Sky & Telescope - June 2021 - 19

VISIBILIT Y CO MPA RISON: PATRICK SEIT ZER (UNIV ERSIT Y OF MICHIG A N, A NN A RBOR);
V ER A RUBIN OBSERVATORY: RUBIN OBSERVATORY / NSF / AUR A

Number of satellites visible

as well as professional and amateur astronomers and dark-sky
advocates. They worked together to assemble guidelines for
both satellite and telescope operators to mitigate the threat
that numerous, bright satellites pose to astronomy, wide-ﬁeld
astrophotography, and stargazers.
In addition to the blanket 7th-magnitude limit, the group
also proposed an altitude-dependent brightness limit (starting with 7th magnitude for satellites at 550 km) and recommended that satellites ﬂy no higher than 600 km.
OneWeb's satellites are above that limit, at 1,200 km.
They're correspondingly fainter than Starlinks at ﬁrst glance,
with a median magnitude of 7.9. But due to their greater distance, OneWeb satellites travel at slower speeds and therefore
appear more in focus to telescopes. According to Tyson, the
surface brightness of a 7.9-magnitude, 1,200-km OneWeb
satellite streaking across an image is actually the same as a
7th-magnitude Starlink satellite at 500 km.
What's more, OneWeb's higher altitude means a signiﬁcant fraction of these satellites will remain visible throughout
the entire night during summer months. Lower altitudes are
therefore actually beneﬁcial to professional astronomy, as
they guarantee at least some unobstructed hours.
Despite the recommendation, OneWeb is unlikely to
change altitude: It already has approval from the Federal
Communications Commission to ﬂy there. The company did,
however, signiﬁcantly reduce the total number of satellites it
plans to launch, in part due to new ownership under the UK
government following a Chapter 11 reorganization.
Besides OneWeb and Project Kuiper, which together with
SpaceX could launch a combined tally of 54,000 satellites,
smaller companies are also looking to edge in on the market. " We have dealt primarily with the big three constellations, " Seitzer said. " But sliding under the telescope cover, so
to speak, there are many smaller constellations of 30 to 50
satellites. My concern is that we will get blindsided by them
as their numbers grow and grow and grow. "
Astronomers and even satellite operators agree that
depending on the goodwill of individual companies will not
sufﬁce in the long term. International regulations will be
key, in part to prevent companies from moving operations to
countries with more lax restrictions.
To that end, Connie Walker (NSF's NOIRLab) and colleagues have recently written up a report to be submitted to
the United Nations Committee on the Peaceful Uses of Outer
Space, outlining recommendations to protect dark skies and
including guidelines for satellite operators. The UN is not a
regulatory body, so if the plan is ultimately approved, it would
then go to member nations for policy and enforcement.
While academia, federal agencies, and international bodies
move along at an " Entish " pace, as panel participant Aparna
Venkatesan (University of San Francisco) put it for Tolkien
fans, the satellite-constellation industry continues to develop

140

1,000 km

Darkest time of night

120
100
80
60

500 km

40
20
0
-1

Universal time (hours)

S NIGHTTIME VISIBILITY This illustrative plot shows satellites' visibility
throughout a summer night at 30° latitude depending on their distance
above Earth's surface. At each altitude, the calculations divide 10,000
satellites amongst 100 orbital planes with orbital inclinations of 53°.
Observed altitudes are assumed greater than 30°. The gray area marks
when the night is darkest, between the hours of astronomical twilight.

at a fast clip. SpaceX aims to provide near-global coverage by
the end of the year, and OneWeb plans to have its ﬁrst-generation network of 648 satellites aloft by mid-2022.
Also in 2022, the Rubin Observatory is due to start full
operations, beginning the observations intended to generate
a decade-long movie of the night sky. But it will have to do so
while dealing with satellites too numerous to dodge and as
yet too bright to completely remove from observations.
At the AAS session, Tyson took umbrage at the idea that
observatories can take actions now to protect themselves in
the future as near-Earth space becomes more crowded. " This
notion of future-prooﬁng is a charming idea, " he said. " But
very frankly, we did a lot of simulations and found that there
is no combination of mitigations that we know of that can
correct for the lost science - particularly the discovery of the
unexpected. "

ANTHONY MALLAMA is a retired astronomer working with
the American Astronomical Society to measure satellite brightness. MONICA YOUNG is Sky & Telescope's news editor.

X UNDER CONSTRUCTION The Telescope Mount Assembly has been
assembled inside the Vera Rubin Observatory in Cerro Pachón, Chile. A
sliver of blue sky peeks through the dome above.
sk yandtelescope.org * JUNE 2 021

http://www.skyandtelescope.org

Sky & Telescope - June 2021

Table of Contents for the Digital Edition of Sky & Telescope - June 2021