SpaceX Starlink sat streaks now present in nearly a fifth of all astronomical images snapped by Caltech telescope

Annoying, maybe – but totally ruining this science, maybe not


SpaceX’s Starlink satellites appear in about a fifth of all images snapped by the Zwicky Transient Facility (ZTF), a camera attached to the Samuel Oschin Telescope in California, which is used by astronomers to study supernovae, gamma ray bursts, asteroids, and suchlike.

A study led by Przemek Mróz, a former postdoctoral scholar at the California Institute of Technology (Caltech) and now a researcher at the University of Warsaw in Poland, analysed the current and future effects of Starlink satellites on the ZTF. The telescope and camera are housed at the Palomar Observatory, which is operated by Caltech.

The team of astronomers found 5,301 streaks leftover from the moving satellites in images taken by the instrument between November 2019 and September 2021, according to their paper on the subject, published in the Astrophysical Journal Letters this week.

“In 2019, 0.5 per cent of twilight images were affected, and now almost 20 percent are affected," Mróz said.

The streaks appear as ugly bright marks in astronomers’ views of the night sky. They’re mostly visible during dusk and dawn, when they reflect the most light from the Sun. When it's nighttime, they’re mostly hidden from the ZTF. There are about 1,800 Starlink satellites orbiting our planet at this time.

Up to 15 satellites can crop up in each of its images if the astronomers are unlucky. On average, there are about 1.09 satellite streaks per affected image, the paper stated. The researchers also found that Starlink satellites sporting a visor to deflect sunlight appear about five times fainter compared to the standard design. They estimate that if SpaceX meets its target of growing its fleet of broadband satellites to 10,000 by 2027, all images captured by the ZTF will contain a Starlink streak. SpaceX has a long-term goal of 42,000 orbiting birds.

Operations at the ZTF aren’t derailed much by Starlink devices at the moment, however. In fact, the team reckon they shouldn’t be too much of a nuisance in the near future either.

“Having a streak in an image does not necessarily mean it is destroyed,” Mróz told The Register on Wednesday. A single streak only blocks out about less than one-tenth of a percent of the pixels in a ZTF image. It’s rare that these annoying bright flecks directly cover up an object astronomers are trying to study.

Having a streak in an image does not necessarily mean it is destroyed

"There is a small chance that we would miss an asteroid or another event hidden behind a satellite streak, but compared to the impact of weather, such as a cloudy sky, these are rather small effects for ZTF," said Tom Prince, an emeritus professor of physics at Caltech.

“Although the number of satellite streaks is rising,” Mróz added, "we have not come across any case in which a satellite streak would impact the discovery or monitoring of an asteroid."

The satellite streaks may not be so disruptive for astronomers using the ZTF since they study moving objects and phenomena. They can snap more images, and observe, say, asteroids moving out of the way of the satellites’ lines. Although the impact on the ZTF is minimal, that’s not the case for all observatories. The effect may be concerning for the upcoming Vera C. Rubin Observatory, under construction in Chile, which has a wider view of space.

“Different satellite constellations may impact astronomical observations differently,” Mróz told us. "In the paper, we demonstrated that even if SpaceX deploys the full constellation of 42,000 satellites, they won’t significantly impact science operations of ZTF, including studying asteroids, because less than one per cent of our pixels would be masked. However, this may not be true for other observatories."

It’s difficult to model the effect these devices will have on astronomy. And there are companies other than SpaceX planning to launch thousands of satellites that will occupy orbits around Earth.

The research community is trying to develop new techniques and strategies to prevent the birds from potentially destroying science. Many have suggested building open-source tools based on machine-learning algorithms to automatically detect and remove bright pixels leftover from streaks in images. Others are racing to update international space policies to force countries to regulate commercial satellite companies. ®


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