Starlink Satellites pass overhead near Carson National Forest, NM, photographed soon after launch. Credit: Mike Lewinski / CC-BY 2.0
In the last several years, some of my research and service work has focused on the subject of satellite 'megaconstellations' — coordinated groups of hundreds to tens of thousands of Earth-orbiting satellites — and the effect they may have on astronomy. This page summarizes the issue, astronomers' concerns, actions taken to date to deal with the problem, and some scholarship my co-authors and I have contributed to the debate.
Background
The changing human uses of near-Earth space
Humans first launched objects into outer space a little over 65 years ago. Nearly seven decades later, the region of outer space near the Earth is changing. Many thousands of satellites now orbit our planet, accompanied by vastly more pieces of space debris, often called "space junk". Satellites have proven useful to our species in providing platforms for Earth observation and global telecommunications.
But the space around our planet is quickly growing crowded, and that risks catastrophe. A new era in the human use of outer space began in 2019. Private commercial space companies began launching massive groups of satellites often referred to in news stories as "megaconstellations". While there is no exact definition of that word, the number of satellites to which it refers ranges from hundreds to tens of thousands. Satellites in these large constellations are intended to function together to serve a common purpose, such as delivering broadband Internet access. Companies use large numbers of satellites in low orbits to ensure uniform coverage of the Earth and to achieve high network speeds.
Since the start of the megaconstellation era, the number of functional satellites has doubled to almost 5000. Space companies have publicly announced plans for the launch of up to 400000 satellites by 2030. More than one million pieces of debris larger than 1 centimeter in size may already exist in orbit around our planet. Hostile events, such as destructive antisatellite weapons tests, threaten to increase these numbers. And each of those pieces of debris threatens to collide with other objects, potentially setting off a cascade of further debris-generating events.
But the space around our planet is quickly growing crowded, and that risks catastrophe. A new era in the human use of outer space began in 2019. Private commercial space companies began launching massive groups of satellites often referred to in news stories as "megaconstellations". While there is no exact definition of that word, the number of satellites to which it refers ranges from hundreds to tens of thousands. Satellites in these large constellations are intended to function together to serve a common purpose, such as delivering broadband Internet access. Companies use large numbers of satellites in low orbits to ensure uniform coverage of the Earth and to achieve high network speeds.
Since the start of the megaconstellation era, the number of functional satellites has doubled to almost 5000. Space companies have publicly announced plans for the launch of up to 400000 satellites by 2030. More than one million pieces of debris larger than 1 centimeter in size may already exist in orbit around our planet. Hostile events, such as destructive antisatellite weapons tests, threaten to increase these numbers. And each of those pieces of debris threatens to collide with other objects, potentially setting off a cascade of further debris-generating events.
Satellites threaten astronomy
Shortly after the launch of the first satellites in the SpaceX "Starlink" constellation in 2019, astronomers began to see a distinct increase in the number of instances in which satellites affected their images of the night sky. Orbiting high above Earth, they are seen as bright objects on the night side of the planet as they remain in sunlight. Reflecting that sunlight back down to the ground, they can quickly overwhelm the faint light of distant objects astronomers try to photograph with their telescopes. Not even space telescopes are immune; the Hubble Space Telescope, for example, orbits below many of these satellites and sees them in its images, too.
But the concern doesn't end with the appearance of bright streaks or trails of light in astronomical images. There is some evidence that small debris, in particular, represents a further threat to astronomy. Millions of pieces of space debris too small to be individually resolved by telescopes still reflect light that telescopes sense. The effect is to raise the brightness of the surrounding sky near astronomical objects of interest, making it more difficult to detect those objects.
The problem also isn't limited to visible light. Sunlight heats up satellites and debris, causing them to emit light at infrared wavelengths that interferes with astronomers' observations at the same wavelengths. And communications satellites' radio transmissions to the ground make them bright sources of "light" at longer, radio wavelengths. Radio emissions from satellites can sometimes be sufficient to damage the sensitive detectors of radio telescopes.
But the concern doesn't end with the appearance of bright streaks or trails of light in astronomical images. There is some evidence that small debris, in particular, represents a further threat to astronomy. Millions of pieces of space debris too small to be individually resolved by telescopes still reflect light that telescopes sense. The effect is to raise the brightness of the surrounding sky near astronomical objects of interest, making it more difficult to detect those objects.
The problem also isn't limited to visible light. Sunlight heats up satellites and debris, causing them to emit light at infrared wavelengths that interferes with astronomers' observations at the same wavelengths. And communications satellites' radio transmissions to the ground make them bright sources of "light" at longer, radio wavelengths. Radio emissions from satellites can sometimes be sufficient to damage the sensitive detectors of radio telescopes.
Astronomers get organized
Astronomers quickly began studying the effects of megaconstellations to understand the risks to the night sky. They engaged directly with companies that launch satellites, international bodies like the United Nations, and the broader community of night-sky stakeholders. These efforts led to some spacecraft design modifications that yielded some reductions in satellite brightness. At the same time, the pace of launches continues to increase. Risks to both the night sky and near-Earth space are great unless these resources are carefully managed. But right now, there are few national or international laws or regulations that put meaningful limits on the acceptable amount of harm that satellites can cause to the night sky.
In 2020 and 2021, astronomers held a series of four workshops to better understand the problem and chart a path forward. Two "SATCON" meetings were convened by the U.S. National Science Foundation and focused on the American aspects. This was important because most commercial space launches now take place from U.S. territory. In a parallel process, an international group led by the International Astronomical Union put on two "Dark and Quiet Skies for Science and Society" events under the auspices of the United Nations Office of Outer Space Affairs. These were broader in scope than the SATCONs and included ground-based light pollution concerns. The final report (PDF) from each of these events is linked below.
In 2020 and 2021, astronomers held a series of four workshops to better understand the problem and chart a path forward. Two "SATCON" meetings were convened by the U.S. National Science Foundation and focused on the American aspects. This was important because most commercial space launches now take place from U.S. territory. In a parallel process, an international group led by the International Astronomical Union put on two "Dark and Quiet Skies for Science and Society" events under the auspices of the United Nations Office of Outer Space Affairs. These were broader in scope than the SATCONs and included ground-based light pollution concerns. The final report (PDF) from each of these events is linked below.
SATCON 1 (29 June-2 July 2020)
SATCON 2 (12-16 July 2021)
Dark and Quiet Skies I (October 2020)
- Report and Recommendations
Dark and Quiet Skies II (October 2021)
Dark and Quiet Skies resulted in an ongoing process at the United Nations Committee for the Peaceful Uses of Outer Space ensuring that astronomy concerns stay on the Committee's agenda each year. It also led to the creation of the IAU Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference. More on that below.
An image of the NGC 5353/4 galaxy group made with a telescope at Lowell Observatory in Flagstaff, Arizona, USA on the night of Saturday 25 May 2019. The diagonal lines running across the image are trails of reflected light left by more than 25 of the 60 recently launched Starlink satellites as they passed through the telescope’s field of view. Credit: Victoria Girgis/Lowell Observatory.
Learning more about this issue
Before jumping into a description of my work on this issue, here are some links to places where readers can learn more about large satellite constellations and their effects. These are mostly non-technical summaries and primers.
McKinsey & Company: "Large LEO satellite constellations: Will it be different this time?" (2020)
U.S. National Science Foundation: "JASON Report on the Impacts of Large Satellite Constellations" (2020)
U.S. Government Accountability Office: "Large Constellations of Satellites: Mitigating Environmental and Other Effects" (2022)
Olivier Hainaut (European Southern Observatory): "Large Satellite Constellations & their Impact on Astronomy" (2022)
U.S Congressional Budget Office: "Large Constellations of Low-Altitude Satellites: A Primer" (2023)
Jonathan McDowell (Harvard-Smithsonian Center for Astrophysics): "Large LEO Satellite Constellations"
U.S. National Science Foundation: "JASON Report on the Impacts of Large Satellite Constellations" (2020)
U.S. Government Accountability Office: "Large Constellations of Satellites: Mitigating Environmental and Other Effects" (2022)
Olivier Hainaut (European Southern Observatory): "Large Satellite Constellations & their Impact on Astronomy" (2022)
U.S Congressional Budget Office: "Large Constellations of Low-Altitude Satellites: A Primer" (2023)
Jonathan McDowell (Harvard-Smithsonian Center for Astrophysics): "Large LEO Satellite Constellations"
My research work
I became involved in researching satellite megaconstellations immediately after the first Starlink launch in 2019. In particular, I'm interested in the policy, historical, and cultural background and implications of megaconstellations as they interact with astronomy. In the years since, my colleagues and I have published a few papers on these topics.
Megaconstellations and rising diffuse night sky brightness
For this project I worked with Miro Kocifaj and František Kundracik (Comenius University, Slovakia) and Salva Bará (now retired from the University of Santiago de Compostela in Galicia, Spain). We wondered whether the collective contribution of scattered and reflected light from many small objects could raise the apparent brightness of the sky background. This is a separate effect from individual streaks of light in astronomical images due to objects are spatially resolved, and thus appear as discrete light points moving across the sky.
One way to think about this is by imaging a camera photographing a landscape. Given a lens with a sufficiently narrow field of view and a detector consisting of many millions of pixels, a sharp and well-defined image will be captured. Now expand the field of view of the lens; the images are all still recognizable. But then start reducing the number of detector pixels. Individual objects become unrecognizable even though the same amount of light reaches the detector. In the most extreme case, the number of pixels decreases to one. How bright is it, and what color is it? The individual objects in the field of view are now fully impossible to recognize, but their collective light is all somewhere in that one pixel.
This is essentially the situation when we use devices like the Sky Quality Meter (SQM) to measure the brightness of the night sky. The device uses a small block of semiconductor material that absorbs light coming in through an entrance aperture. It "counts" the light based on the frequency of an electric signal running through it. The SQM version commonly in use employs a lens to fix the amount of sky the semiconductor "sees". In that sense it is like a camera with a single pixel. The cartoon below shows an SQM at lower left; the cone projecting away from it toward the upper right represents the field of view of that pixel. The cloud of satellites at upper right is a hypothetical group of satellites that fills the field of view at one time.
One way to think about this is by imaging a camera photographing a landscape. Given a lens with a sufficiently narrow field of view and a detector consisting of many millions of pixels, a sharp and well-defined image will be captured. Now expand the field of view of the lens; the images are all still recognizable. But then start reducing the number of detector pixels. Individual objects become unrecognizable even though the same amount of light reaches the detector. In the most extreme case, the number of pixels decreases to one. How bright is it, and what color is it? The individual objects in the field of view are now fully impossible to recognize, but their collective light is all somewhere in that one pixel.
This is essentially the situation when we use devices like the Sky Quality Meter (SQM) to measure the brightness of the night sky. The device uses a small block of semiconductor material that absorbs light coming in through an entrance aperture. It "counts" the light based on the frequency of an electric signal running through it. The SQM version commonly in use employs a lens to fix the amount of sky the semiconductor "sees". In that sense it is like a camera with a single pixel. The cartoon below shows an SQM at lower left; the cone projecting away from it toward the upper right represents the field of view of that pixel. The cloud of satellites at upper right is a hypothetical group of satellites that fills the field of view at one time.
While the SQM doesn't record anything like an image, it is affected by the light of whatever happens to be in that cone. As compared to a view of the sky without any satellites, the light of the satellites in this example raises the brightness of the sky as it is distributed more or less uniformly within the cone.
The same principle applies to the pixels in any camera when the brightness of objects is below the threshold for them to detect a light source as a discrete unit. The light still makes it to the detector, but it is not spatially distinct as in the case of a streak or trail. As objects become too small, and/or too dim, for a detector to sense them as discrete units, the "budget" of their light contribution to the image is transferred from trails to the diffuse background.
The same principle applies to the pixels in any camera when the brightness of objects is below the threshold for them to detect a light source as a discrete unit. The light still makes it to the detector, but it is not spatially distinct as in the case of a streak or trail. As objects become too small, and/or too dim, for a detector to sense them as discrete units, the "budget" of their light contribution to the image is transferred from trails to the diffuse background.
What we did
We aimed to figure out how much of an effect this would be for any photometer or imager. To model the situation, we assumed that satellites and debris orbiting the Earth formed a uniform ring illuminated by the Sun.
We had to make some assumptions about the size of those objects, their reflectivity, and their distribution in space to come up with a number representing their brightness as seen from the night side of the Earth. Then we used radiative transfer theory to follow that light to the ground, adding up how much would be seen in the night sky.
What we found
We calculated that at the zenith (i.e., the "top of the sky") at any given location on Earth, the effect described above was adding about 20 microcandelas per square meter to the apparent brightness of the night sky. In the absence of light pollution on the ground, natural sources of light in the night sky add up to about 200 microcandelas per square meter. Therefore, if our estimate is correct, it means that satellites and space junk are adding 10% on top of that figure. Coincidentally, 10% over the natural background is a critical threshold adopted in 1979 by the International Astronomical Union as its definition of a "light-polluted" site for astronomical observatories. However, it's a very small effect compared to the contribution of cities to skyglow, which can be as high as 300000 microcandelas per square meter. In other words, it amounts to much less than 1% of the brightness of the night sky in or near a city.
The diffuse light contributed by space objects seen from the ground should reach a peak after the onset of darkness and then rapidly decrease. This is because for most objects in low-altitude orbits, they disappear into the shadow of the Earth due to the planet's rotation. Only a tiny amount of light remains later, mainly contributed by satellites in geostationary orbits. They are up so high that they remain in sunlight all night long.
The diffuse light contributed by space objects seen from the ground should reach a peak after the onset of darkness and then rapidly decrease. This is because for most objects in low-altitude orbits, they disappear into the shadow of the Earth due to the planet's rotation. Only a tiny amount of light remains later, mainly contributed by satellites in geostationary orbits. They are up so high that they remain in sunlight all night long.
No one has yet directly measured this effect in order to test the predictions of our model. It would be difficult to make such a measurement from near the surface of the Earth because faint light emitted by the atmosphere at night is a factor of five brighter than the space debris effect. A possible way to measure it is to put a sensitive camera aboard a platform like the International Space Station, which orbits above most of the atmosphere but below the majority of satellites and space debris.
A few months later, some of our European colleagues published a paper in which they argued that the effect of objects raising the diffuse brightness of the sky was most sensitive to the production of the smallest pieces of debris -- literal dust. The authors write that "the macroscopic satellites composing the constellations discussed in this paper will not contribute much to the diffuse sky brightness provided they are not ground into microscopic debris." This provides further evidence that to protect astronomy, we must ensure that satellites and their associated launch hardware don't become the sources of significant new (small) debris.
A few months later, some of our European colleagues published a paper in which they argued that the effect of objects raising the diffuse brightness of the sky was most sensitive to the production of the smallest pieces of debris -- literal dust. The authors write that "the macroscopic satellites composing the constellations discussed in this paper will not contribute much to the diffuse sky brightness provided they are not ground into microscopic debris." This provides further evidence that to protect astronomy, we must ensure that satellites and their associated launch hardware don't become the sources of significant new (small) debris.
Publication
The proliferation of space objects is a rapidly increasing source of artificial night sky brightness. Kocifaj, M., Kundracik, F., Barentine, J. C., & Bará, S. (2021). Monthly Notices of the Royal Astronomical Society: Letters, 504(1), L40-L44.
doi:10.1093/mnrasl/slab030
doi:10.1093/mnrasl/slab030
Re-imaging the landscape of global space policy
To come...
Astronomical data "lost in the noise"
To come...
Work with AAS and CPS
To come...