ROOM
19
Special Report
preventing the proliferation of space debris and
enabling the sustainable use of space.
Space debris mitigation
In 2002 the Inter-Agency Space Debris
Coordination Committee (IADC) drafted a set of
guidelines for international space debris mitigation,
aimed at limiting the generation of debris in the
environment in the short-term, through measures
typically related to spacecraft design and operation,
and the growth of the debris population over the
longer-term, by limiting time spent in the LEO
region after the end of mission to 25 years.
In view of the long-term mitigation guidelines,
the orbital altitude selected by most of the
constellations is a concern. While the remnant
atmosphere in near Earth space automatically
limits the orbital lifetime through the decelerating
forces of atmospheric drag, this effect diminishes
with increasing altitude.
Spacecraft above 600 km typically cannot rely
on natural decay but have to actively reduce their
orbital lifetime by manoeuvring to lower altitudes
after the end of mission. Failing to do this has led
to the current deteriorated environment in the
800-1000 km zone. The peak spatial density of
man-made objects in this region is a result of the
absence of the atmospheric cleansing effects in
combination with frequent use.
The operating altitudes now selected by most
of the mega-constellations correspond to quasi-
eternal orbital lifetimes. They are not fail-safe in
the sense that satellites have to rely on a technical
function that actively lowers the orbital altitude
by several hundreds of kilometres after the end
of mission – and the success of this procedure
cannot be guaranteed.
Analysing environmental effects
European Space Agency experts together
with space debris specialists from UK, France,
Germany and Italy, analysed the response of
the orbital population to the introduction of a
hypothetical large constellation. The constellation
comprised 1080 satellites, each weighing 200 kg
with a cross-section of one square metre and
a five-year design life, distributed in 20 orbital
planes at an altitude of 1100 km and an inclination
of 85 degrees. It was assumed to be complete and
operational for 50 years from 2021 to 2071. This
constellation was taken to be indicative, but not
an exact representation of envisaged systems,
so that it would remain valid in view of changing
designs and would avoid a focus on particular
constellation proposals.
For the purposes of the simulations, the
background (non-constellation) population
consisted of all objects larger or equal to 10 cm
in size, wholly residing in or crossing the LEO
region on 1 January 2013, and derived from the
ESA Meteoroid and Space Debris Terrestrial
Environment (MASTER) model.
To study the effect of existing guidelines in view
of the constellation satellite traffic and behaviour
in isolation, in a first step, all other simulated
objects were assumed to have a high post-mission
disposal (PMD) success rate. A Monte Carlo (MC)
approach (using repeated random sampling)
was used to simulate the evolution of the object
population over a period of 200 years from 1
January 2013 and under different post-mission
disposal requirements.
The results revealed that the current mitigation
measures are suited to level-off the growth in the
number of objects greater than 10 centimetres
in the background space-traffic. Adding the
constellation scenario to this, various phases
could be distinguished. The first phase was
a steep population increase, which extended
from the launch of the first satellites of the
constellation until these satellites started to re-
enter due to post-mission disposal manoeuvres
0
20000
40000
60000
80000
100000
2000
2050
2100
2150
2200
Number of Objects in LEO > 10cm
Year
Background 90% PMD + Constellation 50% PMD
Background 90% PMD + Constellation 80% PMD
Background 90% PMD + Constellation 90% PMD
Background 90% PMD + Constellation 100% PMD
Background 90% PMD - No constellation
Some future scenarios suggest the space debris
population has reached a tipping point such that
collisions will increase in frequency even in the
absence of new space traffic
Simulation results for
the evolution of the
population of objects
greater than 10 cm with
the background space
traffic implementing
mitigation measures
(with 90 percent success)
and the effect of a
mega-constellation
implementing mitigation
measures at various
success rates.
