Will Elon Musk’s Starlink satellites harm astronomy? Here’s what we know.

If SpaceX CEO Elon Musk has his way, Earth’s skies will soon be spangled with about 12,000 false stars—the speeding, reflected gleams from a mega-constellation of telecommunications satellites collectively called Starlink. Once complete, perhaps in the mid-2020s, Starlink will ostensibly connect the entire planet to the internet, providing fast broadband access to remote areas as well as planes, ships, and cars.

Already, Musk has put the first phase of his plan into action. On May 23, a SpaceX Falcon 9 rocket carried 60 Starlink satellites into orbit. But almost as soon as the fleet deployed, it ignited conversations about the ethics of a single company unilaterally changing the night sky’s appearance. (Find out more about the hazards of light pollution, from bird deaths to human health impacts.)

Despite Musk’s initial assurances that the satellites will be barely noticeable, the train of spacecraft has been captured marching across skies around the globe, and some online tracking services can even calculate when and where someone can spot them sailing by overhead.

Astronomers have also raised concerns about the constellations’ effect on ground-based astronomy and how the minifridge-size satellites will add to an already jammed orbital environment.

What exactly is Starlink, what are its potential impacts, and is there a way to prevent it and similar projects from contaminating our celestial vistas? We’ve got you covered.

What is the Starlink master plan?

Ultimately, Musk intends to launch nearly 12,000 Starlink satellites into low-Earth orbit, potentially occupying altitudes between 217 miles and 740 miles. Already, 60 satellites are circling Earth, gradually firing krypton-powered thrusters every 90 minutes to raise them into their intended orbits; within a year, Musk plans to put roughly 720 satellites into space.

Once in place, the complete orbital array will provide fast, uninterrupted internet to the entire planet, Musk says.

How do the satellites work?

In space, the solar-powered, 500-pound satellites communicate with one another through both optical and radio links; the entire network will then be connected with user terminals on the ground that can be installed basically anywhere with a view of the sky. With a large enough array of satellites passing overhead, internet service should be continually available, unlike the delays in connectivity that are common with current Iridium satellites and other networks.

As well, SpaceX says each satellite will be “capable of tracking on-orbit debris and autonomously avoiding collision,” and that 95 percent of the satellites’ components will quickly disintegrate in Earth’s atmosphere during de-orbit at the ends of their lifetimes.

That sounds like a useful goal. Why are astronomers upset?

For starters, low-Earth orbit is already quite crowded. As Musk himself has noted, nearly 5,000 satellites currently swarm the planet’s immediate environment, and Starlink alone is poised to triple that number. Though the Starlink satellites are designed to not survive re-entry through Earth’s atmosphere, they could still pose a substantial problem if they are damaged in orbit, or take a while to come apart.

“Even at that altitude, debris can stay in orbit for a long time,” Flinders University researcher Alice Gorman posted on Twitter. “I don’t trust glib reassurances from satellite operators without precise details.”

How visible will they be from Earth?

At first, estimates of the satellites’ visibility suggested they’d be among the brighter objects in evening skies—around apparent magnitude 2, only a bit dimmer than Polaris, the north star. Now, though, estimates are somewhat more favorable to those who value dark skies.

“The latest reports indicate that the satellites spend most of their time around magnitude 5, possibly as faint as magnitude 7,” says astronomer Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics. (In astronomy, higher apparent magnitude numbers indicate dimmer objects.)

However, like the Iridium satellites that preceded them, Starlink satellites can sometimes “flare” if their solar arrays are angled just right to throw a burst of reflected sunlight toward Earth, briefly boosting their apparent brightness to potentially rival Venus or Jupiter.

And even these revised estimates, which are based on the satellite’s final orbital altitudes and inclinations, suggest that they’ll be visible to unaided eyes from very dark sites.

“It’ll be weeks or months before we can fully assess the situation,” McDowell says.

Will this be a problem for astronomers?

Likely, yes. For starters, a portion of the satellites will be operating at, or very close to, frequencies that radio astronomers use to study the cosmos. Normally, these frequencies are used primarily by scientists, but leakage and interference from orbiting satellites can make it tricky for ground-based telescopes to continue scrutinizing faraway objects.

“As a general principle, radio astronomy facilities are particularly vulnerable to satellite downlinks and to airborne uses, as radio telescopes cannot be protected from high-altitude transmissions through geographical shielding alone,” says Indiana University’s Liese van Zee, chair of the National Academy of Science’s Committee on Radio Frequencies, or CORF. She says it is CORF’s understanding that a coordination agreement with Starlink is currently in the works, and that historically such agreements have balanced the interests of science and telecom companies.

Both SpaceX and OneWeb—another company with plans to launch a fleet of communications satellites—have been working out such details with the National Science Foundation and the National Radio Astronomy Observatory, adds Harvey Liszt of the NRAO.

“At the same time,” Liszt says, “they keep changing the parameters of their satellites without telling us.”

Ground-based optical instruments, especially the large survey telescopes of the future, will have to contend with these satellites streaking through their research images.

“Professional astronomers like myself may need to prepare for streaky skies ahead. I can’t say I’m looking forward to that,” writes Monash University astronomer Michael Brown.

Stanford University’s Bruce Macintosh notes that one of the premier projects of the next decade, called the Large Synoptic Survey Telescope, will likely have to deal with between one and four Starlink satellites in every image within an hour or two of twilight. The LSST will be continually scanning the sky from its perch on a mountaintop in Chile, which means the number of Starlink-contaminated images really adds up.

“For astronomers I think this is more of a nuisance than a disaster, but changing the sky for every human needs talking about,” Macintosh writes on Twitter.

Did SpaceX have to get any legal approval to launch Starlink?

Yes. SpaceX has already received approval from the International Telecommunications Union and the U.S. Federal Communications Commission to launch a total of 11,943 satellites. And assuming the Federal Aviation Administration continues to grant launch licenses for rockets with Starlink satellites on board, there’s no regulatory machinery in place to prevent the company from seeing its strategy through to completion.

To be fair, SpaceX is not the only company with plans to assemble clouds of space-based internet satellites. Among those, Amazon is planning its 3,000 satellite-strong Project Kuiper, and OneWeb (which has already launched six satellites) intends to eventually send as many as 2,000 spacecraft into orbit.

Has Musk responded to these concerns?

Actually, yes. Despite his initial insistence that Starlink’s components will be invisible and unproblematic for astronomy, Musk has said he’s directed his team to think about ways of reducing Starlink’s reflectivity, or albedo.

“Sent a note to the Starlink team last week specifically regarding albedo reduction,” he tweeted on May 27, after acknowledging that SpaceX cares “a great deal about science.”

For now, though, the general consensus among the science community seems to be that discussions about the effects of such projects—whether that means altering humanity’s view of the cosmos, or making ground-based astronomy more difficult—ought to take place before hardware is put into orbit.