BBC Sky at Night Magazine
Under a Starlink sky
How will SpaceX’s plans to launch thousands of satellites into low-Earth orbit affect views of the night sky?
By day, Jonathan McDowell is an X-ray astronomer, part of the team who keep the Chandra space telescope running and productive, staring at some of the distant Universe’s most energetic phenomena. In his spare time, he keeps track of objects much closer to home, cataloguing the population of satellites that clutter low Earth orbit.
He’s therefore the right person to weigh in on SpaceX’s rapidly growing constellation of ‘Starlink’ satellites, and in a new paper he does just that. Starlink’s initial plans were for just over 1,500 satellites, but SpaceX has permission to launch as many as 12,000, and maybe many times that. Even with 1,500 in orbit, McDowell explains the effect will be dramatic.
That might seem surprising. There are, after all, nearly 5,000 satellites already in orbit, so the initial Starlink deployment accounts for only a small addition. But to be bright – visible with the naked eye – you need a satellite to be both large and in low Earth orbit, and there just aren’t that many satellites for which this is true. In higher orbits debris and defunct satellites can last a long time, but anything large that orbits under 600km above the surface will burn up in the atmosphere before too long.
“Starlink satellites are both large and low, and already account for the majority of active large satellites in low Earth orbit”
Starlink satellites – which are both large and low – already account for the majority of active large satellites in lower orbits, and a large proportion of the satellites that are visible to the naked eye. If we’re heading for even only 1,500 Starlinks in the near future, four in every five bright satellites will belong to the constellation. I’ve already spotted some of them myself, and as the paper makes clear, they will be visible for long stretches of the night.
Using real observations from a network of volunteer observers (satobs.org), the paper builds a model for how the constellation of satellites will look when deployed. They’re typically between magnitudes
4 and 6 – in naked-eye visibility range from a dark site. From almost any observatory location, hundreds are above the horizon at all times and, during the summer months, they will be bright throughout the night. During the winter, from most locations, there are hours of respite either side of midnight, but much of the night will still be Starlink-streaked.
For the biggest surveys astronomers are planning, which use wide-field cameras to cover much of the sky, it seems possible that every image taken will have a satellite streak due to a Starlink satellite. For casual observers, who may care more about the view near the horizon, or those hunting near-Earth asteroids by scanning the twilight sky, the situation is worse. Can anything be done? SpaceX themselves have experimented with changing the design of the spacecraft, and one special, darkened satellite is currently in orbit. Initial observations suggest that the changes might work, taking the satellite out of naked-eye visibility, but more careful monitoring is needed. The warning from this paper is that our night sky might be changing, and fast – and if so it will never be the same again.
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