BlueWalker 3: bright, loud, and causing angst amongst astronomy dark sky communities

Concern is growing about the impact on radio astronomy from the massive BlueWalker 3 satellite which unfurled its antenna array last month.

The International Astronomical Union’s (IAU) Center for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference (CPS) says the satellite could interfere with astronomy and other scientific research, and that its radio frequencies could interfere with astronomy-related radio telescope observations.

Bluewalker 3 64-square-metre communications array
AST SpaceMobile’s BlueWalker 3 test satellite is has a 64-square-metre communications array. Credit: AST SpaceMobile

BlueWalker 3 was launched on September 10th this year, developed by AST SpaceMobile, as a low-cost, highly automated satellite platform designed for high-altitude operations and telecommunications.

Its 64-square-metre antenna system is adept at reflecting light and is easily visible to the naked eye, becoming one of the brightest objects in the night sky, according to the IAU CPS.

Bluewalker 3’s large size and bright reflective surfaces could interfere with astronomical observations, as its light could be mistaken for stars or interfere with the ability to detect dimmer objects. Additionally, the satellite’s large size could block out a portion of the night sky, making it difficult for astronomers to observe certain objects.

“The night sky is a unique laboratory that allows scientists to conduct experiments that cannot be done in terrestrial laboratories,” says the IAU’s CPS. “The pristine night sky is also an important part of humanity’s shared cultural heritage and should be protected for society at large and for future generations.”

Trails in the night sky left by bluewalker 3
Trails in the night sky left by BlueWalker 3 are juxtaposed against the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, a Program of NSF’s NOIRLab. The lights from Tucson, Arizona, are seen in the background. Credit: KPNO/NOIRLab/IAU/SKAO/NSF/AURA/R. Sparks

The IAU CPS is also worried about the radio frequencies BlueWalker 3 uses for communications. The satellite is equipped with a number of radio frequency transmitters which operate in the same bands as terrestrial mobile phone networks.

As the satellites orbit the Earth, they are likely to traverse areas of sky which are terrestrial ‘radio quiet’ zones around radio astronomy observatories, such as the Square Kilometre Array Observatory (SKAO) which consists of two enormous radio telescopes, one in Australia and the other in South Africa.

“Astronomers build radio telescopes as far away as possible from human activity, looking for places on the planet where there is limited or no cell phone coverage,” says SKAO Director-General Philip Diamond. “Frequencies allocated to cell phones are already challenging to observe even in radio quiet zones we have created for our facilities. New satellites such as BlueWalker 3 have the potential to worsen this situation and compromise our ability to do science if not properly mitigated.

“This a key reason why the SKAO is deeply involved in the IAU CPS and promoting the equitable and sustainable use of space,” he said.

Unfortunately, when transmitters or passing satellites cause interference, all of the data in that frequency channel is flagged and thrown away. “The contamination is so great that it just wipes out our data,” says Dr Jack Line, a Research Associate with ASTRO 3D at the International Centre for Radio Astronomy Research at Curtin Institute for Radio Astronomy in Western Australia .

“The biggest problem with satellites is that they don’t broadcast with one constant brightness or frequency, making it hard to tell which parts of the data are contaminated, and which aren’t,” he said.

The ionosphere of the Earth bounces around signals of astrophysical sources, affecting where they look like they are located in the sky. “Disentangling the real signals plus satellite effects is hard”, says Line.

Sky trail left by bluewalker 3 over mcmath-pierce solar telescope.
Trail left by BlueWalker 3 over McMath-Pierce Solar Telescope. Credit: KPNO/NOIRLab/IAU/SKAO/NSF/AURA/R. Sparks

And for some astrophysical events, there’s only one chance to see them.

“For a science case like fast radio bursts (FRBs) or other transient objects, this is a nightmare, because some of them only ever go off once,” explains Line. “If one of these transmitters happen to be passing through the sky at the same time, we just miss them entirely.”

AST SpaceMobile has suggested it would take steps to minimise the satellite’s interference, including reducing the satellite’s brightness by using surfaces designed to be less reflective and avoiding radio astronomy areas.

The IAU is still monitoring the situation closely, but says more data is needed to fully understand the impact. They are encouraging anyone who has visual or telescopic observations of BlueWalker 3 to submit a report to SatHub, a worldwide public observing initiative of the IAU CPS, noting details about the observation such as location, altitude, weather conditions and brightness (or magnitude) and length of observation.

Correction and Clarification: Dec 6, 2022: AST Spacemobile has asked that a sentence be removed from the original article. Cosmos reported that the company said: “…it will ensure that the satellite’s operations do not interfere with astronomy in any way.” The company says it has never made this claim, and Cosmos has removed the statement.

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