Space scientists have long known that the cloud of manmade objects circling the Earth pose a risk to space launches and satellites.
But they also pose a growing problem to ground-based astronomy.
A few months ago, the American Astronomical Society (AAS) Board of Trustees issued a formal statement of concern, and last week, at an AAS meeting in Hawaii, attendees were discussing the proliferation of satellites as an “existential threat” to their entire field.
It’s a problem that came to the fore last May, when SpaceX launched the first cluster of 60 of its Starlink satellites, the vanguard of a plan to blanket low-earth orbit with 12,000 to 42,000 more such satellites in an effort to provide orbit-based wireless Internet service to the entire globe.
SpaceX Starlink satellites seen lighting up the sky above the Netherlands on May 24, 2019.
CREDIT: MARCO LANGBROEK
What caught everyone’s attention at the time, says Jeffrey Hall, director of Lowell Observatory in Arizona, US, was the sheer brightness of the things – which on initial deployment shone like a string of pearls roughly as bright as the main stars in the Big Dipper and the Southern Cross.
Prior to Starlink, says Patrick Seitzer, an astronomer at the University of Michigan, there were only about 200 satellites visible to the naked eye.
But if you add to this the ones already launched, plus another 1584 scheduled for launch this year, “that’s a nine-fold increase in less than one year” – not to mention future Starlink launches, plus an unknown number of other such “constellations” that might be launched for competing networks, if other companies get in the game.
“You can understand why this got everyone’s attention,” Hall says.
The problem, according to Seitzer, isn’t so much that bright satellites leave streaks across the type of time-exposed images astronomers use to capture dim, far-away objects. Rather, it’s that if these streaks are bright enough, they can saturate the detector, messing up its electronics.
That can do everything from producing ghost images to creating residual images the next time the same detector is used for another image.
“Even if [satellites] are not visible to the unaided eye, they could still cause problems,” he says.
It’s a particular issue, Seitzer says, for astronomers trying to do sky surveys using wide-field-of-view images, especially if they require long time exposures in order to detect faint objects.
“The large surveys of the sky basically came to an end in the early ‘90s because it was impossible to take a [long] exposure without having one or more satellite trails,” he says.
But there is good news.
SpaceX declined to be part of the public discussion at the AAS meeting, but that didn’t mean it wasn’t concerned. “We have had eight telecoms with SpaceX engineers and public relations people,” Hall says, adding that the company says it’s looking forward to finding a way to solve the problem. “I was happy to hear them say that.”
Seitzer notes that the first goal is to look for a way to reduce the brightness of future Starlink satellites. It will help, he adds, that the initial deployment of these satellites is only 350 kilometres above the surface, but they will, from there, migrate to 550 kilometres – enough to make them far less visible to the naked eye.
At that elevation, he says, “any effort to reduce their brightness should make them invisible to the unaided eye”. “That appears doable,” he says.
The second goal is to get them below saturation level on large telescopes. I don’t know the threshold for that,” Seitzer says, “[but] we’re trying to collect the information.”
“They [SpaceX] are on record saying they want to solve the situation,” Hall adds.
Harvey Liszt, a radio astronomer with the US National Radio Astronomy Observatory, adds that radio astronomers have been dealing with satellites since the 1970s, in their case because the transmissions they beam back to Earth – important for everything from weather monitoring to terrain mapping – can burn out expensive radio-telescope detectors if they happen to hit at a vulnerable moment.
“Some are so strong that even the backscatter from the ground will burn out a receiver,” he says. “These are things we’ve been dealing with for a long time.”
Ruskin Hartley, executive director of the International Dark Sky Association, agrees.
If you’d approached him about “light pollution” in the night sky a year ago, he says, he’d have been thinking about the backwash from urban lighting, which can so thoroughly blot out the view of the night sky that there are billions of people who’ve never seen the arc of the Milky Way across a truly dark sky.
“Eighty percent of the people in the world live under a light-polluted sky,” he says.
But the intrusion of manmade objects into the sky is an equally bad problem, he says. “How will it change our relationship to the world, and the universe?” he asks. “Is the future going to mediated by this false canopy of moving, artificial stars?”
Hartley admits that SpaceX’s goal of global internet connectivity is a gift to humanity. “It’s democratisation,” he says.
But somehow, he says, that must be done without permanently changing the night sky. “It’s the ultimate commons.”
Luckily, Hall adds, nobody is likely to be scared off by the difficulty of finding a win-win solution. “Neither astronomers nor space scientists are strangers to difficult problems.”