State of Knowledge
Space debris is already an important part of the space environment which must be considered during planning and operation of a satellite or constellation of satellites. If larger objects are approaching the satellite, it is sometimes necessary to perform avoidance manoeuvres. The orbits of larger objects, those greater than ten centimetres, are tracked using ground based sensors. The orbital data catalogue maintained by USSTRATCOM contains only a small amount of the actual space debris population. The number of smaller, untrackable objects is very high. The population of sub-millimetre sized objects is derived from the analysis of impacts on retrieved satellites and, therefore, knowledge about the small particle population is limited to the orbit and the time span when these spacecraft were on orbit. Clearly, such a measurement regime does not provide a complete picture of the debris population. The total number of particles and their orbits cannot be determined solely from measured data. This information must be estimated by a model.
Space Debris Model
A model called MASTER-20091 has been developed at the Technical University (TU) of Braunschweig on behalf of the European Space Agency (ESA). This model reasonably predicts the particle flux for all orbits up to an altitude slightly above the geostationary orbit. The particle flux quantitatively indicates the number of particles that strike the surface of a satellite over a one year period – a normalized surface measure of one square metre is used. Measured data are used to validate the model. This modelling approach indirectly gives a good estimation of the expected particle count over a specified timeframe.
What is involved in modelling? At the TU Braunschweig a very sophisticated approach has been chosen. Every space debris-generating event ever documented in the history of space flight is simulated within the model. A forecasted debris cloud is generated which encompasses all particles larger than one micron in size. During a fragmentation event, each simulated piece of debris is assigned to its own orbit. Taking into account all perturbing forces that can occur in outer space, the orbits of these objects are propagated to a reference epoch. A huge debris population is represented for which orbital parameters are assigned to each particle. From this population, particle fluxes on satellite-based surfaces can be predicted. The model estimates many debris characteristics and parameters including: how many particles, from which direction they will impact a satellite, with what speed, and the size class.
The scientific challenge in the design of the MASTER 2009 model was threefold. First was consideration of higher orbital mechanics; the theory of orbital perturbations, necessary to represent the dynamic behaviour of debris distributions around the earth. Perturbing forces cause changes to the shape and the orientation of orbits. The second scientific task involved the understanding of and consideration for all known space debris sources. There exist many different sources of space debris which uniquely and / or differently contribute to specific orbits or specific debris components. For example, objects larger than one centimetre are predominantly the result of fragments from spacecraft explosions or collisions. The second largest source is slag particles from solid rocket motors. Furthermore, a very unusual source of space debris was observed that occurs only at 900 km altitude; liquid metal droplets which have been released from the cooling systems of space-borne nuclear reactors. Space debris less than a millimetre in size originates predominantly from other sources. For example, in the 100-micron class, the majority of particles on Low Earth Orbit (LEO) are so-called ‘ejecta’: particles generated by smaller space objects or micrometeorites impacting or colliding with larger debris or even satellites. Such collisions produce small craters, from which material is ejected. Their number is so high because they are permanently produced. The next dominant source of debris in this class is paint flakes, which continuously erode or flake away from spent rocket upper stages or satellites. Our research shows that every debris source has its own release mechanism, each based on unique or specific physical effects. These release events must be described by different models or model elements which are integrated. Only then it is possible to model the complete particle environment predictably with reasonable accuracy.
The third scientific challenge is to validate the modelled space debris environment using measured data and newly discovered debris sources. The aim is to achieve that the observed space debris environment is accurately reproduced by the model, a goal which requires many years of experience and continuous collection empirical data. Each source must be researched carefully, with their release or fragmentation mechanisms understood and modelled so to agree with the observations.
Model Estimations and Findings
The most important estimations stemming from our research in the field of space debris and the development of the model are summarized below:
- About 29,000 objects larger than ten centimetres are on earth orbit.
- The number of objects greater than five centimetres is approximately 60,000.
- About 700,000 objects larger than one centimetre orbit the earth.
- The millimetre population is close to 200 million particles.
- The number of sub-millimetre class particles is in the order of magnitude of some trillions.
Perhaps the most important finding was that the orbits facing the highest risk of collision today range from 800 km to 900 km in altitude. In almost all size classes, the largest quantity of objects occurs in this altitude band. This means that those space vehicles operating in these orbits are exposed to the highest risk of collision. The orbits at 800 km altitude are especially important for earth observation missions, since they are used by sun-synchronous satellites. Most avoidance manoeuvres must be performed at these altitudes. This orbital altitude requires special attention in terms of space situational awareness activities.
Besides the high probability of collision, simulations revealed that the spatial density of debris at the altitude close to 800 km is so high that a collisional cascading effect is plausible. This type of fragmentation event is called a ‘catastrophic collision’. The debris generated during such a collision could again trigger a new catastrophic collision and so on. Considering a typical LEO collision velocity of ten kilometres per second, an object with a diameter of ten centimetres has enough kinetic energy to completely destroy a spacecraft. This could result in a self-driven or cascading effect releasing further space debris, an effect known as the Kessler-Syndrome. A principle cause for concern is larger spent spacecraft with long remaining orbital lifetime and their potential for collision with large elements of debris.
To date, catastrophic collisions have not significantly contributed to debris generation. They occur, statistically, about every five to nine years. However, as the number of objects continues to accumulate on orbit, such collisions become more likely. The ability to model the space debris environment is not only a necessary scientific step to close space observation gaps, but it also provides an avenue to create awareness about the expected evolution of the risk to operating in space in the future.