Science 28Feb2020

SCIENCE sciencemag.org 28 FEBRUARY 2020 • VOL 367 ISSUE 6481 965

PHOTO: VICTORIA GIRGIS/LOWELL OBSERVATORY

S

tarting in 2022, the Vera C. Rubin

Observatory will survey the entire

night sky every few nights from a

mountaintop in Chile, using a giant

mirror to capture faint, fast-changing

objects. But something much more

mundane is likely to streak into view: thou-

sands of low-flying communications satel-

lites glinting in the Sun. The threat, evident

ever since rocket company SpaceX launched

its first batch of 60 Starlink satellites in May

2019, has come into sharper focus—as has a

potential way to limit the damage.

Unpublished studies, some seen by

Science, suggest satellite trails could ruin

about one-third of the images from the Ru-

bin Observatory during parts of the night.

Observatory staff say the problem cannot

be avoided with software tweaks or shrewd

telescope pointing. The best remedy seems

to be to darken the satellites themselves,

something SpaceX and the Rubin Observa-

tory are already exploring.

“Darkening is the name of the game,”

says Tony Tyson, chief scientist of the Ru-

bin Observatory. “I’m cautiously optimistic

we will get there with SpaceX.” But with

other companies starting to build similar

megaconstellations , astronomers are rac-

ing to keep ahead of the problem.

After the first launch of the Starlinks,

which aim to provide internet access to re-

mote parts of the globe, skywatchers could

see strings of bright dots with the naked

eye as the table-size satellites spread out

290 kilometers up. “We were totally sur-

prised by the brightness,” says Patrick

Seitzer of the University of Michigan, Ann

Arbor, a member of a working group set up

by the American Astronomical Society to

study the problem.

Just how big the problem will become is

hard to predict. Companies keep tight hold

of spacecraft design details, says aerospace

engineer Hugh Lewis of the University of

Southampton, who has adapted a space de-

bris model to help understand the bright-

ness of Starlinks. “Ideally we would want

to know everything about the spacecraft,

but that’s not the world we live in,” he says.

Astronomers also don’t know how big the

constellations will be: SpaceX, for exam-

ple, is aiming for an initial constellation of

about 1600 but has applied to the Federal

Communications Commission to loft as

many as 42,000.

Brightness measurements made by Las

Cumbres Observatory, a global network of

small telescopes, and others showed that

the Starlinks dimmed as they rose to their

working orbits 550 kilometers up. They are

brightest when they’re low on the horizon,

around twilight. Most telescopes can avoid

the trails, because they observe high in the

sky and have a narrow field of view. But

long exposures, a large mirror, and a wide

field of view all make a telescope vulner-

able. “Combine two or three of these and

you’re in trouble,” says Olivier Hainaut,

who is studying the problem for the Euro-

pean Southern Observatory.

The Rubin Observatory, with an 8.4-meter

mirror that will take pictures of the sky the

size of 40 full Moons in 30-second exposures,

“is the perfect machine for running into

these satellites,” Tyson says.

He and his team conducted simulations

that suggested the track of a satellite image

across their camera would saturate each

camera pixel as it passes, and cause leak-

age into neighboring ones. The resulting

artifacts “cannot be removed in software.

We have failed in doing that,” Tyson says.

The team looked at altering schedules to

avoid satellite trails, but with such a wide

field of view, avoiding thousands of satel-

lites would end up as “a wild goose chase,”

he says.

So Tyson is pinning his hopes on SpaceX

darkening its future satellites. He and his

team speak several times a week with en-

gineers at SpaceX, which launched one

darkened satellite in January that is just

now reaching its final orbit. Tyson’s team

calculated that if the company can reduce

reflections by a factor of 15, the issue will

be manageable. Images would still contain

trails, but they wouldn’t saturate pixels and

could be removed digitally. SpaceX and its

chief, Elon Musk, are “totally committed

to solving this problem,” Tyson says, and

his team has worked with them to “narrow

to a design that may work.” Several satel-

lites with this updated dark design will be

launched in coming weeks. SpaceX did not

respond to requests for comment.

Starlink is, however, just the first con-

stellation to get started. “It’s safe to say, the

number will rise dramatically,” says Connie

Walker of the National Optical-Infrared As-

tronomy Research Laboratory, who chairs

an International Astronomical Union com-

mission looking at the threat. The com-

pany OneWeb launched 34 satellites on

7 February—a first installment in an ini-

tial constellation of 648. The satellites are

smaller and orbit higher than Starlinks, so

they are fainter, but their altitude means

they’re illuminated by the Sun most of the

night. “They’ll need to darken them,” says

Tyson, who is in contact with OneWeb and

other operators, including Amazon, whose

Project Kuiper envisions 3200 satellites.

Tyson has his fingers crossed. The con-

stellations won’t kill the Rubin Observa-

tory, but they will make its job harder and

could jeopardize its chances of making

discoveries. “It’s there that the existential

threat is,” he says. j

Satellite megaconstellations

menace giant survey telescope

ASTRONOMY

Darkening the orbiters could limit damaging light trails

in wide-field images from the Vera C. Rubin Observatory

At twilight, streaks from Starlink satellites could ruin

about one-third of Vera C. Rubin Observatory images.

By Daniel Clery

Published by AAAS