SpaceX has found a new way to prevent Starlink satellites from interfering with astronomy: Bring them closer to Earth.
The satellites can reflect sunlight and accidentally photobomb astronomical observations. But last week, SpaceX told the Federal Communications Commission that its effort to operate over 300 Starlink satellites at lower altitudes could help keep them out of view.
“Analysis shows nearly a 60% reduction in Vera Rubin Observatory images containing an illuminated satellite when operating equivalent constellations at 350 km (217 miles) vs. 550 km,” the company tells the agency.
In November,the FCC granted SpaceX permission to operate 400 second-generation Starlink satellites closer to the planet. This can reduce the latency of Starlink’s internet speeds, but another side benefit is minimizing the satellite’s visibility in the night sky.
“The main effect is that if a satellite is lower, it is above the horizon less of the time from a given location,” says astronomer Jonathan McDowell, who monitors Starlink launches.
Another issue is that satellites can be seen in the night sky during the first few hours after dusk or before dawn. While it may be night on the ground, the satellites are high enough to catch and reflect sunlight. “The higher the satellite is, the more likely it is to see the sun,” McDowell says.
Satellite sun reflections (Credit: SpaceX)
Lowering a satellite can increase its brightness if it does catch sunlight. But in SpaceX’s letter to the FCC, the company found that “peak brightness from nadir surfaces is only marginally increased when operating at 350 km, due to a higher eclipse fraction and angular rate.”
SpaceX conducted the analysis with the US National Science Foundation, which is working with the company to reduce Starlink’s impact on astronomy. In a statement to PCMag, the NSF said that “lower altitudes may be beneficial to ground-based optical and infrared astronomy as satellites move faster through a telescope’s focal plane and are more out-of-focus, reducing overall brightness per pixel.”
“At lower altitudes, satellites are also blocked from sunlight by the Earth for longer portions of the night, not reflecting towards the ground. This enables the goal of satellites appearing fainter than approximately 7th magnitude (the human naked-eye limit) to be achieved for large periods of the night,” the foundation added.
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Still, there are “trade-offs” to the lower altitudes, such as the increased brightness of the satellites during twilight hours, the NSF said. The foundation plans to continue to monitor the Starlink satellites and “looks forward to working with SpaceX in the upcoming year to verify,” it said.
SpaceX didn’t respond to a request for comment, but the company has been devising numerous ways, including design changes, to prevent Starlink satellites from reflecting too much light. Meanwhile, astronomers have lobbied regulators to prevent “mega constellations” from polluting the night skies.
For now, SpaceX only has permission to operate 400 Starlink satellites at the 350km orbits in coordination with NASA through a study. But the number could be expanded to 7,500 second-generation Starlink satellites and thousands more if the space agency and the FCC sign off in the future. In October, the company made a new push to receive regulatory approval to operate nearly 30,000 second-gen Starlink satellites, including at 350km orbits.
Editor’s note: This story has been updated with comment from the NSF.
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About Michael Kan
Senior Reporter
