Space-based Internet service will revolutionize the Internet and provide high-speed connections to countless communities around the world. Programs like SpaceX’s Starlink paint a picture of a bright future for the world’s citizens. Like many revolutionary technological advances, Starlink has a dark side.
The constellation of hundreds (and eventually thousands) of satellites reflects light back to Earth and affects the darkness of the sky for professional astronomers and stargazers alike. Astronomers report that images and data are disrupted by bright streaks from the satellites passing through their fields of view. One possible solution to this problem is to apply a dark coating to the reflective antennas on the bottom side of the satellites. In January 2020, SpaceX launched the experimental DarkSat to test the effectiveness of such a coating. Astronomers around the world watched the new satellite. In December 2020, a team from the National Astronomical Observatory of Japan (NAOJ) published an article in the Astrophysical Journal showing detailed measurements of DarkSat’s effectiveness.
What were the results of the study? Is DarkSat an Effective Solution to Starlink’s Astronomical Problem? As is often the case in such studies, the answer is a bit complicated.
The 105 cm Murikabushi telescope that Darksat was imaged with. Photo credit: NAOJ
In a conversation with the main author Dr. Takashi Horiuchi from Ishigakijima Astronomical Observatory / NAOJ the results were highlighted. Dr. Horiuchi, an astronomer who spends most of his time observing quasars billions of light-years away, highlighted the fact that his study of reflectivity was broken down into different wavelengths by DarkSat. In the visible part of the electromagnetic spectrum, the DarkSat appears half as bright as uncoated Starlink satellites. The coating reduced the apparent size of the satellite to about 7. This size is just below the lower brightness limit at which the naked eye can pick up an object in the night sky, but is still far brighter than many astronomically significant targets. Dr. Horiuchi put it that way; “DarkSat is a strength of 7, but a typical Quasar is a strength of 18.” It is noteworthy that on the magnitude scale, the higher the number, the darker the object becomes. A quasar of strength 18 is about 23,000 times weaker than a DarkSat of strength 7.
Dr. Takashi Horiuchi reports on a long exposure from the Ishigakijima Astronomical Observatory, which is part of the National Astronomical Observatory of Japan. Photo credit: NAOJ
Dr. Horiuchi further explained that the light energy absorbed by the coating is then converted into heat. All objects with measurable temperature radiate in the infrared. Since the DarkSat is heated more than a typical Starlink satellite due to its coating, DarkSat radiates more in the infrared. In the IR, DarkSat is essentially brighter than a typical Starlink.
Does the higher brightness in the infrared range of the spectrum affect astronomical observations? Professor Masatoshi Ohishi, director of the Spectrum Management Office at NAOJ and another author on the paper, stated, “The higher infrared flux would negatively affect the infrared observations.” Ohishi went on to explain that this was not a surprise and was exactly what was expected from the coating.
A demonstration of the dark coating on DarkSat’s reflective antennas. Image Credit: SpaceX
It seems clear that the DarkSat solution for Starlink satellite strips disrupting astronomical observation is flawed at best. The good news for the astronomical community is that SpaceX has already implemented a new solution, VisorSats, which employs panels similar to the sun visors that are used to keep parked cars cool in the summer. Dr. Horiuchi assured me that an in-depth study of the new VisorSats for his observatory is in the works in 2021, even if his paper is still fresh off the press. “I want to continue these observations … we feel it is important to continue to discuss this with the astronomical community of ground observations. “
Another positive aspect of the situation is a growing report between SpaceX and the astronomical community. Professor Ohishi commented on his meetings with SpaceX and spoke about the importance of programs like Starlink. “I would like to emphasize that we respect the technical progress of SpaceX. Her project aims to make internet connections much better for everyone in the world. This is very important. At the same time, we need to work with space x to keep our environment as clean or calm as possible. “
Dr. Horiuchi reflected on the importance of the dark sky and the preservation of the night sky for astronomers and ordinary people alike. “Dark skies are beautiful and a treasure for all people all over the world.” Fortunately, given SpaceX’s continued commitment, it appears to be problems with light pollution with the ever-expanding Starlink constellation and the tireless work of brilliant astronomers like Dr. Tackling Horiuchi, likely that the regions of the earth fortunate enough to have clear, dark skies still persist for long time enjoying cosmic vistas.
Main image: A streak against a background of stars shows the decrease in brightness of the dark coating on a SpaceX DarkSat Starlink satellite. Credit NAOJ
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National Astronomical Observatory of Japan
Simultaneous multicolored observations of Starlinks Darksat with the Murikabushi telescope with MITSuME
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