Debris removal

A first analysis on the stability of the current environment independent of human measures was conducted by NASA in 2009, which examined a future scenario in which no further objects are added to the space environment (i.e. no launches, no debris release).

The results, which are confirmed by ESA’s simulations, show that the number of debris objects would continue to grow even under these idealised conditions – under which a collision rate of once every 10 years can be assumed.

This is a clear indicator that the population of large and massive objects has reached a critical density in LEO. In turn, this means that the number of large and massive (mostly physically intact) objects must be reduced.

3300 intact objects in LEO

The current LEO environment contains about 3300 intact objects. An ESA analysis shows that the (lower) level  of around 2500 intact objects (i.e. the status in the mid 1990s) would have a 50% probability of decreasing the overall debris population. If this is considered to be a desirable goal for remediation, the number of intact objects has to be reduced while the world’s spaceflight activities continue – which sees the placement of about 72 objects into the LEO environment per year (average over the eight years 2004-12). 

However, limiting the launch rate and a further reduction of the allowed lifetime (which are two options to reduce the number of intact objects) cannot be mandated and would not be very efficient.

Therefore, the only remaining option is to actively remove large objects now in orbit, which would provide several benefits:

  • The most critical objects can be removed from the environment first (the standard 25-year lifetime rule does take into account to environmental criticality)
  • Decommissioned objects can also be removed
  • A controlled de-orbit can be performed (as large removal targets typically are also most critical in terms of on-ground risk)

Studies at NASA and ESA show that with a removal sequence planned according to target mass, the environment can be stabilised when on the order of 10 objects are removed from LEO per year. Active removal can be more efficient in terms of the number of collisions prevented versus objects removed when the following principles are applied for the selection of removal targets:

  • The selected objects should have a high mass (they have the largest environmental impact in case of collision)
  • Should have high collision probabilities (e.g. they should be in densely populated regions)
  • Should be in high altitudes (where the orbital lifetime of the resulting fragments is long)

Long-term environment simulations can be used to analyse orbital regions that are hotspots for collisions. The most densely populated region in LEO is around 800- to 1000-km altitude at high inclinations. The collision hot spots can be ranked by the number of collisions predicted to occur under a ‘business as usual’ scenario.

High ranking hot spot regions are at around

  • 1000 km and 82 deg inclination
  • 800 km and 98 deg inclination
  • 850 km and 71 deg inclination

The concentration of critical objects in narrow orbital bands offers the advantage that removal missions can be designed for one orbit type and then repeatedly launched to perform their clean up. Besides the criteria above, an additional criterion would be the number of objects of one type contained in each of the hot spot regions.

Active debris removal
Active debris removal

The concentration of critical objects in narrow orbital bands offers the advantage that removal missions can be designed for one orbit type and then repeatedly launched to perform their clean up. Besides the criteria above, an additional criterion would be the number of objects of one type contained in each of the hot spot regions.

While removal targets should be selected from a global perspective, legal constraints dealing with the ownership of space debris objects, and the validation thereof, cannot be neglected.
Also it should be kept in mind that legal responsibility for a coupled remover/target stack (i.e. when a removal spacecraft attaches itself to a inoperative body for deorbiting) is shared. While removal technology should be generic, i.e. applicable to a wide range of removal targets that may also include non-ESA objects, special emphasis on firm agreements with the owners of the object are required.

Actions to counter the exponential growth of space debris, such as mitigation and active removal, are most effective when they are applied as soon as possible. The further the number of critical intact objects in the environment deviates from a sustainable level, the more objects will have to be removed to suppress any additional growth and the multiplying effects thereof.
ESA’s internal studies have shown that continuous removal actions starting in 2060 will only have 75% of the beneficial effect compared to an immediate start.

Last update: 19 April 2013

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