ROOM
21
Special Report
However, small satellites of between 100 kg
and 1000 kg, which are often equipped with
propulsion systems, had a success rate of less
than five percent for actively clearing higher
orbits. It is a concern that the number of objects
in this size class is expected to rise in view of the
announced constellations.
A post mission disposal success rate of such low
order is also observed in currently operated large
constellations in higher LEO orbits, such as those
of Iridium and Globalstar. As previously shown,
low success rates have dramatic effects on the
environment and success rates of 20 percent or lower
even caused difficulties to the simulation models
due to the intensified onset of collisional cascading
leading to unmanageable fragment numbers.
A quantum leap in the reliability of the employed
post mission disposal technology and operations
paradigms is required in order to raise the current
level of less than 20 percent to almost 100 percent.
Background traffic influence
Simulations shown so far assumed that all other
spacefarers (the background traffic) adhered to
post mission disposal guidelines with a success
rate of 90 percent. Examining recent performance
records, however, revealed that the currently
achieved rate is actually around 60 percent.
The simulation was therefore repeated using
the more realistic assumption of 60 percent PMD
success rate for the background space traffic.
When combined with a mega-constellation
adhering to the guidelines with 90 percent success
(i.e. the inverted scenario), the simulation showed
that the background traffic played a notable role
in the future trend. However, with the background
achieving 90 percent and the constellation 60
percent, the influence of the mega-constellation on
the environmental trends was much stronger than
the influence of the remaining spaceflight activities.
In other words, there is a considerable and
unprecedented responsibility for a single
operator to implement the technologies
and operations paradigm to preserve the
environment for all spacefarers.
Break-ups
In addition to PMD the technology and reliability
to be developed for the constellations will have
to address the prevention of break-ups. Even
modern spacecraft can break up e.g. in the case
of total failure preventing passivation (removal of
any internal energy). Large series of spacecraft
bear the additional risk that faulty parts will be
present in several spacecraft. Such parts can
0
20000
40000
60000
80000
100000
2000
2050
2100
2150
2200
Number of Objects in LEO > 10cm
Year
Background 60% PMD + Constellation 60% PMD
Background 60% PMD + Constellation 90% PMD
Background 90% PMD + Constellation 90% PMD
Background 90% PMD - No constellation
often reveal their detrimental behaviour long
after the mission, after the last of the spacecraft
series has been launched.
Such a chain of events was observed with the
NOAA Defense Meteorological Satellite Program
5D series, which were all found to be equipped
with a battery charger that made batteries
susceptible to explosive rupture.
Such a design flaw in a large series of mega
constellation satellites that are quickly launched
one after another would have disastrous effects
for the space environment in LEO.
Finally, it should not be forgotten that even
apparently high PMD success rates of 95 percent
still mean a five percent failure rate, often leading
to the loss of a non-passivated spacecraft. For
a mega constellation this could mean dozens of
break-ups per year with severe effects for the
constellation itself, but also every other spacefarer
in LEO.
We may conclude that international space
debris mitigation guidelines are adequate to
manage the space environment but in the
presence of one or more mega constellations
they only generate the desired effect when they
are perfectly adhered to. Today’s practice is far
away from the required success rates. Failing to
meet them will have detrimental consequences
for all future spacefarers.
About the author
Dr Holger Krag is head of ESA’s Space Debris Office at ESA/ESOC in
Darmstadt, Germany, and he represents ESA in the IADC. The Space
Debris Office plays a leading role in the research and development
of space surveillance technology in Europe and since 2015 it has
been studying environmental issues in relation to upcoming mega-
constellations jointly with European expert groups.
Simulation results for
the evolution of the
population of objects
greater than 10cm with
both the background
space traffic and a
mega-constellation
implementing mitigation
measures at various
success rates.
Some 46
satellites get
‘stranded’ in
space each
year
