Dark SpaceX satellites may still disrupt astronomy, new research finds

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. That’s an improvement, but not yet sufficient, according to the team led by astronomer Takashi Horiuchi of the National Astronomical Observatory of Japan. These “DarkSatsAs they are called, also continue to cause problems at other wavelengths of light.

Launched in May 2019, the initial batch of 60 Starlink satellites raised concerns that large constellations of satellites in low Earth orbit would interfere with astronomical observations. And indeed, this appeared to be the case, with the long- photobombing Starlink satellitesexhibition photos of nearby galaxies and comets, for example. Alerted to the problem, the astronomers described different ways that SpaceX satellites could interfere with scientific research, including surgery of the future Vera C. Rubin Observatory in Chile.

First batch of orbiting Starlink satellites are brighter than 99% of objects in low Earth orbit. It’s a huge concern, given Elon Musk’s desire to launch more than 12,000 Starlink satellites and possibly up to 42,000. Starlink’s goal is to provide high-speed Internet access to customers around the world.

Discouragingly, comments made by the CEO of SpaceX in March 2020 seemed incongruous with the emerging reality, in which Musk claimed that Starlink “will not cause any impact on astronomical discoveries, zero”. Encouragingly, however, he also said SpaceX “will take corrective action if it is greater than zero.” The company responded by deploying DarkSats, in which the 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 this dark coating.

To do this, Horiuchi and his colleagues observed the satellites using the Murikabushi telescope at the Ishigakijima Astronomical Observatory.. The team observed the DarkSats in the same way the original version, known as Starlink-1113, at several 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 reflective objects to benchmark stars. In total, the team made four different observations from April to June 2020.

Scientists found that “DarkSat’s albedo is about half that of STARLINK-1113,” as they wrote in their article. It’s a decent improvement in the visual spectrum, but still not great. In addition, problems persist at other wavelengths.

“The darkening paint on DarkSat halves the reflection of sunlight compared to regular Starlink satellites, but [the constellation’s] the negative impact on astronomical observations remains ”, Horiuchi Told Physics World. He said the attenuation effect is “good in the UV / optical region” of the spectrum, but “the black coating increases the surface temperature of DarkSat and affects intermediate infrared observations.”

A third version of Starlink is said to be even darker. Called “VisorSats”, they have a sunshade that “darkens the satellites once they reach their operational altitude”, according to to the sky and to the telescope. SpaceX launched some VisorSats last year, but the degree to which their albedo is decreased from the original version is still not known, or whether those versions will exhibit high surface temperatures.

Horiuchi said to physics World that SpaceX should seriously consider raising the altitude of the Starlink constellation to further reduce the brightness of these objects. Starlinks currently orbit at Heights reaching 340 miles (547 km). Compare that to OneWeb, a SpaceX competitor, whose constellation of satellites will orbit 1200 km (750 miles), and therefore will be considerably darker.

Back in January 2020, Jonathan McDowell, astronomer at the Harvard-Smithsonian Center for Astrophysics and satellite expert, Told me that “SpaceX is making a good faith effort to fix the problem” and that it believes the company “can dim satellites more than the naked eye can see.”

For the sake of astronomers around the world, I hope he’s right on both counts.