017-02-09
24) "Call for a sustainable future in space," ESA, 21 April 2017, URL: http://m.esa.int/Our_Activities/Operations/Space_Debris/Call_for_a_sustainable_future_in_space
25) "International consensus on debris threat," ESA, 21 April 2017, URL: http://m.esa.int/Our_Activities/Operations/Space_Debris/International_consensus_on_debris_threat
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).