Ask Ethan: How Are Mega-Constellations Of Satellites Changing The Night Sky?

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This image shows the first 60 Starlink satellites launched into orbit on May 23, 2019. They are still in their stacked configuration, just prior to being deployed. You can clearly see that these satellites are quite reflective and quite large; continuing to launch such satellites even after many legitimate concerns and solutions have been brought to SpaceX's attention has raised many issues and questions of interest among the general public and the astronomy community.

For countless millennia, human beings have gazed up into the abyss of the night sky, mesmerized by the natural wonders of the planets, stars, and Universe beyond our world. Beginning with Sputnik in 1957, however, humanity began contending with artificial points of light streaking through the sky: satellites. With the push towards mega-constellations involving thousands of new satellites, many have expressed concerns, from casual skywatchers to astrophotographers to professional astronomers. This includes Mark Bailey, who writes in to ask the following:

I’m worried sick about Elon Musk’s crazy satellite constellation fiasco. I watched a string of them drift by brightly the other morning predawn as I was wrapping up my telescope observing for the night. They outshined most stars in the sky and it hasn’t yet begun. [...] I’ve always relied on the heavens for solace and inspiration. The thought of one man ruining OUR constellations—the constellations that our ancestors watched in awe for eons—sickens me in a way like nothing before. What can be done to stop this foolish ripoff of our rightful heritage?

I sympathize with this position, but it's important to understand how these satellites actually will and won't impact our view of the skies. Here's where we are today.

On November 18, 2019, a constellation of Starlink satellites passed through the observing frame of the Dark Energy Camera aboard the 4m telescope at CTIO. Any technique that we'd use to subtract out these trails would hinder our ability to detect potentially hazardous asteroids or measure variable objects in the Universe.

The motivation. You can do things from space that you cannot do from Earth's surface. These include:

We've been doing this with satellites for a long time, both for telecommunications and for GPS. However, we're fundamentally limited by the physics of electromagnetic waves in this endeavor.

Thousands of human-made objects—95 % of them “space junk”— occupy low and medium Earth orbit. Each black dot in this image shows either a functioning satellite, an inactive satellite, or a large-enough piece of debris. The current and planned 5G satellites will vastly increase both the number and the impact that satellites have on optical, infrared, and radio observations taken from Earth and taken of Earth from space, and raise the potential for Kessler syndrome.

The limitations. If all you wanted was continuous coverage from space of the entire surface of the Earth, a small number of geosynchronous (orbiting at the right distance so that they're always over the same point on Earth's surface) satellites would do the job. This is a fine location for many Earth-observing satellites, as well as many satellites that only need to send-and-receive a small amount of data. However, there are two fundamental limitations to geosynchronous satellites.

  1. Geosynchronous orbits require an altitude of ~36,000 kilometers (~22,000 miles), which requires light to take about a quarter of a second to complete a round-trip journey from Earth: about 50-100 times the latency of a low-Earth orbiting satellite.
  2. Because all electromagnetic waves spread out in proportion to the distance squared, a geosynchronous satellite, at about 50-100 times the altitude of a low-Earth orbiting satellite, can only achieve ~0.01%-to-0.04% the data throughput as low-Earth orbiting satellites.
The brightness distance relationship, and how the flux from a light source falls off as one over the distance squared. A satellite that's twice as far away from Earth as another will appear only one quarter as bright, but the light-travel time will be doubled and the amount of data throughput will also be quartered.

The new application. That's the explanation for why the coming explosion of satellite mega-constellations is all but inevitable. If you want to transmit large amounts of data to-and-from Earth's surface without laying ground-based infrastructure, you need continuous satellite coverage from a network of low-altitude satellites. Those satellites need low latencies and high throughputs, which means low-Earth orbit is the way to go.

There are many potential problems with implementing such a network, however, and the most obvious one is that this is going to interfere with the night sky as never before. Instead of seeing an occasional satellite, we might have dozens or even hundreds populating the skies for all observers on Earth simultaneously. Even if they're rendered dim enough to be invisible to the naked eye, there might even be more satellites than stars through a pair of binoculars. And then, on top of it all, there's the cost to astronomy.