At small (sub-millimetre) particle sizes, meteoroids can prevail over space debris in some orbital regions, in particular during intense seasonal meteoroid streams, with peak activities close to perihelion passes of the related source comets (e.g. the Leonids in 1966 and 1999). The ESA Interplanetary Meteoroid Model (IMEM) describes the resulting meteoroid environment, also outside Earth-bound orbits.

DELTA: Assessing mitigation effectiveness

In order to study the effectiveness of debris mitigation measures on the debris population stability, long-term forecasts are required to determine future trends as a function of individual mitigation actions. This kind of analysis can be performed with ESA's DELTA tool (Debris Environment Long-Term Analysis).

DELTA is a 2- to 3-dimensional, time-dependent, dynamic debris model, with detailed traffic model and release event data, and with statistically generated collision events, based on local object densities and collision probabilities. It is built on the mathematical principles of MASTER. Starting from a current MASTER population, DELTA analyses the effects of traffic variations and different debris mitigation measures (e.g. explosion prevention, re-orbiting at end of mission, de-orbiting at end of mission, lifetime reduction) on the future evolution and stability of the space debris environment up to super-GEO altitudes.

The time spans covered in such projections are typically 100 to 200 years. The particle sizes considered by DELTA are larger than 1 mm (as compared to 1 micrometre for MASTER), since only these are relevant for risk assessments. DELTA uses a corresponding MASTER debris population as a starting point for its projections. An underlying traffic model statistically distributes the contributions of launches, explosions and solid rocket motor firing events into different orbit regimes of the Earth environment.

The resulting collision risk is dynamically determined from the actual status of the environment at each epoch, and in each altitude regime. Collision events are statistically triggered - and fragments are added to the environment - if a given kinetic energy threshold is exceeded. The so-called 'business-as-usual' scenario (unchanged operational practises) forms the baseline for comparisons with alternative approaches in which mitigation measures are applied. It can be shown that business-as-usual space activities lead to a progressive, uncontrolled increase of object numbers, with collisions becoming the primary debris source within less than 50 years. The removal of mass from orbit turns out to be the most effective way of preventing this collisional cascading process from setting in.

To make their analyses more accessible by mission planners, and by spacecraft designers and manufacturers, ESA has published a "Space Debris Mitigation Handbook." This comprehensive document provides an overview of all major space debris research disciplines, with a large number of tables and charts to characterise the space debris environment, to determine related risks, and to outline effective protection and mitigation measures.