SpaceX Satellite Streaks Cross Nearly a Fifth of Telescope’s Twilight Photos

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The swath of a Starlink satellite appears in this image of the Andromeda galaxy taken by the Zwicky Transient Facility, or ZTF, during twilight on May 19, 2021.

Caltech/ZTF

This week, SpaceX is set to launch its 2,000th Starlink satellite In three years. While not all flying routers are still in orbit, the researchers say the growing constellation’s impact on certain astronomical observations is clear.

“In 2019, 0.5% of twilight images were affected and now almost 20% are affected,” Przemek Mróz, a former Caltech researcher who is now at the University of Warsaw in Poland, said in a statement. It is important to note, however, that further research on the images was based on observations from just one instrument among many around the world and in space, the Zwicky Transient Facility at the Palomar Observatory near San Diego.

The table-sized satellites appear as straight lines in an image of the night sky when ZTF exposures capture a Starlink moving along its orbital path. The bands tend to show up more in observations taken near dawn and dusk, because that’s when satellites are at their highest reflectivity levels due to the geometry of the Earth and the sun at those times.

“We don’t expect Starlink satellites to affect non-twilight images, but if the constellation of satellites from other companies enters higher orbits, it could cause problems for non-twilight observations,” Mróz said.

Mróz is the lead author of a study published Monday in The Astrophysical Journal Letters that analyzed images from the ZTF. The instrument is designed to scan the entire night sky every two days, looking for cosmic objects that change in some way over time, such as supernovae or even near-Earth asteroids.

The team found that as Starlink’s footprint in low Earth orbit grew from 2019, the number of bands seen in images over a 10-day period grew to over 200 by mid-2021.

The study appears to confirm the fears of several astronomers after the famous Starlink “trains” began appearing in the night sky almost immediately after the first batch of satellites launched in 2019. But study co-author Tom Prince, professor emeritus of physics at Caltech , notes that a Starlink strip affects less than one-tenth of one percent of the pixels per image.

“There’s a small chance that we’ll miss an asteroid or other event hidden behind a string of satellites, but compared to the impact of weather, like a cloudy sky, these are pretty small effects for the ZTF.”


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Prince is optimistic that the software can help mitigate the problems through coordination between astronomers and SpaceX, something Elon Musk and the company have also suggested. The software can also reduce or mask the impact of streaks on images after the fact.

SpaceX did not immediately respond to a request for comment, but the company has taken steps to reduce the reflectivity of the satellites, equipping each with a “visor” to reduce glare.

The study also looked at the effectiveness of the viewfinders and found that they significantly reduced glare, although they fell short of the standards for satellite constellations described by the astronomical community in 2020.

However, upcoming next-generation sky surveys such as the Vera C. Rubin Observatory in Chile are expected to be more adversely affected, and SpaceX has signaled that it may increase its Starlink constellation by up to 20 times its current size or most.

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