Things Are Looking Brighter! But Not The Stars

Growing up in Montana I remember looking out at night and seeing the Milky Way, reminding me of my insignificance in the universe. Now that I live in a city, such introspection is no longer easy, and like 1/2 of humanity that also lives in urban areas, I must rely on satellites to provide the imagery. Yet satellites are part of the problem. Light pollution has been getting worse for decades, and with the recent steady stream of satellite launches and billionaire joyrides we have a relatively new addition to the sources of interference. So how bad is it, and how much worse will it get?

Looking up at the night sky, you can usually tell the difference between various man-made objects. Planes go fairly slowly across the sky, and you can sometimes see them blinking green and red. Meteors are fast and difficult to see. Geostationary satellites don’t appear to move at all because they are orbiting at the same rate as earth’s rotation, while other orbit types will zip by.

SpaceX has committed to reducing satellite brightness, and some observations have confirmed that new models are a full magnitude darker, right at the threshold of naked-eye observation. Unfortunately, it’s only a step in the right direction, and not enough to satisfy astronomers, who aren’t looking up at the night sky with their naked eyes, naturally.

The satellites aren’t giving off the light themselves. They are merely reflecting the light from the sun back to the earth, exactly the same way the moon is. Thus something that is directly in the shadow of the Earth will not reflect any light, but near the horizon the reflection from the satellites can be significant. It’s not practical to only focus our observatories in the narrow area that is the Earth’s shadow during the night, so we must look closer to the horizon and capture the reflections of the satellites.

What Would Satisfy Astronomers?

Sunlight reflects off space debris and satellites and reflects onto the night side of Earth. []
While it’s entertaining to imagine astronomers as crotchety old men shaking their fist at the sky and yelling “get off my lawn” at the passing satellites, their concerns are valid, and the impacts are measurable and potentially catastrophic.

When photographing the stars, the galactic paparazzi are using complicated and sensitive equipment. The exposure times are very long in order to gather enough light. But when a satellite passes over, its brightness can saturate the CCD, which doesn’t just ruin one pixel in the image, but a whole line as the satellite streaked through the shot. Further, the camera is focused way beyond low earth orbit, so the satellite is blurry, making the thin line into a wide gash across the image. Thus in any given image, the efficiency of the image, or how many pixels are usable, is significantly impacted by any satellite passing overhead.

The streak of a satellite not only saturates pixels, but the focus interrupts nearby pixels, and inter-CCD crosstalk creates additional errors. []
Even if the satellites were completely black, though, their passing in front of stars would cause the stars to appear have brief dips in brightness. Rather than having a bright gash across the image it would be a dark gash. Astronomers use dips in brightness for everything from exoplanet detection to estimations of nebula density.

The relative movement of all of the celestial bodies, and the rotation of Earth and position of the observatory are all calculated by a scheduler that determines when the best time is to take a photo of a specific part of the sky. It’s certainly possible to add the positions of satellites into that calculation to determine when is the optimum time to take a photo without interference. But with thousands of satellites already in space, and tens of thousands more planned, the windows are getting shorter and shorter, the delays until the right time is available are getting longer and longer, and the calculations become not about avoiding images with satellites but just reducing the number of gashes.

Astronomers can calculate the exact loss of efficiency caused by the satellites. At the Rubin Observatory LSST, they have found that with 48000 LEO satellites in orbit, about 30% of all LSST images would contain at least one satellite trail, and at least 1% of pixels would be lost. Further, because multiple images must be taken to compare, and a lot more math has to be applied to pixels to mathematically erase the trails (as long as the pixels hadn’t reached saturation), the extra effort required would extend surveys by several months.

The effects could be catastrophic. When it’s so bright in here that we can’t see out, we can’t identify external threats, like approaching asteroids. We already know where many of the nearby stars are and the mechanics of their motion, but we don’t often know about the much smaller, much dimmer asteroids that may have paths we don’t know about, that could intersect with ours. Being able to detect them requires constant attention to the vastness of our surroundings, and they could easily be masked by a passing satellite, delaying their detection.

So what’s the right amount for astronomers? The idealists might say only theirs. The pragmatic want to work with the space companies to take measures to reduce their impact. And the fearful worry that if efforts aren’t put in from the beginning then it will become a free-for-all with nobody bothering to put in the work to reduce their impact.

What About Us Plebs?

It’s one thing for the individual satellites to be visible to the naked eye, but even if they were darkened, the cumulative effect of tens of thousands of satellites scatters enough light that the night sky becomes brighter overall. With all the objects currently in the sky it’s estimated to be about 10% brighter already than it was in the 70s. If you live in the city and can’t already see the stars, maybe this doesn’t affect you at all. But for everyone else, this may mean the difference between seeing the Milky Way and not. Being able to see it in Montana was profound; for the whole world to be denied that so that some people can get better Internet is disappointing.

[Banner photo: “Castle Geyser & Milky Way”, NPS photo by Neal Herbert, Public Domain]

37 thoughts on “Things Are Looking Brighter! But Not The Stars

  1. Got into an argument about how we detect approaching asteroids by the dip in light of background stars. The dude was dead set on that not being how they are detected. So even if the sats aren’t illuminated they can interfere with detection of a potential asteroid hitting us. If it delays detection by a few days that’s less time to do something.
    Musk needs to outfit a few with cameras facing out and it would be awesome if it was available for all to see.

  2. Watching “The Sky at Night” the other night with the feature about the new ‘biggest ever’ comet. A professional astronomer showed 4 frames, 2 of which couldn’t be used because of the satellite trails across them, I didn’t appreciate why until I read this article so thanks.

    Perhaps the ‘polluter pays’ principle should apply? Professional astronomy telescope time has an associated cost, so every image that has a satellite trail has the sat automatically identified from a database, and the organisation that owns the sat is automatically billed for the wasted time.

    The ongoing cost might start to focus a few minds.

    I can’t believe that the starlink ‘one magnitude less’ will have much effect either, those things were at least mag -1 to start with, so that’s mag 0, still far too bright.

      1. I have seen several, and in an urban environment, they were easy to see, although at the right time of day.

        According to t’internet ISS is -6 (at the right time of day). I’ve seen that too, and it’s MUCH brighter.

        So I suppose you are around 5 different which is at least consistent.

    1. If I made it my project to count the grains of sand on one square meter of a busy blacktop road, should I be able to bill every single passing vehicle for upsetting my count? Most astronomy has just as much practical value as my fictional sand counting project, and deserves just as much legal protection.

  3. First off we need a way to deorbit all the dead satellites up there. Secondly al new satellites need to have a de-orbit feature so when they break down they burn up in our atmosphere. Yeah it will cost $$$ but if you can afford to put a $200 million sat into orbit you can afford a few million for engineering this feature into the sat.

    What would also be nice is a space garbage truck so to speak to collect these dead sats and dispose of them. I’d start with Musks Starlink c**p that is already causing problems. Who cares if it annoys some geeks who can’t watch movies on their computers.

    BTW there is so much space junk up there it is even becoming a hazard to the ISS.

    1. The ‘geeks’ are people in remote regions and third world countries, and that is a lot of humans. If you check the SpaceX prices for launches to high orbit you can see that many institutions and individuals can afford telescopes in space, and they work so much better that it is ridiculous to not make them.

      The Pro-Am astronomy community today does long total exposure by stacking hundreds or thousands of shorter ones and can reject frames that have planes or spacecraft or meteors automatically. The results are spectacular.

      1. “Do it for the poor foreignenrs” is the same lie as “do it for the children”. NO, I don’t want foreigners to be dependent on the foreign multipbillionaire. Stop with the naivety. Capitalists use fake propaganda all the time and it fools way too many people. Light pollution is a major issue. The challenge is stopping the destruction of the environment. Stop with the capitalist propaganda, its destroying all species and ecosystems on the planet.

        1. You stop with the anticapitalistic propaganda. Capitalists are not some evil planet-destroyers, they are providers of goods and services that you buy. Don’t like them? Don’t buy from them. Don’t like how your quality of life looks like without their goods and services? Well, that’s hypocrisy.

    2. I’m not 100% sure on this, but it seems like there is a requirement (by what agency??) that any satellite launched today have a de-orbit plan. In the case of cube-sats, it seems like the plan is generally “this is in a low enough orbit that it will naturally de-orbit within a couple of years”, but for others it is a serious consideration.

      1. I would think solar power for some of the new small and relatively cheap ion thrusters could get it done with enough time. Or it might be much easier to move them to trash orbits and cluster together. How much energy does it take to change orbital orientation compared to altitude?

        Maybe they should all have capture handles or tethers and safe-ing switches for the day when cleaner bots will be tidying up Earth orbit. I have an idea on how to bring them down without fuel. It sounds like it violates some conservation laws and I will have to look at it more closely.

  4. Not that this is a silly thing to attempt. for tv’s and tires to be recycled, by mandate. Musk or someone should by mandate, scoop up old satellites.

    But nearer home, can we not get cities to kill streetlights by 10? Or 12, 2? 230 so the drunks can get home? Cars have headlights. People can carry a flashlight. I ALWAYS have a flashlight and a knife, and now, a phone.

    FIRST – streetlights. THEN, the stars and satellite issues will be more, dare I say, a more visible problem with which to deal. Hard to get folks behind a problem which they cannot see.

    1. Not sure where you live, but some places have already fixed the streetlights and are now trying to fix the light pollution.

      Did you know that streetlights contribute to light pollution, so if we were to turn them off at night we would see the sky much better (if it weren’t for those satellites).

      1. Maybe it is just me, but it seems that if you turn streetlights off at night you don’t need streetlights. OTOH if they are baffled to be seen directly at some limited angle, like angles less than those where street glare begins, it should make a considerable difference. I wonder how much of the light comes from car headlights in busy areas? They are basically pointing into the sky when you go uphill.

  5. “Geostationary satellites don’t appear to move at all because they are orbiting at the same rate as earth’s rotation”

    Patently untrue. It doesn’t take a very powerful telescope to reach the point where the Earth’s rotation is visible, in the form of the stars and planets moving across the viewing field at a perceptible rate. Any geostationary satellite would be quickly noticeable for its NOT having this apparent motion, or in the case of telescopes with clock-driven polar mounts, by their motion against the backdrop of stationary stars, which would be at that same rate.

    1. The paragraph in question states that you can tell it’s a geostationary satellite because of an apparent lack of motion, exactly what you go on to explain in greater detail. How is the article’s statement patently untrue?

      1. Geostationary satellites are exactly that, geostationary. They stay above some point around the equator. This is nice for communication because you can just point a stationary satellite dish at them and not have to worry much about steering. However, the stars are appearing to rotate around us every 23h 56m… so if your telescope is tracking the stars, geostationary objects will be imaged as streaks because they’re moving too slowly across the sky compared to the stars. Starlink and LEO objects on the other hand will cause streaks because they’re moving faster than the stars across the sky. Potentially, there’s some altitude where satellites would orbit exactly in sync with the stars… but they would likely still be blurred because it probably would be on an orbit inclined to the celestial equator.

        1. I understand that a geostationary satellite will look like a streak if you’re tracking the stars. Just like the stars would look like a streak if you just point the telescope at a specific point in the sky and never move it. That’s not in question.

          I was objecting to the claim that the sentence was “patently untrue”. The first line in the sentence’s containing paragraph says “Looking up at the night sky, you can usually tell the difference between various man-made objects.” This paragraph is about stargazing with the naked eye. No mention has yet been made about satellites ruining a telescope’s view of the night sky. That comes later. At this point it’s all about stargazing. And to a human sans telescope, geostationary satellites do indeed appear to be stationary.

          I will say, however, that I don’t really understand the point of that paragraph. Doesn’t seem to add much to the discussion of light pollution.

          1. It makes a difference because being able to discern satellites from stars is important, so making the statement that geostationary satellites don’t move implies that they are indistinguishable from stars, which would be problematic. Also, I didn’t see any indication that the discussion was limited to viewing with the unaided eye.

          2. Really? No indication that the discussion was limited to viewing with the naked eye? When it’s sandwiched in between a paragraph talking about childhood stargazing at the Milky Way, and the following introducing the concept that astronomers have different criteria than us? I feel the context makes it apparent that he’s talking about LOOKING at the night sky, not photographing it with long exposure times.

          3. Well, NOT using a high-powered telescope, but just with a camera with a normal telephoto lens mounted on a tripod, motion of the stars is quite obvious. And yes, I DO call this “looking” at the sky. Seriously, do you really have to be that picky with my words?

          4. “with a camera with a normal telephoto lens mounted on a tripod, motion of the stars is quite obvious”

            I agree. With that setup the motion of the stars is quite obvious. And in that setup, geostationary satellites would appear to be motionless, as stated in the article. Check out this 3 minute exposure revealing apparently motionless geostationary satellites:


  6. You can also have some fun with satellites.

    A brother of a brother in law has a hobby of chasing iridium flares ( ) so at the right time he jumped on his bicycle to get to a right spot to see it. Over there he met a stranger who was walking his dog and they got into some conversation. He told the dog walker that he was contacting aliens and was waiting for a signal from them, and at the right time he started a countdown form 10 for when the flare would happen, and of course it happened right at the end of the countdown.

    I’m not sure if he told the dog walker what it really was.

    Also, photo below has 19 satellite streaks over a 333 seconds exposure. Ouwtch!

  7. Interesting article! I tried to reproduce the data from the object population in LEO mentioned in the publication this article is refering to using the state-of-the-art debris population model MASTER 8.0.2 from ESA. You have to know that the population has two parts: One part are (natural) meteoroids dominating the population for smaller particle sizes up to around 1 cm. The other part – artificial space objects – are dominating the population for larger diameters. As far as I understood in the analysis they calculate the net effect for all objects. The scattering effect from meteoroids were already present before humans sent satellites to space. For me, it is not clear what is the additional effect from the artifical objects. But I assume from my data, the effect is marginal.
    Don’t get me wrong: The light polution from artificial space objects is a serious problem (not only) for astronomers exactly as described in this article. But I think, the light scattering by artificial space objects is overestimated here. Nevertheless, we should care for a sustainable use of space and preserve the possibility to enjoy dark skies.

  8. “Being able to see it in Montana was profound; for the whole world to be denied that so that some people can get better Internet is disappointing.”

    Persuasion tip. Don’t casually dismiss the opposing viewpoint, especially when you do so incorrectly. SpaceX is bringing better internet to everyone. Literally everyone. Whether you’re in rural Montana, central Africa, the Andes, or northern India you’ll have good internet. Casually dismissing hat is a great way to get people to ignore you or worse against you. Acknowledge the benefit & ask for help to mitigate the impact.

  9. Starlink has been the only way to get internet at our home and I am extremely grateful for it. It saves us a one way 30 minute drive just to get somewhere for a somewhat reliable packet rate. The entire family works remotely so hopefully the fact that we are using less fuels makes you “less sad.”

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