Effects of Large Satellite Constellations on Astronomy


Many of you will have seen the images back in 2019 which showed  long ‘trains’ composed of up to sixty SpaceX Starlink satellites crossing the sky in a straight line. These pictures hit the headlines not only because of the number of satellites, but because of their brightness.

A ‘train’ of SpaceX Starlink satellites, the satellites are around 300 km in altitude. In the days after launch these trains break up as the satellites position themselves into their final orbits at around 550 km altitude.

The satellites are still being launched in batches of sixty at a time and their purpose  is to provide worldwide fast internet coverage. Traditional satellite internet operators have a small number of satellites in geostationary orbit (GEO), 35 800 km above the  Earth’s surface. This limits response times, coverage and bandwidth. It takes around 0.12 seconds for radio-waves to travel between the satellite and the Earth.

SpaceX’s plan is to surround the Earth with a large number of satellites in Low Earth Orbit (LEO). This offers much better response times and greater capacity than GEO satellites. The plan is that the network of Starlink satellites will grow over the next few years forming a ‘floating internet backbone’, providing a faster alternative to the fibreoptic cables that span the world.

Satellite constellations Rebuilding internet

At time of writing this post,  SpaceX have launched 2282 Starlink satellites, all in orbits around 550 km above the Earth.  2062 of these satellites are still in orbit. (Most of the satellites launched in 2019 were test satellites which have been intentionally deorbited.)  They have been given approval by the U.S. Federal Communications Commission (FCC) to launch up to 12000 satellites  and  are seeking permission to put an additional 30 000 Starlink satellites into orbit.  To put these numbers into perspective, in the sixty five years since the launch of Sputnik-1 back in 1957, nearly 13 000 satellites have been placed into Earth orbit, of which 5300 are still working. So around 40% of  functioning satellites are SpaceX Starlinks!

Satellite constellations satellites in orbit

Source  ESA

Other Operators

High speed satellite broadband is likely to be a big growth area in the future, because it offers access to internet without need for an expensive fixed line infrastructure, which can be difficult and very costly to build in remote areas.   By 2040, it may be worth in excess of 400 billion dollars per annum. So, it is not surprising that other companies are planning similar large LEO satellite constellations. Although none of these companies seem to have plans as ambitious as SpaceX or have launched as many satellites.


OneWeb are British company partially owned by the UK government. They have built an initial constellation of 428 satellites, which may be expanded as demand for their service grows.

Satellite constellations OneWeb2

Unlike SpaceX Starlink, OneWeb are aiming their service to businesses, governments  departments, phone network operators and other organisations rather then individual consumers.

Amazon – Project Kuiper

Amazon plan to launch  a constellation of 3236 satellites. A development known as project Kuiper. As yet no satellites have been launched, the first launches are planned this year (2022). So they are a few years behind SpaceX, but they intend to be a big player in a growing market and Amazon have massive financial muscle.  In terms of turnover Amazon is one of the largest companies on our planet (2021 turnover was $470 billion) and in 2020 it was reported that Amazon would invest $10 billion in Project Kuiper.

Satellite constellations Amazon


The Canadian company Telesat have a constellation of 298 LEO satellite, which it plans to expand to around 1700 satellites in the near future.

Satellite constellations Telesat

Impact on astronomy

There has been much written in the press in the last few years about the impact of these satellite constellations on astronomy (not all of  these reports have been accurate). Compared to astronomical objects and satellites in higher orbits, LEO Satellites move rapidly across the sky and are in general brighter than other satellites because they are closer to Earth.

However LEO satellites spend most of the night in the Earth’s shadow and are only visible in the twilight hours. This is described further in the video at the end of this post.

Satellite constellations LEO

A satellite in a Low Earth orbit will spent roughly half of its orbit in the Earth’s shadow when it will be invisible to an observer because it is not illuminated by the Sun

Space X have been engaging with the astronomical community to reduce the brightness of the Starlink satellites. The  long trains of satellites seen just after launch are a temporary formation, which break up as the satellites disperse and move to their higher orbits 550 km above the Earth. All Starlink satellites launched since August 2020 have a visor fitted which reduces the amount of sunlight reflected back to Earth by a factor of three. A recent study found that the average magnitude of Starlink satellite fitted with the visor was +5.92 – making them only just visible to the naked eye, when viewed by someone with good eyesight at a location without significant light pollution.

In raw astronomical images satellites are sometimes captured when they move across a telescope’s field of view. They appear as streaks, looking a little like meteor trails. However, image processing software used by astronomers (including amateurs) can identify satellite tracks and strip them out of the final images.

This might present more of an issue if the number of LEO satellites were to increase twenty-fold in the next 15 years. Rather than an image containing a single satellite trail, a typical image taken in the twilight hours would contain multiple trails, as numerous satellites crossed the field of view.

The large telescope being constructed at the Vera Rubin observatory may be particularly badly affected. It has an 8.4 metre primary mirror (giving it a very high sensitivity) and also a very large detector giving it a very large of view – so many satellites will be captured in its images. This is a topic I discussed in a previous post. 

LSST Rubin Observatory

Impact on radio astronomy

Radio astronomy is allocated protected bands in which radio transmitters, including those on satellites, are forbidden to operate. Examples of protected bands are

  • the region around 1.420 MHz known as ‘the 21-cm hydrogen line’
  • the region around 1.660 GHz the OH-line
  • the region around 22.235 GHz,  which is a frequency at which interstellar water vapour clouds emit radio waves.

However, radio astronomers sometimes take observations outside the protected bands. They can do this because large radio telescope are often situated in radio quiet zones, often in remote areas, where there is  very low quantity of manmade radio transmissions. If, in the coming decades, there are tens of thousands of LEO satellites beaming powerful radio transmission at all locations on Earth, then in future there may be no radio quiet zones anywhere and many frequencies outside the protected bands will be closed to radio astronomy. This is a topic about which I could write a great deal about but, in the interests of keeping this post at a reasonable length ;-),  I’ll leave the final word to Tony Beasley, director of the National Radio Astronomy Observatory,  who said in 2020.

SpaceX is legally transmitting inside one of their [allocated] bands and there are going to be impacts for anyone trying to do radio astronomy,” ….”These spectrum allocations represent the goals and intent of society. We make [them] to enable commerce and to enable defense and all kinds of activities. We have to come to a solution that satisfies all these to some extent

And Finally……

It will be interesting to see how things develop over the next decade and how big the impact on ground-based astronomy turns out to be. A lot will depend on willingness of the satellite broadband companies to engage with the astronomical community. What is clear to me is that satellite broadband  is likely to become more and more important over the next decade.

For those reader who want a little more information I have made this video on the Explaining Science YouTube channel which expands some of these topics further


When measuring the brightness of objects in the sky, astronomers use the magnitude scale where the lower the magnitude value the brighter the object.  The scale was invented by the ancient Greek astronomers who classified all the stars visible to the naked eye into six magnitudes. The brightest stars were given a  magnitude of 1, the next brightest magnitude 2 and so on. The faintest stars visible to the naked-eye by someone with good eyesight were given a  magnitude of 6.

Values in the magnitude scale were standardised  by nineteenth century astronomers  to make an increase in magnitude of 5 a decrease in luminosity of factor of 100. So for example, a star of magnitude 6 appears one hundred times fainter than one of magnitude 1.  The brightest star in the  sky Sirius has magnitude -1.4. The first artificial satellite Sputnik had a magnitude of around 6 – making it just about visible to the naked eye when viewed at a location without significant light pollution

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