Surging satellite numbers threaten to dazzle even space telescopes
Starlink is growing fast, and SpaceX has filed paperwork for as many as 42,000 satellites.
One of the reasons astronomers like to put telescopes in space is that the “seeing” is better. With no turbulent atmosphere in the way the stars shine steadily, rather than twinkling as they do when seen from the ground. But it is not just an atmosphere that can ruin the seeing. The rapidly growing number of satellites in low-Earth orbit is already causing problems for ground-based telescopes, which find their images contaminated by streaks of sunlight reflected off passing satellites. Now, a paper published in Nature suggests that could soon become a serious problem for space-going telescopes as well.
Alejandro Borlaff and his colleagues at NASA’s Ames Research Centre, in California, scoured regulatory filings to work out just how many satellites might be in orbit by the end of the next decade. Numbers are already exploding. In 2018 there were around 2,000 satellites in orbit. These days there are more than 9,000 of SpaceX’s Starlink internet satellites alone.
Starlink is growing fast, and SpaceX has filed paperwork for as many as 42,000 satellites. Others such as OneWeb, Amazon, Qianfan and GuoWang are also building “mega-constellations”. In total Dr Borlaff reckons there could be around half a million satellites in low orbits—below about 2,000 kilometres—by 2040.
His team fed information about the size and orbits of dozens of planned mega-constellations into a computer simulation to work out how they might affect four space telescopes in particular. The Hubble Space Telescope and SPHEREx are both run by NASA, and are in orbit already. China’s Xuntian space telescope is due to launch next year, while the European Space Agency’s ARRAKIHS instrument is slated to be in space by 2030.
Exactly how bothersome the satellites will be depends on factors such as how high a telescope flies and whether it has a wide or narrow field of view. Crunching the numbers, the researchers concluded that around a third of Hubble images could be affected. That sounds bad enough. But if anything, the Hubble gets off lightly. Around 96% of exposures from SPHEREx, ARRAKIHS and Xuntian could be contaminated, with an average of 5.6, 69 and 92 streaks per image respectively. (The picture on the previous page shows a simulated picture taken by ARRAKIHS, streaked by orbiting satellites.)
What to do? SpaceX has tried to make its satellites less reflective, though with limited success, notes Dr Borlaff. But business pressures push in the opposite direction. Starlink’s satellites have grown in size over time, in order to serve more customers and offer higher connection speeds. Bigger satellites are usually brighter ones.
Co-ordination and data-sharing can help. Astronomers armed with details of a mega-constellation’s orbits can time their observations to minimise interference. But that solution does not scale well, says Dr Borlaff: eventually the sky becomes so crowded that avoidance is impossible.
What to do? The same cheap rockets that have made mega-constellations possible could also make it easier to loft future space telescopes above the growing swarms of satellites. The James Webb Space Telescope, for instance, is 1.5m km from Earth, far too distant for low-flying satellites to pose a problem. But flying low can offer advantages for telescopes just as it does for communications satellites, says Dr Borlaff, especially for space agencies that lack NASA’s budget. Earth’s magnetic field helps shield low-flying telescopes from cosmic rays, for instance. And flying low makes downloading data easier.
Dr Borlaff hopes diplomacy and regulation can help. He cites the Montreal Protocol, agreed in 1987, in which the world pledged to stop using chemicals that damage the ozone layer. In 2022 the International Astronomical Union set up the Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference. One of its jobs is to allow astronomers and mega-constellation operators to find compromises—limiting satellite numbers or orbits, for example. In the middle of a commercial and international space race, it will have its work cut out.
One of the reasons astronomers like to put telescopes in space is that the “seeing” is better. With no turbulent atmosphere in the way the stars shine steadily, rather than twinkling as they do when seen from the ground. But it is not just an atmosphere that can ruin the seeing. The rapidly growing number of satellites in low-Earth orbit is already causing problems for ground-based telescopes, which find their images contaminated by streaks of sunlight reflected off passing satellites. Now, a paper published in Nature suggests that could soon become a serious problem for space-going telescopes as well.
Alejandro Borlaff and his colleagues at NASA’s Ames Research Centre, in California, scoured regulatory filings to work out just how many satellites might be in orbit by the end of the next decade. Numbers are already exploding. In 2018 there were around 2,000 satellites in orbit. These days there are more than 9,000 of SpaceX’s Starlink internet satellites alone.
Starlink is growing fast, and SpaceX has filed paperwork for as many as 42,000 satellites. Others such as OneWeb, Amazon, Qianfan and GuoWang are also building “mega-constellations”. In total Dr Borlaff reckons there could be around half a million satellites in low orbits—below about 2,000 kilometres—by 2040.
His team fed information about the size and orbits of dozens of planned mega-constellations into a computer simulation to work out how they might affect four space telescopes in particular. The Hubble Space Telescope and SPHEREx are both run by NASA, and are in orbit already. China’s Xuntian space telescope is due to launch next year, while the European Space Agency’s ARRAKIHS instrument is slated to be in space by 2030.
Exactly how bothersome the satellites will be depends on factors such as how high a telescope flies and whether it has a wide or narrow field of view. Crunching the numbers, the researchers concluded that around a third of Hubble images could be affected. That sounds bad enough. But if anything, the Hubble gets off lightly. Around 96% of exposures from SPHEREx, ARRAKIHS and Xuntian could be contaminated, with an average of 5.6, 69 and 92 streaks per image respectively. (The picture on the previous page shows a simulated picture taken by ARRAKIHS, streaked by orbiting satellites.)
What to do? SpaceX has tried to make its satellites less reflective, though with limited success, notes Dr Borlaff. But business pressures push in the opposite direction. Starlink’s satellites have grown in size over time, in order to serve more customers and offer higher connection speeds. Bigger satellites are usually brighter ones.
Co-ordination and data-sharing can help. Astronomers armed with details of a mega-constellation’s orbits can time their observations to minimise interference. But that solution does not scale well, says Dr Borlaff: eventually the sky becomes so crowded that avoidance is impossible.
What to do? The same cheap rockets that have made mega-constellations possible could also make it easier to loft future space telescopes above the growing swarms of satellites. The James Webb Space Telescope, for instance, is 1.5m km from Earth, far too distant for low-flying satellites to pose a problem. But flying low can offer advantages for telescopes just as it does for communications satellites, says Dr Borlaff, especially for space agencies that lack NASA’s budget. Earth’s magnetic field helps shield low-flying telescopes from cosmic rays, for instance. And flying low makes downloading data easier.
Dr Borlaff hopes diplomacy and regulation can help. He cites the Montreal Protocol, agreed in 1987, in which the world pledged to stop using chemicals that damage the ozone layer. In 2022 the International Astronomical Union set up the Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference. One of its jobs is to allow astronomers and mega-constellation operators to find compromises—limiting satellite numbers or orbits, for example. In the middle of a commercial and international space race, it will have its work cut out.
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