Currently there are about 150 million artificial objects with diameters of one millimeter or more in Earth orbits. Even the smallest objects pose a threat to manned and unmanned space flight due to high relative velocities. The impact of a space debris object on a satellite can cause its damage or failure. Thus, space debris forms an economic risk for the satellite operator. It is important for space agencies and satellite operators to know the hazard potential caused by space debris. This caused the development of the MASTER model (Meteoroid and Space Debris Terrestrial Environment Reference Model) at the Institute of Aerospace Systems under ESA contract.
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The modelling of motion and distribution of artificial objects on Earth orbits (space debris) forms a research focus at the Institute of Aerospace Systems (TU Braunschweig). In the recent past the institute worked intensely on this field in the framework of the MASTER project. MASTER is the European model for the assessment of risks caused by hyper-velocity impacts of space debris on satellites. Led by the Institute of Aerospace Systems MASTER has been developed in coorporation with several European partners under ESA contract. MASTER is based on a very complex model of the space environment to calculate the spacial density and the velocity distribution of space objects, including natural meteorides.
Sources and sinks of space debris
Fragments from explosions form the part of the space debris population with the highest spacial density and therefore pose the highest thread to many orbits. Such objects originate from unintended explosions of remaining fuel from burnt out rocket stages, from chemical reactions in non-discharged batteries and from deliberately destruction of military satellites, a procedure which had been carried out often in the past.
Further important sources of space debris are slag and dust from solid-rocket motors. Common solid-fuel engines include about 18 percent aluminium to improve the efficiency and to reduce instabilities during combustion. The aluminium is converted to aluminium oxide and at the end of combustion slag particles emerge from the aluminium oxide which can have diameters of up to the centimeter regime. Furthermore, dust particles with a diameter of up to 50 micrometers are produced during the combustion process.
The simulations at the institute have shown that longterm measures for avoidance and reduction of space debris are able to keep the space debris density constant in the future. Moreover, in some orbits a reduction is possible. The simulations also revealed that without mitigation measures the collision rate between space debris objects would increase and a cascade effect would set in. This chain reaction would rapidly increase the space debris density. Space flight would become impossible. To avoid this, the following measurements can be carried out:
reduction of mission-related objects (latches, rocket parts, caps)
avoidance of rocket stage explosions with release of residual fuel and discharge of batteries at the end of a mission
removal of spent stages and satellites at the end of a mission
Space debris is located mainly in two areas: On low orbits several hundreds of kilometers above Earth, and in the geostationary orbit. The geostationary orbit is also called GEO or 24-hour-orbit and is situated in 36,000 kilometers height above the equator. Space debris particles on low orbits have a velocity of about 7 kilometers per second. In a collision of a space debris object and a satellite the collision velocity is about 10 kilometers per second on average. Space debris objects with a diameter of one millimeter can damage a satellite. Space debris objects with a diameter of one centimeter destroy a satellite. Centimeter objects trike through every hull, even if it is surrounded by additional protection hulls. At the moment the risk of a collision with space debris on a particular mission is not dramatic yet. However, it is necessary to implement measures for space debris mitigation, even if the mission costs rise as a consequence. An uncontrolled increase of the space debris population can be avoided with these measurements.
Contributions to the space debris environment
Space debris consists of artificial objects with a variety of sizes, compositions and origins (see Figure 1). Well-known contributions to space debris are discarded satellites, mission-related objects and fragments from numerous explosions and a few collisions.
Figure 1: Objects in near-earth space and their respective contribution to space debris environment, including meteorides
Mission-related objects are parts, such as strings, bolts and caps, which are set free during operational processes (stage separation, starting of optical sensors and engines on orbits). A portion of the space debris, especially on low orbits with a size of 10 centimenters or more, can be observed continuously from the ground with radars. The trajectories of these objects are tracked and recorded.
Their trajectories, the so-called "Two Line Elements" (TLE), are published in a catalogue edited by the "United States Strategic Command". This radar catalogue contains all objects with known orbits. At the moment, 9,000 objects are listed. The estimated total number of objects with a diameter of greater than 10 centimeters at all heights including the geostationary orbit is around 20,000. About 12,000 objects have originated from explosions. The remainder are mission-induced objects and discarded space vessels. These objects, including the 500 active satellites, are called "TLE-Objects" or "radar catalogue objects".
In addition to the space debris mentioned before there is a contribution from liquid metal droplets, which consist of an alloy of two alkali metals, sodium and potassium (NaK). The liquid sodium-potassium alloy had escaped from nuclear reactors, where it was used as coolant. Sodium-potassium droplets only occured in the period between 1980 and 1988. They have a size of up to 200 μm. Computer simulations show that small droplets with diameters of few millimeters have descended and are not in space any more.
Slag particles from solid-fuel rocket engines also contribute to the space debris environment. To a large portion they consist of aluminium oxide (Al2O3). In addition to the slag particles, a very fine Al2O3-dust emerges from solid-fuel engines.
The smallest space debris particles are, aside from the Al2O3-dust, the so-called "ejecta" and paint particles. Ejecta are ejected particles generated from the impact of small objects on surfaces. The detachment of paint particles is a consequence of surface degradation, which are caused by atomic oxygen and the effects of solar radiation. This can lead to the detachment of small particles, such as paint particles or pieces of multi-layer insulation.
Clusters of short and thin copper wires are situated in an orbit close to 3,600 kilometers. They originate from two experiments in the framework of the West Ford project in the early sixties. The wires were supposed to serve as dipole antennas and are also called "West Ford Needles". The clustering was an unintentional accompanying effect of the release processes. The contribution of the clusters to the space debris population is considered very low.
Simulation of the space debris distribution
The description of the total orbital population based on measurements seems to be most acceptable, as solid measured facts are used. However, continuous ground-based radar measurements are only available for objects with diameters of 10 centimeters or more. These objects are recorded in a radar catalogue. In contrast, the distribution of smaller objects has been measured merely sporadically. Data were collected during special radar measurement campaigns and from the analysis of impacts on satellite hardware, which had been carried back to earth. Thus, the population of smaller objects were merely estimated for specific times and orbits. In between there are huge temporal and spacial gaps. However, the emergence and disappearence of orbital objects, their temporal and spacial distribution is a dynamical process, which cannot be described adequately with isolated measurements. This process is consequently simulated on a computer by a mathematical model. Data from measurements are used to support the model. The model is based on the simulation of events, in which space debris has been produced (e.g. explosions of space vehicles). Object clouds are generated during the simulation and orbit elements are assigned to the objects. The different orbits of the simulated objects are calculated with all disturbing forces for a specified period, the so-called reference period.
Hazards caused by space debris
The risk potential of space debris lies in the high kinetic energy, which can be released at the high velocities during a collision. These collision velocities are in the order of 10 kilometers per second. Objects with a size of one millimeter or larger are a hazard for space vehicles. They can damage the satellite structure. Objects with a size of one centimeter pose a special risk, as they are able to destroy a space vehicle. They pass through any structure, even if it is surrounded by multiple hulls for protection. These objects release the energy of a hand grenade in the case of a collision. It is not possible to observe these objects, the orbits of objects smaller than 10 centimeters are unknown. Thus, they are very hazardous. A collision with a space vehicle cannot be predicted.
Liquid-metal droplets in space
Space debris consists of artificial objects of different size, composition and origin. Well-known contributions to space debris are spent satellites, objects which were released on a satellite mission (protection caps etc.) and fragments from numerous explosions and a few collisions. Further contributions were discovered in recent years and included in the MASTER model. One of these is the contribution from liquid-metal sodium-potassium droplets.
Liquid-metal droplets were released during the operation of nuclear reactors in the eighties. These reactors with the Russian name "Buk" (English "beech") were used for power generation on RORSAT satellites, which used radar for ocean monitoring. After their mission the reactors were transferred to higher orbits between 900 and 950 kilometers height to remain there. The nuclear fuel was separated from the reactor and ejected into space. In particular, the reactor container opened and ejected the reactor core, which consisted of a small package of 37 uranium fuel rods. The total number of reactor core ejections was 16. During the opening of the reactor container also the primary cooling cycle was opened. The reactor container and the cooling cycle were pressurized. The pressure was probably released instantaneously during the opening of the container. The liquid coolant, a liquid-metal alloy which consists of sodium and potassium (NaK), escaped into space. The liquid formed spherical droplets which are still in space. The liquid-metal alloy has a very low evaporation rate and the droplets can exist in space. Sodium-potassium droplets were released from 1980 to 1988. They have sizes from a tenth of a millimeter up to 5 centimeters. Computer simulations show that small droplets with diameters of few millimeters have descended and are not in space any more.
Today, the centimeter-scale droplets can be found mainly in a narrow belt in an orbit at 900 kilometers height. In contrast, most of the other contributions to space debris can be found in all heights up to the geostationary orbit. At the beginning of the nineties 50 percent of the centimeter-scale space debris in this orbit were liquid-metal droplets. The droplets decay due to atmospheric drag and finally reenter in the upper layers of the atmosphere. The number of droplets in space is decreasing continuously due to this "self-cleaning effect".
Copper needles in space
Several hundred million short thin copper wires were released in the early sixties in orbits at about 3,600 kilometers height. This happened during two experiments of the West Ford project. The wires were supposed to serve as dipole antennas and are also called "West Ford Needles". The goal of the West Ford project was to create a belt of dipole antennas around the globe to reflect radio waves. These experiments were used for communication purposes. The experiments were launched as secondary payloads in the years 1961 and 1963 on the satellites Midas 4 und Midas 6. The clustering was an unintentional accompanying effect of the release procedures. On the chosen orbits the solar pressure continuously reduces the closest point to the surface, the so-called perigee, until the dipoles reach the upper atmospheric layers and leave their orbit. This fine adjustment between orbit and the direction of the sun is called resonance. The resonance condition is met on certain orbits only. However, the needle clustering reduced the ratio of area and mass (A/m) of the objects. The impact of the solar pressure on the descending behaviour of the clusters was lower than expected. The lifetime of the needles increased and many of the objects are still in space and contribute to space debris. Computer simulations have shown that the clusters of the first experiment have very long lifetimes. The total mass of all clusters in space is estimated to be 60 grams. The total number of clusters is 40,000 with a total of 750,000 needles. The needles are razor-thin and have a length of about two centimeters.
Two reasons were identified for the durability of the clusters. The reduction of the A/m ratio after the clustering of the needles and an incorrect orbit in the case of the first experiment, where the resonance condition is not met. The clusters from the first experiment are expected to stay in the orbit for a very long time. The contribution of the clusters to space debris is negligible small. However, the fine copper wires can form long chains which can be visible through ground-based radar.