SpaceX claims some success in darkening Starlink satellites

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WASHINGTON — SpaceX says it will take more steps to reduce the impact of its Starlink satellite constellation on astronomy, although astronomers disagree with statements by Elon Musk that the system will have “zero” effect on their work.

SpaceX launched its latest set of Starlink satellites March 18, the fourth such launch this year and sixth overall. The company has placed 362 Starlink satellites into orbit, counting two experimental satellites launched in 2018, and nearly all of them remain in orbit.

Since large-scale Starlink launches started in May 2019, astronomers have warned that the satellites, far brighter than expected, could interfere with their observations, particularly if SpaceX proceeds with plans to launch 12,000 or more such satellites in the next several years. In response to those concerns, SpaceX included an experimental “DarkSat” among the 60 Starlink satellites launched Jan. 6, with portions of the satellite darkened to reduce its reflectivity and hence brightness.

During the company’s webcast of the latest Starlink launch, SpaceX claimed some success with that effort. “Preliminary results show a notable reduction,” said Jessica Anderson, one of the hosts of the webcast. She added that the company had “a couple of other ideas that we think could reduce the reflectivity even further.”

One of those, she said, was a “sunshade” that would deploy like a patio umbrella from the satellite. That will be tested on a future Starlink mission, but she didn’t give more details about either the sunshade itself or when it would be flown.

SpaceX Chief Executive Elon Musk also mentioned the sunshade option during an onstage interview at the Satellite 2020 conference here March 9. “We are working with senior members of the science community and senior astronomers to minimize the potential for reflection from the satellites,” he said. “We’re running a bunch of experiments.” That includes a sunshade and other steps “to minimize the potential for any impact.”

However, the company’s claim that DarkSat has achieved a “notable reduction” in brightness is not necessarily supported by recent observations. In a paper posted to the online preprint server arXiv March 17, astronomers using a small telescope in Chile measured the brightness of DarkSat and compared it to another Starlink satellite without darkening treatments. They found DarkSat was about 0.88 magnitudes, or 55%, dimmer than the ordinary Starlink satellite.

That falls far short of what many astronomers are seeking. In a March 11 panel discussion organized by the American Astronomical Society, Tony Tyson, chief scientist for the Vera Rubin Observatory under construction in Chile, said that simulations of the Starlink satellites showed that not only would the satellites make bright streaks on images taken by the telescope, but create other image artifacts by saturating pixels in the detector.

“If we could make those particular spacecraft, the Starlinks, darker by 10 to 20 times, it may remove many of these artifacts,” he said. “It won’t remove the main trail — it will always be there — but it would remove the artifacts so that we might be able to get the science out of the data.”

At the time of that event, there was still little information available about how much dimmer DarkSat was than the rest of the Starlink constellation. While launched in early January, the spacecraft reached its operational orbit only in late February, allowing for accurate comparisons of its brightness.

“This is a continuing experiment,” Tyson said of the DarkSat observations, noting that measurements of its brightness were taken just the night before. The data from the small Chilean telescope analyzed in the arXiv preprint came primarily from a single night of observations in early March after DarkSat reached its operational orbit.

Tyson, though, emphasized the cooperation between SpaceX and the astronomy community to reduce the brightness of future Starlink satellites. “We’ve had a really delightful collaboration going now for a couple months with SpaceX engineers,” he said. “There are a lot of ideas on the table for darkening their satellites. This is just the first.”

Musk, in his Satellite 2020 appearance, claimed that the Starlink constellation will ultimately have no effect on ground-based astronomy. “I am confident that we not cause any impact whatsoever in astronomical discoveries,” he said. “Zero. That’s my prediction. We will take corrective action if it’s above zero.”

Astronomers like Tyson, though, would not go so far to say that Starlink will have “zero” effect on their observations. “My hope in the future is that they will be darkened sufficiently, just as I mentioned, just to get out of this region where our detectors are impacted very negatively,” he said. “The trail will always be there, of course, but maybe we can salvage some of the science.”