Sky & Telescope - March 2020 - 20

Satellites vs. Stars

The Space Debris Problem
Megaconstellations' potential impact on astronomy is outweighed by their potential to contribute to the growing space
debris problem (S&T: July 2018, p. 34). Only about 1,950
active satellites were orbiting Earth in early 2019, but SpaceX
alone aims to increase that number by 80% by the end of
2020, and by a factor of seven within the decade. These satellites, as well as their accompanying deployment structures,
are virtually guaranteed to leave debris behind.
They would challenge a tracking system that's already
stretched thin. The U.S. Air Force 18th Space Control Squadron uses radar to track more than 20,000 pieces larger than
10 cm, large enough to shatter satellites and contaminate
the space environment. Based on this number, statistical
models run by the European Space Agency (ESA) indicate
that there are 900,000 pieces between 1 cm and 10 cm. Even
one of these smaller but speedy pieces could end a mission.
"The energy equivalent of a collision with a 1-cm object is the
explosion of a hand grenade," says Holger Krag, head of the
ESA's Space Debris Ofﬁce.
Dead megaconstellation satellites would only add to the
current menagerie. With its ﬁrst launch of 60 satellites,
SpaceX reported that three had lost communication with
ground stations before they reached operational altitude and
were deorbiting naturally. Reentry is expected within a year,
in accordance with NASA's guidelines. SpaceX says two additional satellites are being intentionally de-orbited.
However, that doesn't seem to be the full story. "When
you look at the reality, the objects' current altitude, there
are a number of objects that behave oddly," Langbroek notes.
Observations by the SeeSat-L group show that 10 of the satellites had not reached operational altitude by the end of October, and SpaceX has not commented on the ﬁve unaccountedfor satellites that have not reached their station.
Even a small percentage of failure is too much, Krag says.
"The sheer number, even with good behavior on individual
satellites, causes worry."
Another concern is coordination between satellites.
Starlink satellites are equipped with autonomous collision
avoidance, a feature SpaceX argues puts it on the leading edge
of on-orbit debris mitigation. Krag says that ESA's operators perform about one debris avoidance maneuver per year
per satellite. Meanwhile, the Starlink satellites had already
performed more than a dozen automatic maneuvers within a
couple months after launch.
But that works best when facing something inert;
encounters between active satellites will require coordination between satellite operators. "Today, there is not even a
phone number that I can call," Krag says. "This trafﬁc is not
organized at all." That lack of direct communication caused
the ESA to redirect its Aeolus wind-monitoring satellite in
t SPACE DEBRIS This artist's impression of objects in low-Earth orbit is
based on actual density data, but the size of the objects has been exaggerated to make them visible at the scale shown.

MARCH 2 02 0 * SK Y & TELESCOPE

ESA

lites, though, will use portions of the Ka (26.5-40 GHz) and
Ku (12 to 18 GHz) bands for communication, and growing
megaconstellations mean this frequency space will increasingly have interference.
To deal with this encroachment, astronomers will have to
pursue a combination of technology development and coordination. Right now, for example, most receivers saturate when
they come within tens of degrees of a radio-emitting satellite.
So National Radio Astronomy Observatory (NRAO) scientists
are working to build new receivers that are more resistant to
interference.
Coordination with satellite companies will likely play an
important role as well. So-called dynamic sharing, in which
satellites stop transmitting a particular frequency when
passing over ground-based radio telescopes, could enable
radio astronomers to access parts of the spectrum that might
otherwise be unavailable. "I think the technology that we're
seeing in these new satellite systems provides a lot of opportunities to not only protect radio astronomy but also expand
access to the spectrum," Zauderer muses. "Lots of potential
for win-win for everybody here."
It's worth noting, though, that the technology - and
therefore the opportunities - vary between satellite constellations. OneWeb, for example, cannot dynamically share
radio frequencies because its satellites' beams are both ﬁ xed
and, due to its satellites' higher orbits, necessarily much
larger than Starlink's steerable beams.

Sky & Telescope - March 2020

Table of Contents for the Digital Edition of Sky & Telescope - March 2020