SpaceX has revealed new details on its plans to darken the Starlink satellites.

Starlink satellites
In this conceptual view, each Starlink satellite unfurls its single solar panel.

The Starlink megaconstellation, which will consist of at least some 12,000 satellites and possibly up to 42,000, will be serving up broadband internet from low-Earth orbit. But it has worried astronomers, professional and amateur alike, after the first launch in 2019 proved what a bright multitude it would be. Now, with 420 satellites in orbit and more to come soon, SpaceX is showing that it's taking astronomers' concerns seriously.

The new document, posted on the Starlink website, clarifies why the satellites are so bright, and what can be done to mitigate the problem. Namely, the company is attempting a software fix that will dim satellites as they raise their orbits early on, and a hardware fix — a Sun visor — that will dim the satellites once they reach their operational altitude.

The first "VisorSat" will launch with the next Starlink batch. By June, SpaceX says, all Starlink satellites will be equipped with deployable Sun visors.

Darksat Details

SpaceX's first attempt to darken the Starlink satellites, dubbed "DarkSat," launched in January. While the company had announced it would put a dark coating on parts of this test case in order to reduce its brightness, the details were vague.

DarkSat, an earlier experiment, had a dark coating on the phased array and parabolic antennas. However, while DarkSat was dimmer than its comrades, the coating did not reduce the reflections enough and also resulted in thermal balance issues.

The new document clarifies which pieces were darkened. The brightest parts of the satellites are the white phased array antennas on the bottom, the white parabolic antennas on the sides, and the white backing of the solar array. For DarkSat, engineers had darkened the phase array and parabolic antennas. (Surprisingly, the solar panels don't present as much of a problem. They do glint in the sunlight, but for the vast majority of the time that glint is facing a direction other than down.)

Ultimately, studies showed that the paint job did not do enough to reduce the satellite's brightness, and it caused thermal problems to boot.

Knife-Edge Reflection

While the satellites' brightness at operational altitudes to worry astronomers, they made the news on a regular basis in the weeks following a launch. A "string of pearls" would appear in the night sky and show up in photographs as the satellites raised their orbits.

During the orbit-raising period, the satellites appear brighter because not only are they at lower altitudes, but their solar panels are positioned differently, to reduce the drag the spacecraft feels. The white back of the solar panels reflected diffuse light to observers on the ground. Now, SpaceX is testing a roll maneuver that will put the solar panel in line with the Sun, presenting a "knife's edge," and ultimately reflecting less sunlight.

SpaceX cautions that there are some limiting factors, that might prevent a satellite from always presenting that knife's edge to the Sun. But the good news is, the maneuver is a software change. Once engineers know that satellites can roll without damaging their ability to communicate with the ground or collect solar power, it will be straightforward to have all raising-orbit satellites do the same thing.

The Sun Visor

But while the time a satellite spends raising its orbit gets more attention — because the satellites are closely packed and bright — it's the satellites at operational altitude that still pose a threat to professional astronomers.

Artist's conception of VisorSat, where a Starlink satellite will be equipped with a deployable visor that shades the antennas from sunlight. The visor will be transparent to radio frequencies.

The Vera C. Rubin Observatory in Chile, due to start collecting science data in 2022, faces the worst-case scenario because its detectors are both very sensitive and have incredible fields of view. The Rubin Observatory, after all, is designed to video the entire night sky every few days, ultimately collecting a decade's worth of reel in which astronomers hope to discover asteroids, supernovae, and many other transient events that might otherwise slip them by.

The Rubin Observatory therefore presents SpaceX with its biggest challenge, as Starlink satellites at their current brightness would saturate detectors and ruin whole images. For that reason, Tony Tyson (University of California, Davis) and others with Rubin Observatory have been communicating directly with SpaceX engineers, including providing them with target numbers to hit.

While coating the bright parts of the satellite didn't work, SpaceX hopes to do better with its plan for a Sun visor (initially called a Sun "umbrella). The visor provides the benefit of black paint by blocking sunlight from all of the white parts of the main body, as well as the antennas, while avoiding the thermal balance problems.

The first VisorSat will launch in May, and if all goes according to plan all satellites from June forward will be equipped with the visor.

SpaceX has also said that, in addition to providing orbital elements to help astronomers (and others) track its satellites, it's also now providing predictive data ahead of launch.

"While it will not be possible to create satellites that are invisible to the most advanced optical equipment on Earth," the document reads, "by reducing the brightness of the satellites, we can make the existing strategies for dealing with similar issues, such as frame-stacking, dramatically more effective."

"I'm cautiously optimistic," Tyson told the American Astronomical Society, “but it will be months before we can validate joint solutions to mitigating many of the effects of even the darkened Starlinks on the Rubin Observatory’s wide-field survey data.”

Further Reading

Read the full SpaceX document.

Peruse presentations from the Astro2020 Decadal Survey Optical Interference from Satellite Constellations Meeting.




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May 8, 2020 at 4:47 pm

Starlink reminds me of Globalstar and Iridium...only on a colossal scale. Remains to be seen if anyone really needs satellite based internet in the middle of the African continent or the Brazilian jungle...Is there really a market for this or just another Musk pipe-dream?

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May 9, 2020 at 9:17 am

Not sure about remote locations like the African continent, but I, and others I know, could really use good internet where we live in Texas. Looking to replace high priced, low speed and unreliable internet in Texas with Starlink. Hopefully Starlink will force other providers to improve prices, speeds and provide competition to markets like DFW area in Texas where the monopolies that the telcos have leaves customers paying too much for poor service.

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Andrew James

May 9, 2020 at 7:33 pm

A bit of a specious argument, because the 'carrot' of better and faster internet services does not equate to better or other alternate technological methods to provide that same service. e.g. Using lesser satellites or new ground-based stations.
Also if the "monopolies that the telcos" are true in Texas, then the problem is your governmental legislators allowing the telcos to provide such poor or expensive services.
In a nutshell, 5% of Earth's population is imposing a orbital system that affects 100% of that population - all on the assumption that its intent is altruistic; when it in fact the motivation is ultimately for political and/or economic reasons.
Note: Such views might seem anti-SpaceX (or even anti-American), but I don't think Americans or Musk understand the deep resentment that exists within the international astronomical community - exacerbated them because they are mostly powerless to fix or influence the decisions being made.

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Anthony Barreiro

May 8, 2020 at 6:17 pm

Thanks Dr. Young for your ongoing timely reporting on this issue.

It's encouraging to know that SpaceX is working closely with astronomers to mitigate the harmful effects of these satellites. But it's worrisome that this collaboration depends entirely on the good will and ingenuity of a private company. As more businesses enter this market, will they all be motivated and resourced to make similar efforts? One unethical or incompetent actor would have (literally) disastrous consequences.

This problem cries out for a binding international agreement with meaningful guidelines and zealous enforcement. Is anybody talking about negotiating a new provision of the Outer Space Treaty? In order to be effective, a revised treaty would need to be ratified and enforced by every nation that launches satellites. Unfortunately our current US administration and Senate are actively hostile both to international cooperation and to government regulation of private industry.

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Andrew James

May 8, 2020 at 7:53 pm

Sorry. Still not good enough, as all these issues should have been sorted out before the Federal Communications Commission (FCC) approve it. In saying: "Now, with 420 satellites in orbit and more to come soon, SpaceX is showing that it's taking astronomers' concerns seriously." But what consequences will exist if it doesn't? The decision is purely economic an internal and political one, was seemingly made regardless of the impact upon the international community. In the non-US astronomical community the whole situation created here is palpable because their techological assets are being compromised - and they can do little about avoiding its damage.
Q: Perhaps Musk should have adopted less damaging geosynchronous satellites placed over North America?

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Andrew James

May 8, 2020 at 7:57 pm

Correction "The decision is purely economic and a internal political one that was seemingly made regardless of the impact upon the international community."

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May 11, 2020 at 2:27 pm

Geosynchronous satellite Internet access already exists. However, the much more distant orbit and the limited speed of light results in much slower response times which keeps many Web functions from working properly. Notice how the SpaceX system points out its "low latency" - which is just what that means.

Also, geosynchronous satellites orbit over the equator, so "over North America" means most of the Western hemisphere. Not a huge improvement. And still doesn't provide connectivity to the other end if it's in an unserved area.

Finally, the radio free-space loss equation results in needing more power, larger user antennas or both for geosynchronous satellites instead of lower orbit ones. That also limits the possible uses of same.

So there are many technical reasons for Starlink's approach. As to whether they will pay out long term remains to be seen. Living where high speed low latency internet access is very limited, with many homes having no access at all, I understand both sides.

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May 10, 2020 at 10:34 pm

Why doesn't Space-X etch the solar panel glass so it's more of a matte like surface that is anti glare vs a smooth, glossy, highly reflective surface?

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Monica Young

May 11, 2020 at 2:24 pm

As noted in the article, the glint of sunlight off solar panels isn't what's making the satellites bright — the white antennas and backing of the solar panels reflect sunlight in a diffuse manner, and that's what makes the satellites so bright. So reducing the brightness of these multiple components is crucial to making the satellites appear dimmer in our night skies.

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January 23, 2021 at 11:25 am

Just a reminder that one of the strongest motivations for the low earth orbit is lower latency which allows financial transactions to "outrun" transactions based on some other communication methods or more distant satellites such as geo stationery. This lower latency can make millions upon millions of dollars of difference to its users who will pay BIG BUCKS to access the high speeds available with lower latency. Providing access to the internet for folks with no internet connectivity available now is a fringe benefit, a collateral side effect but not the chief source of revenue.

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Andrew James

January 31, 2021 at 5:49 pm

"Providing access to the internet for folks with no internet connectivity available now is a fringe benefit, a collateral side effect but not the chief source of revenue."

Who or whom decided this? Who ask them? This view is based on spin not actual evidence. Musk and the FCC are imposing a unilateral decision on the rest of the world, leaving other nations to deal with the impacts on their environment without choice. Is that fair? (Worst, this isn't an historical precedent either. e.g. Nuclear weapon testing.) Interesting too, in the long run, one country profits, and the rest them pays for the privilege. "All the skies are mine" is not good foreign policy decisions, are they?

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