SpaceX’s attempt to reduce the reflectivity of Starlink satellites is working, but not to the degree required by astronomers.
Starlink satellites with an anti-reflective coating are half as bright as the standard version, according to research published in The Astrophysical Journal. It’s an improvement, but still not good enough, according to the team, led by astronomer Takashi Horiuchi from the National Astronomical Observatory in Japan. These “DarkSats,” as they’re called, also continue to cause problems at other wavelengths of light.
Launched in May 2019, the initial batch of 60 Starlink satellites prompted concerns that large satellite constellations in low Earth orbit would interfere with astronomical observations. And indeed, this appeared to be the case, with Starlink satellites photobombing long-exposure shots of nearby galaxies and comets, for example. Alerted to the problem, astronomers described different ways in which the SpaceX satellites might muddle scientific research, including the operation of the upcoming Vera C. Rubin Observatory in Chile.
The first batch of orbiting Starlink satellites is brighter than 99% of objects in low Earth orbit. This is a huge concern, given Elon Musk’s desire to launch upwards of 12,000 Starlink satellites and possibly as many as 42,000. The purpose of Starlink is to provide broadband internet to customers around the world.
Discouragingly, comments made by the SpaceX CEO in March 2020 seemed incongruous with the emerging reality, in which Musk claimed that Starlink “will not cause any impact whatsoever in astronomical discoveries, zero.” Encouragingly, however, he also said that SpaceX would “take corrective action if it’s above zero.” The company has responded by deploying some DarkSats, in which Starlink satellites were given a darker coating to reduce albedo, or reflectivity. These DarkSats, known as the Starlink-1130 version, were included in a batch of satellites launched by SpaceX on January 7, 2020. The new study aimed to evaluate the effectiveness of that dark coating.
To do so, Horiuchi and colleagues observed the satellites using the Murikabushi Telescope at the Ishigakijima Astronomical Observatory. The team observed the DarkSats along with the original version, known as Starlink-1113, at multiple wavelengths of light. This telescope allows scientists to make simultaneous observations in the green, red, and near-infrared bands. The team also compared the brightness of the reflective objects to reference stars. In total, the team conducted four different observations from April to June 2020.
The scientists found that the “albedo of DarkSat is about a half of that of STARLINK-1113,” as they wrote in their paper. That’s a decent improvement in the visual spectrum, but still not great. What’s more, problems persist at other wavelengths.
“The darkening paint on DarkSat certainly halves reflection of sunlight compared to the ordinary Starlink satellites, but [the constellation’s] negative impact on astronomical observations still remains,” Horiuchi told Physics World. He said the mitigating effect is “good in the UV/optical region” of the spectrum, but “the black coating raises the surface temperature of DarkSat and affects intermediate infrared observations.”
A third version of Starlink is supposed to be even dimmer. Called “VisorSats,” they feature a sun visor that will “dim the satellites once they reach their operational altitude,” according to Sky and Telescope. SpaceX launched some VisorSats last year, but the degree to which their albedo is lessened compared to the original version is still not known, or if these versions will exhibit elevated surface temperatures.
Horiuchi told Physics World that SpaceX should seriously consider lifting the altitude of the Starlink constellation to further reduce the brightness of these objects. Starlinks currently orbit at heights reaching 547 km. Compare that to OneWeb, a SpaceX competitor, whose satellite constellation will orbit at 1,200 km, and as a consequence be considerably darker.
Back in January 2020, Jonathan McDowell, an astronomer at Harvard-Smithsonian Centre for Astrophysics and an expert on satellites, told me that “SpaceX is making a good-faith effort to fix the problem,” and that he believes the company “can get the satellites fainter than what the naked eye can see.”
For the sake of astronomers around the world, I hope he’s right on both counts.