SpaceX is trying to reduce the reflective properties of Starlink satellites — but despite measured success, its results don't cut it for astronomers, according to a recent study published in The Astrophysical Journal.
Darkened Starlink satellites could still disrupt astronomy, says study
Starlink satellites with anti-reflective coating are already half the brightness of the standard models, according to the study. While this is an improvement, the darkened models don't lower the "noise" of light pollution, according to the team of scientists from the National Astronomical Observatory in Japan — under the leadership of Takashi Horiuchi.
Called "DarkSats," the SpaceX-launched internet satellites also continue to be a problem at other light wavelengths, Gizmodo reports.
The initial pack of 60 Starlink satellites was launched in May 2019, but instantly met concerns from the astronomical community regarding the constellations' propensity in low-Earth orbit (LEO) to disrupt astronomical study of the night sky.
SpaceX could launch up to 42,000 Starlink satellites
Indeed, this proved to be correct, as Starlink satellites photobombed long-exposure images of nearby galaxies, comets, and more. Once informed of the problem, astronomers explained a manifold of ways SpaceX satellites can mess up scientific research — including the operations of the forthcoming Vera C. Rubin Observatory in Chile.
This might sound like minutiae to entrepreneurs, but SpaceX's first batch of Starlink satellites is more bright than 99% of the objects in LEO. Since SpaceX CEO Elon Musk intends to launch around 12,000 Starlink satellites — perhaps even 42,000 — this is a serious threat to Earth-based astronomy as we know it.
Recent study evaluated DarkSat satellites' reflectivity
Sadly, Musk's comments on the matter in March 2020 seemed to break with this reality — as he claimed Starlink "will not cause any impact whatsoever in astronomical discoveries, zero." On the bright side, he also suggested SpaceX might "take corrective action if it's above zero."
SpaceX's answer to avoiding astronomical interference was the DarkSats, which coated the original Starlink satellites with a darker finish to reduce albedo — or their propensity to reflect light. Called the Starlink-1130 version, these DarkSats were included in a batch launched on Jan. 7, 2020 — and the latest study evaluated how effective this alteration was at lowering the reflectivity.
Scientists compared Starlink satellites to reference stars
The test involved Horiuchi and colleagues looking at the satellites with the Japanese Murikabushi Telescope, based in the Ishigakijima Astronomical Observatory. Together, the team evaluated both the DarkSats and the original version — called Starlink-1113 — through several light wavelengths.
The telescope enables scientists to make observations in the red, green, and near-infrared bands, and also helped the team compare the brightness of each reflective object with reference stars. In sum, the team of scientists carried out four distinct observations from April to June of 2020.
DarkSat Starlink satellites' black coating 'raises surface temperature'
The Japan-based scientists described how the "albedo of DarkSat is about a half of that of STARLINK-1113," according to the paper. This is a substantial improvement in de-cluttering the visual spectrum, but 50% is not a passing grade. And worse, problems remain unsolved in other, non-visible 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," said Horiuchi to Physics World. He also emphasized how the mitigating progress 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."
SpaceX could raise Starlink satellites' altitude
Another forthcoming version of Starlink aims for an even dimmer solution. Called "VisorSats," the new version came with a sun visor capable of dimming "the satellites once they reach their operational altitude," Sky and Telescope reports. Some of these models were launched in 2020, but their albedo reduction rate compared to the initial Starlink model remains to be seen.
While it's too soon to say if SpaceX's latest generation of reflection-reducing Starlink satellites will make the cut for astronomers to have a clear view of the night sky, Horiuchi also told Physics World the company should give thought to raising the altitude of the internet satellites. As of writing, they orbit up to 340 miles (547 km) high, but at the same altitude as OneWeb — a SpaceX rival — at 750 miles (1,200 km), the reflective disruption of astronomy will be substantially lessened.