ROOM
20
Special Report
(30 years after their launch in this scenario). In
the short second phase, from 2051 to 2071, the
satellite replacement rate and the re-entry rate
were almost the same (differing only because 10
percent of the constellation satellites failed to
meet the disposal requirement).
The third phase showed a steep population
decrease, which started at the end of the
constellation mission and ended when the last de-
orbited satellite re-entered 25 years later and the
final post-constellation phase showed a gradual
population growth that depended only on the
behaviour of the background population and on
the effects caused by the constellation satellites
that did not meet the disposal requirement.
To analyse the effect on the distinct phases
identified above and arising from different
implementation levels by the constellation
satellites of a 25-year PMD requirement, we
compared the trends for PMD implementation
levels of 50 percent, 80 percent, 90 percent and
100 percent. It was very apparent that the long-
term evolution of the LEO population beyond
this phase was greatly influenced by the extent of
successful implementation of the PMD guidelines
among the constellation satellites.
The lower the success rate, the higher the
number of constellation satellites that were
left in orbit. This accumulation of satellites,
and their interaction with the background
population, resulted in a detrimental evolution of
the population e.g. a doubling of the population
by 2071 and a five-fold increase by the end of
the projection period for the 50 percent PMD
implementation level.
The successful implementation of post-
mission disposal by an average of more than 90
percent of the constellation was required for an
invariant population in the long-term. If such
invariant behaviour was to be guaranteed even
for a scenario with more than one constellation, a
success rate of 100 percent was needed.
Post mission disposal
The analysis above raises the question of the
success levels that can actually be expected to
come with the mega-constellation - which is a
difficult one to answer. A first indication could,
however, be obtained from a review of the success
rates observed in the recent past.
ESA experts regularly report on the global
achievements in post mission disposal. This
information is gathered from the automated
analysis of surveillance data on LEO and
geostationary (GEO) objects. About 114 satellites in
LEO reach end of life per year. Since the beginning
of the analysis efforts in 1990 the annual PMD
success rate has wavered around a level of about
60 percent, which means that roughly 46 satellites
get ‘stranded’ in space each year.
A closer look at the successful cases reveals
that the clear majority never had to implement
any post mission disposal, but the original mission
orbit was low enough to ensure natural re-entry.
Of those satellites on higher orbit only a small
fraction performed a de-orbiting attempt and
only half of the attempts actually lowered the
altitude sufficiently. In recent years there has
been a weak trend towards implementing post
mission disposal attempts that reached a success
rate of only 20 percent.
Breaking down this analysis for satellites per
mass class reveals that satellites of less than 10
kilograms (typically cubesats) obviously benefit
from the low mass-to-area ratios and large
satellites (greater than one tonne) benefit from the
capability to perform manoeuvres and the fact that
most of these missions occur in low altitude orbits
(e.g. Cosmos-type Earth observation satellites).
Currently observed
practice in implementing
post mission disposal for
spacecraft in LEO (less
than 2000 km) and
historical trends (top: all
spacecraft, bottom:
spacecraft that require
active disposal
manoeuvres).
The operating
altitudes
now selected
by most of
the mega-
constellations
correspond
to quasi-
eternal orbital
lifetimes
