Questionnaire on Impact to Optical Astronomy of Large Constellations of Low-Earth Orbit Satellites

Dear Colleagues,

We write to request your prompt feedback on a potentially serious threat to ground-based astronomy. We would like to receive your input by 19 December 2019.

As you no doubt know, numerous projects are underway internationally to launch large "constellations" of low-earth orbiting (LEO) satellites. As of this writing, SpaceX has launched two sets of 60 Starlink satellites each, out of a total of 24 launches planned with 1584 total satellites by the end of 2020, and with the Federal Communications Commission's permission to launch 2,825 more satellites. The Starlink satellites orbit at a final altitude of 550 km, after initial deployments launched around 280 km. Meanwhile, other broadband connectivity satellite operators including OneWeb, Amazon, Samsung, Telesat, as well as private companies in India and China have stated plans to launch a total of over 25,000 LEO satellites in the next few years. SpaceX has also filed plans to launch up to 30,000 additional LEO satellites.

If all those plans are carried out, the number of operating and defunct artificial satellites and large debris in LEO could jump from roughly 15,000 — accumulated over the course of 60 years — to more than five times that number in just five years.

What will be the impacts on astronomy of these new large constellations of LEO satellites?

Observations by professional and amateur astronomers of the first two "tranches" of 60 SpaceX Starlink satellites indicate that after reaching final orbiting altitude of 550 km, the satellites appear to have visual brightness around apparent magnitude 5, i.e., visible to the naked eye under non-light-polluted clear dark skies. They appear much brighter — as bright as magnitude 0 or even brighter — immediately after launch, when they are at lower altitude; they can take up to several weeks to reach final operating altitude. They cross the observer's sky in typically a few minutes.

Several teams in the US and abroad through the International Astronomical Union (IAU), including at the Large Synoptic Survey Telescope, are working with SpaceX to characterize their impact on optical astronomy to aid ongoing efforts to mitigate the visibility of upcoming satellite deployments. In particular, we are aiming to model the expected appearance of the full suite of 1,584 SpaceX Starlink satellites and their possible impact on ground-based astronomical research. The appearance — geometric visibility in the sky, speed, and apparent brightness — is a strong function of location on Earth, time of day, season of the year, and the details of the satellite orbits including altitude and orbital plane, as well as the attitude (orientation), albedo, shape, and size of the satellites.

For example, early results of modeling for 21 December at Cerro Tololo Inter-American Observatory (CTIO) in Chile indicate that for the final Starlink constellation of 1,584 satellites at 550 km altitude in 24 evenly-spaced orbital planes, there will be 6-9 satellites illuminated directly by the Sun or in the Earth's penumbra visible at elevation 30 degrees or more at any time during about an hour after 12-degree evening twilight and an hour before 12-degree morning twilight. At that latitude, the transition between 12-degree and 18-degree twilight lasts about half an hour, but the satellites remain illuminated by the Sun for about twice that time. A few Starlinks will be visible up to two hours after the start of evening 12-degree twilight, and again starting two hours before morning 12-degree twilight. Since the satellites cross the sky in roughly four minutes, to be replaced by another 6-9 satellites as long as they are illuminated, CTIO could see 90-135 Starlink satellites cross the sky during each of those evening and morning twilight+dark hours. Of course those numbers will climb as more satellites are launched to similar altitude, potentially by a factor of five or more in the next five years.

See the AAS statement on satellite constellations from June 2019 here and the IAU statement on satellite constellations here.

Now we seek your feedback. Could you please fill out the questionnaire below by 19 December 2019? Results will be compiled and analyzed by the AAS Committee on Light Pollution, Radio Interference, and Space Debris, and shared at the 235th AAS meeting in Honolulu, Hawai‘i, in January 2020. Your feedback is crucial to our evolving understanding of this threat to ground-based astronomy, and will directly inform our ongoing conversations with SpaceX and possibly other satellite operators as well as decision-makers in the US and internationally.

Thank you for your input.


Assume the numbers and brightness of LEO satellites as described above:

  • Consider satellites to be visible only when altitude ≥ 30 degrees and in direct Sunlight or Earth's penumbra
  • magnitude 5
  • satellites cross sky in 4 minutes
  • 6-9 satellites visible at any time during 1-hour starting (ending) at 12-degree evening (morning) twilight
  • spatial density on sky ~ 7E-4 deg-2
  • angular speed across sky ~ 0.5 - 1 degree / sec
Contact Information