As satellites ramp up, so do concerns

The upcoming launches of large “constellations” of satellites, including 1600 expected to be orbited by the end of this year by SpaceX to provide broadband internet access from space, could ruin many astronomical observations, scientists say.

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This annotated image shows the sky at ESO’s Paranal Observatory around twilight. Blue lines mark degrees of elevation above the horizon. Small green circles mark satellites that are low in the sky and thus faint. Those marked in red would be above 30 degrees of the horizon, the part of the sky where most astronomical observations take place. Credit: ESO/Y Beletsky/L Calçada

Hardest hit, suggests Olivier Hainaut, an astronomer with the European Southern Observatory (ESO) in Germany who has investigated astronomers’ concerns, will be wide-field surveys, which monitor big swathes of sky in the hope of detecting supernovae and other short-lived astronomical events. (Such telescopes can also be used to spot potentially dangerous asteroids.)

Hainaut calculates, for example, that if projects under development by Amazon, OneWeb and others join the SpaceX satellites, there will soon be enough satellites in orbit to damage a third to half of observations from the US National Science Foundation’s Vera C Rubin Observatory, currently under construction on a mountaintop in Chile.

Narrower field-of-vision telescopes, such as the ESO’s €1 billion Extremely Large Telescope, now under construction on a different Chilean mountaintop, would see fewer observations ruined. 

But as many as 3% of time exposures lasting 1000 seconds or more (about 16 minutes) would be wrecked by streaks of light from passing satellites, especially shortly after sunset or before sunrise, when they are more likely to be visible. (In the middle of the night, most satellites are in the Earth’s shadow, and therefore invisible.)

The problem, Hainaut says, isn’t new. Already, there are about 2300 active satellites and several thousand inactive ones in orbit, along with tens of thousands of pieces of debris larger than 10 centimetres. 

But the new wave of mega-constellations, his team reports in the journal Astronomy & Astrophysics, could add another 26,000 satellites to the total, about 1600 of which would be in the sky above any given observatory at any given time. 

The problem becomes worse if the new satellites are brighter than existing ones – a possibility, Hainaut says, because many will be in low orbits, closer and therefore brighter.

“Currently, at twilight,” he says, “there are 1-2 bright satellites in the high sky (that is, above 30 degrees of elevation, and 5-10 faint ones. With the 26,000 [new ones], there would be in addition, 3-5 bright ones and 10-15 fainter ones.” 

Already, at least one organisation, concerned about preserving the pristine beauty of the night sky, is petitioning to halt the launches. 

But for astronomers, there are less draconian ways to mitigate the problem. 

The simplest is to collaborate with satellite builders to make the satellites darker, and less obtrusive. But doing this isn’t as easy as it sounds, Hainaut says. 

“If a satellite is completely black, it would get too warm, and fail. So, the idea is to darken the part of the satellite that is facing down.” SpaceX, he adds, is experimenting with coatings and panels to do exactly that.

Another solution, Hainaut says, is for observatories to keep track of the motion of all satellites and either point their telescopes in directions where there will be no interference, or close down the exposure just before a satellite crosses the field of view, then resume once it is gone. 

“The satellites are moving very fast, so the interruption would be very short,” he says. “It is not that hard,” he adds, quipping, “it is just rocket science…”

This would work very well for narrow to medium field-of-view images, he says, but for wide field-of-view instruments “it becomes problematic.” 

“The number of satellites crossing the view could be large,” he says, “so you need to interrupt too often.” And, “for very wide cameras, the shutter is a very large mechanical piece, and opening/closing/opening it very often could become problematic.”

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