Brightest object in night sky not a star: BlueWalker 3 satellite for cellular broadband interferes with astronomical studies

BlueWalker 3, a test satellite launched on September 10, 2022, is as bright as the brightest stars in the 89 constellations of Canis Minor and Eridanus, according to a new study.

Its brightness makes it hard for astronomers using ground-based telescopes to peer into the night sky, the study published in Nature stated.

BlueWalker 3 is 693 square feet in size. It uses its large phased-array antenna to generate power from space and deliver cellular broadband directly to mobile phones.

“Amateur astronomers first flagged BlueWalker 3 as a potentially bright object. This allowed us to alert our international network of observers to capture the satellite when it was unfurling its 64 square metre phased array antenna, which, as it reflected sunlight, made it one of the brightest objects in the night sky,” Siegfried Eggl from the University of Illinois at Urbana-Champaign told Down To Earth.

Satellites such as BlueWalker 3 could also present an additional source of noise for radio astronomy. Eggl likens bright satellites in the field of view of telescopes to staring at the sun without sunglasses.

Eggl and professional astronomers from Chile, the United States, Mexico, New Zealand, the Netherlands and Morocco conducted an international observing campaign to assess the impact of brightness on radio astronomy.

Observations made before BlueWalker deployed its antenna on September 10, 2022 suggested that this satellite would be particularly bright. 

A telescopic observation campaign confirmed visual observations, suggesting that the satellite’s brightness jumped from about +6 to +0.4 as bright as Procyon and Achernar, the brightest stars in the 89 constellations of Canis Minor and Eridanus, respectively. 

For comparison, the unaided eye at a 90 dark sky site will see stars of magnitude +6, the paper said.

“Size certainly matters. The larger the object the more sunlight it can reflect down to Earth,” Eggl explained.

Predicting brightness, he explained, is complicated because satellites change their position and altitude with respect to the observer and the sun continuously. 

The material used in the satellite also adds to the complications. They do not reflect the same amount of light in all directions. 

Mitigating the impact of satellites on radio astronomy has not been successful despite many efforts made by the aerospace industry, policymakers and astronomers.

Space X has already launched nearly 5,000 Starlink satellites so far. On October 5, 22 more will lift off from Florida to the low-Earth orbit.

Further, 16 other companies have filed their intent to launch constellations with the Federal Communications Commission in the United States. 

All those companies together, he said, plan to launch more than 400,000 satellites in the near future. 

“If all those satellites are as bright as constellation satellites are now, this will completely change the night sky! This will make astronomical observations very difficult, especially during the beginning of the night and near the end, when the satellites are most likely to reflect sunlight,” he pointed out.

The trend towards the launch of increasingly larger and brighter satellites continues to grow, the paper warned.

“Researchers and constellation operators such as SpaceX work together on identifying how exactly their satellites reflect sunlight. The better we understand how that works the more insightful mitigation strategies we can come up with,” he noted. 

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