M
ost ground-based observatories require a dark
night sky to uncover answers to some of the
most fundamental questions about the nature
of our Universe. However, a number of com-
panies and governments are in various stages
of planning or deploying bright satellites in
low-Earth orbit (or LEOsats) in greater num-
bers than ever before. These “megaconstellations” will
fundamentally change astronomical observing at visible
wavelengths. Nighttime images will be contaminated by
streaks caused by the passage of Sun-illuminated satel-
lites. If proposals calling for 100,000 or more LEOsats
are realized, no combination of mitigations will be
able to fully avoid the negative impact on astronomy.
This threat comes at a time when new technology of-
fers unprecedented scientific opportunities, all requiring
access to dark skies. One example is
the Vera C. Rubin Observatory, which
is nearing completion. Its Legacy Sur-
vey of Space and Time (LSST) will
soon offer a dramatic new view of
the changing sky. Rubin Observatory
will employ the 8.4-m Simonyi Sur-
vey Telescope and the 3200-mega-
pixel LSST Camera to capture about
1000 images of the sky, every night,
for 10 years. A single 30-s exposure
will reveal distant objects that are
about 40 million times fainter than
those visible with the unaided eye.
The observatory’s combination of a
large light-collecting area and field of view is unpar-
alleled in the history of astronomy, which is why the
project was the top ground-based priority for U.S. as-
tronomers in the 2010 National Academies Decadal
Survey of Astronomy and Astrophysics. LSST six-color
images will contain data for about 20 billion ultrafaint
galaxies and a similar number of stars, and will be used
for investigations ranging from cosmological studies
of the Universe to searches for potentially hazardous
Earth-impacting asteroids. However, the discoveries an-
ticipated from Rubin and other observatories could be
substantially degraded by the deployment of multiple
LEOsat constellations.
The most exciting science to come out of current and
planned astronomical facilities may be the discovery
of types of objects and phenomena not yet observed or
predicted. Such profound surprises have the potential
to revolutionize our understanding of every field from
exobiology to cosmology. Rubin Observatory’s LSST, for
example, opens the prospect of observing how ultra-faint objects change over time. It is precisely this kind
of astronomy that is most at risk from image artifacts
arising from LEOsat megaconstellations. These satel-
lites scatter sunlight for several hours after sunset or
before sunrise, are relatively close and bright, and thus
can affect ground-based telescopes observing at visible
wavelengths. Constellations in orbits well above 600 km
will be illuminated by the Sun all night long.
Astronomers worldwide are seeking ways to diminish
the satellites’ most damaging effects—the focus of a re-
cent virtual workshop* sponsored by the U.S. National
Science Foundation—and are collaborating with SpaceX
(in particular, the Rubin Observatory), the first operator
to launch a substantial constellation of LEOsats. SpaceX
has shown that satellite operators can reduce reflected
sunlight through satellite orientation, Sun shielding,
and surface darkening. A joint ef-
fort to obtain higher-accuracy pub-
lic data on the predicted location of
individual satellites could help as-
tronomers point their instruments
to avoid some of the interference.
Although all of these measures are
helpful, there are no guarantees, and
the research community is left to
hope for good corporate citizenship.
Future constellations owned and
operated by foreign governments
pose a different sort of challenge. Al-
though there are international regu-
lations covering radio-frequency
interference, there are no such regulations in place for
visible-frequency light pollution from space.
Earth orbit is a natural resource without environmen-
tal protections, and we are now witnessing its industri-
alization. Currently there are about a thousand bright
LEOsats, but that may be just the beginning. Proposals
to expand telecommunications and data relay to serve
new technologies like self-driving cars could lead to a
100-fold increase in the number of LEOsats in the next
decade. The American Astronomical Society is work-
ing with astronomy stakeholders, commercial satellite
operators, and international organizations to begin to
forge policy on light pollution from space. It is unclear
how long this will take and how effective it can be. What
is clear is that without productive industry-observatory
collaboration, voluntary operator compliance with best
practices for mitigation, and subsequent regulatory ac-
tion, we are slated to lose a clear view of the Universe
and its secrets.
–Anthony Tyson and Joel ParriottDark skies and bright satellites
Anthony Tyson
is the Rubin
Observatory chief
scientist and a
professor of Physics
and Astronomy at
the University of
California, Davis, CA,
USA. tyson@physics.
ucdavis.eduJoel Parriott
is the deputy
executive officer
and director
of Public Policy
at the American
Astronomical
Society, Washington,
DC, USA. joel.
[email protected]*C. Walker et al., Bull. AAS 52 (2020); https://doi.org/10.3847/25c2cfeb.346793b8. 10.1126/science.abeSCIENCE sciencemag.org 25 SEPTEMBER 2020 • VOL 369 ISSUE 6511 1543EDITORIAL
“...we are slated
to lose a clear
view of the
Universe and its
s e c r e t s .”