Figure 1. Detection of the six ODERACS spheres by radar
Contractors:
Battelle Institute, (D),
CERT-ONERA/DERTS, (F),
FGAN (D), Zeiss Jena (D)
Max-Planck Institut für Kernphysik (D)
SAS (B),SIRA Ltd.(UK),
Technical Univ. of Brunswick (D),
University of Bern (CH)
Univ. of Kent at Canterbury (UK)
Funding:
General Studies, Basic Technology Research Programme, General Supporting Technology Programme.
The space environment which surrounds the Earth is becoming increasingly polluted with debris. Currently, the US Space Surveillance Network is using ground-based radar, optical and infrared sensors to track more than 7500 objects. The minimum size of a trackable object is about 10 cm for a low Earth orbit, and about 1 m for the geostationary orbit. Only about 6% of these catalogued objects are active satellites. Over 40% are fragments of disintegrated satellites and upper stages of rockets.
The vast majority of man-made debris comprises objects smaller than 10 cm which are not tracked during routine operations. Estimates of the numbers of objects larger than 1 centimetre range from 30 000 to 100 000. The basic hazard in space caused by debris is damage or even destruction by collision. Current risk levels are small, but are steadily increasing. For some space missions space debris has already become a safety issue and active protection through shielding will be needed; the International Space Station is an example.
Current debris control practices aim at reducing the growth rate. The total amount and mass of debris is still rising, since rate of removal of debris by natural mechanisms, such as air-drag perturbations, is exceeded by the rate of deposition in space of man-made objects. These practices, if unchanged, will sooner or later lead to a major increase in the amount of debris as a consequence of collisions between Earth-orbiting objects. To avoid the excessive and uncontrolled growth of space debris, ultimately more efficient and more costly mitigation measures must be taken. These will likely include the removal of rocket bodies and spacecraft from space upon completion of their missions. Such measures will have far-reaching implications for the design and cost of space systems.
Space debris is an inherently international problem whose solution requires international co-operation. The Inter-Agency Space Debris Coordination Committee (IADC) whose members are ESA, Japan, NASA, and the Russian Space Agency RKA, provides a forum for discussion and coordination of technical space debris issues.
ESA's research into space debris aims at achieving a better understanding of the critical issues, in particular the environment of the Earth and its future evolution, and measures for protection and mitigation. ESA is currently pursuing four main activities regarding the environment:
Figure 1. Detection of the six ODERACS spheres by radar
A mathematical model is under development at the Technical University of Brunswick to describe the debris and meteoroid environment at altitudes between the low Earth orbit and the geostationary orbit. The minimum size of an object is 0.1 mm. The model is based on the catalogued population and on known break-ups of spacecraft and rocket upper-stages in orbit. The initial distribution of fragments is described in terms of their position, velocity, mass. The objects are then propagated forward in time taking into account the relevant perturbations. To improve and validate mathematical fragmentation models, particularly for low-intensity explosions, Battelle Europe performed a series of explosion tests on simplified, scaled models of third stage mock-ups of the Ariane launcher.
The reference model can provide the debris flux for any particular orbit; from this the probability of a collision can be calculated for a given spacecraft. It is also used to predict the flux of debris of a given size, to dimension spacecraft shielding. In addition, the model provides data on the initial debris population, as an input to projections of future populations.
Current space debris models in low earth orbit suffer from significant uncertainties about objects smaller than about 50 cm. This is of particular concern for spacecraft which require protection, since it is not technically feasible to shield against objects larger than 1 cm. The feasibility of detecting and tracking medium-size debris (1 to 50 cm)with a high power radar at the Forschungsgesellschaft für Angewandte Naturwissenschaften (FGAN)in Germany was investigated. Figure 1 shows the signature of six ODERAC spheres taken with this radar at a 2000 km slant range. The diameter of the smallest sphere is 5 cm.
Optical observation methods are best suited for objects in high-altitude orbits. Detectors using charge coupled devices in combination with suitable digital image processing techniques offer promising prospects. For debris related research, ESA is upgrading a1 m aperture Zeiss telescope which will be installed in at the Teide Observatory on the Spanish island Tenerife (Figure 2).
Figure 2. One metre telescope, to be used at the Teide Observatory, Tenerife
The return of a satellite to Earth after exposure to the space environment provides important information on the population of micron to millimetre-size particles in space. Examples are the solar array of the Hubble Space Telescope (Figure 3) and the Eureca spacecraft. Although the LDEF (long duration exposure facility) carried experiments specifically designed to investigate the near-Earth particulate environment, the entire vehicle provides a record of the numerous impacts of meteoroids and space debris it suffered. In general the number of impacts agrees largely with current models. An inherent difficulty is to distinguish betweeen the impacts of meteoroids and man-made debris.
In response to the objectives relating to space debris formulated by the ESA Council in 1989, the Agency is conducting research with the aim of achieving a better understanding of critical issues. As already outlined above, significant progress has been made in preparing measurements, in measuring the sub-millimetre particulate population and in modeling the space-debris environment.
Preparing for the Future Vol. 4 No. 4.