AAS Statement on the Atmospheric Impacts of Spacecraft Reentries and Launches

The American Astronomical Society (AAS) is gravely concerned about the
impacts of emissions and residual effects from extensive space launches
and space object reentries on Earth’s sky and upper atmosphere. Tens
of thousands of new satellites are planned to be launched and then
replaced regularly in order to indefinitely maintain steady-state
populations of large satellite constellations, often referred to as
megaconstellations. The replacement process will involve disposing of
retired satellites by ablation in the mesosphere and stratosphere, and
launching new satellites to replace them. The AAS is concerned about the
potential impacts of the continuous launching of new spacecraft and
vaporizing old ones on our atmosphere, and in turn on ground-based
astronomy. We call on policymakers to secure funding for scientific
research on the aggregate effects of a growing number of launches and
reentries on the Earth’s climate and ozone layer, and on regulators to
incorporate the results of such research into their licensing of space
activities. 

Re-entry of spacecraft into the atmosphere at the end of their lifetimes
has become a common practice to mitigate the crowding of low Earth orbit
(LEO) and the generation of orbital debris. For example, the FCC now
requires satellite operators in LEO to dispose of their satellites
within 5 years of completing their missions, usually by having them
reenter the atmosphere. In planning for spacecraft reentries, the
“design to demise” principle, which involves satellites fully
burning up in the atmosphere, has been advocated for and adopted by many
space operators to minimize the risk of uncontrolled re-entries that
bear casualty risks and the potential to damage homes and infrastructure
(Byers et al. 2022). While this approach to end-of-life satellite
disposal helps reduce debris in orbit, and minimizes damage from
uncontrolled reentries, the choice to instead burn up satellites in the
atmosphere may pose a significant risk to the Earth’s climate and the
ozone layer, through the resulting alteration of atmospheric chemistry. 


A single large satellite constellation, composed of 42,000 satellites,
with steady state replacement on a five year cycle would require 23
satellites to be launched and deorbited each day. Given existing rocket
capacity to carry 20-50 satellites, this would require launches every
1-2 days. Moreover, with a typical satellite weighing of order 1000 kg,
and being primarily metallic, the steady state injection of metals into
the stratosphere by vaporization of satellites would be at least 8000
tons per year. This would exceed the natural injection of metals into
the stratosphere by meteoroids. For example, the aluminum deposition
from satellites would be more than double the natural injection by
meteoroids, which are less than 1% aluminum by mass (Schultz &
Glassmeier 2021, Boley & Byers 2021).

Recent measurements show that metallic inclusions from current rocket
and satellite re-entries are already present in at least 10% of
stratospheric background aerosols (Murphy et al. 2023). This fraction
will soon increase quickly, as the many planned satellite constellations
are deployed and these satellites begin to reach the end of their short
lifetimes. The effects of the expected increase in reentry rates and
associated metal pollution are completely unknown.

Pollution from reentries could potentially increase the opacity of
Earth's atmosphere (Keith 2000), further impacting ground-based optical
and infrared astronomy that is already plagued by satellite streaks. The
lithium deposition from re-entries will exceed natural deposition by at
least two orders of magnitude (Boley & Ostapenko 2023), and could induce
an airglow that increases the night sky brightness, limiting our ability
to study faint astronomical objects. The magnitude and severity of these
effects are still uncertain and need ongoing research to better
understand. This pollution could also cause ozone depletion and modify
the properties of the naturally occurring sulfate aerosols that play a
role in maintaining Earth’s climate, and further research is urgently
needed to understand the severity of the impacts. Reentry shock waves
will also cause atmospheric chemistry changes, with significant
production of nitrogen oxides (NOx), which could also contribute to
ozone depletion (Ryan et al. 2022).

The atmospheric effects of reentries could also be compounded by the
launches that are needed to replace the deorbited satellites. While the
CO2 emissions from an individual launch will not necessarily be
significant, launches deposit large amounts of black carbon, or soot,
into the stratosphere, which would lead to an increase in the
stratospheric temperature, potentially increasing ozone depletion
(Maloney et al. 2022). Launches also deposit considerable water vapor
into the mesosphere, leading to the formation of mesospheric clouds
(Voigt et al. 2013), further impacting ground-based astronomical
observations. Both ascending rockets and deorbiting secondary boosters
create holes in the ionosphere, which create a temporary, but bright,
induced auroral effect. Frequent launches and reentries of large rockets
may begin to consistently impact our ability to carry out astronomical
observations. 

We recommend that regulators and policymakers seriously and
substantively consider the issue of atmospheric pollution from satellite
launches and reentries. We call on policymakers to provide funding to
conduct urgent scientific research in order to provide stakeholders with
a timely assessment of all spaceflight emissions and their effects. Such
research will require coordination with satellite operators and
manufacturers to incorporate accurate projections of re-entry rates and
data regarding the metallic composition of satellites. We further urge
regulators to incorporate the results of this research into their
licensing of space activities. This could include setting a cap on the
flux of reentry aerosols into the stratosphere that is informed by
scientific research on reentry emissions and changes to the global
atmosphere. While such research is being carried out, we strongly
encourage abiding by the precautionary principle.