WHAT ARE SPACE DEBRIS?
According to the general definition proposed by the Inter-Agency Space Debris Coordination Committee (IADC) space debris are all “non-functional objects with no reasonable expectation of assuming or resuming its original function or any other function for which it is or can be expected to be authorized, including fragments and thereof “. This definition includes all non-operational satellites, spent rocket bodies, materials released during planned space operations and fragments generated by satellites and upper stage breakup due to accidental explosions or collisions.
SPACE DEBRIS ENVIRONMENT
From the beginning of the space activities, with the launch of Sputnik-1 in 1957, the number of objects sent in orbit has exponentially increased. Of these objects, only a very small quantity, re-entered back on the Earth, while the greater part is destined to remain in orbit for hundreds, even thousands years.
Presently, there are about 17,127 catalogued objects in orbit. Approximately 3,965 are intact satellites; 1,344, which corresponds to 7% of the entire population, are operational payloads, while 2,621, about the 15% of the entire population, are spent intact objects. The remaining 78% comprehends rocket bodies and fragments of different nature: 11% are rocket bodies, 11% mission related debris and the great amount, and 56% are fragments due to breakup events, accidental or intentional collisions or explosions. In Figure 1.6 it is represented the distribution in the three main orbital bands; as it can be observed, most of the objects, about 73%, are concentrated in LEO regions, where also the largest amount of mass can be found, while 8% are in GEO and 19% are in MEO.
Most of the debris were generated from satellites breakups. Two in particular where quite significant since they caused debris clouds that spread over the most frequented orbits, between 750 and 850 km. On January 11th the Chinese FENGYUN-1C satellite was disrupted by an interceptor missile, and created about 2,500 catalogued small objects. It was the first intentional breakup event ever in the space history. The second occurred on February 10th, 2009 and it was an accidental hypervelocity collision between two satellites, COSMOS and IRIDIUM, one of which was still operational (IRIDIUM). As of July 2012, nearly three and a half years after the event, 90% of the debris from the collision was still in orbit around Earth.
MITIGATION MEASURES AND INTERNATIONAL GUIDELINES
The continuous growth of space debris represents a threat for the future of the space activities. According to the theory of the Kessler Syndrome (1990), it is foreseen the development of a cascading process in which the growth of space debris will be dominated by a self-sustained phenomenon, triggered by initial collisions between intact objects and ultimately sustained by collisions between small fragments. This process, that could not be stopped once began, could make some regions in LEO unsafe for long time. For this reason, Space Agencies from all over the world established some general guidelines, resumed in the official document Nasa Safety Standard (NSS) 1740.14 Guidelines and Assessment Procedures for Limiting Orbital Debris, with the aim to limit and prevent the creation of further debris, intentional or accidental, in the future space activities, through protection and remediation activities.