🔭 astrophysics

Large or bright satellite constellations: Effects on observations, including on the background sky brightness

This study evaluates the impact of proposed satellite constellations on astronomical observations, finding that while dimmer satellites are manageable, mega-constellations and bright reflectors significantly degrade sky brightness and detector performance, necessitating strict limits on both individual satellite brightness (V_550km > 7) and total population (~100,000) to preserve observational quality.

Original authors: Olivier R. Hainaut

Published 2026-04-13

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Olivier R. Hainaut

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the night sky as a giant, pristine canvas that astronomers use to paint pictures of the universe. For centuries, this canvas has been kept as dark and clean as possible, allowing the faintest whispers of light from distant galaxies to be seen.

This paper is a warning from a scientist at the European Southern Observatory: We are about to spray-paint over that canvas with thousands of moving, glowing dots.

Here is the breakdown of the problem, the different types of "pollution" involved, and what the future might look like, explained simply.

1. The Problem: Too Many Satellites, Too Bright

Since 2019, companies like SpaceX have launched thousands of satellites to provide internet to the whole world. But the plans are getting bigger. Instead of just a few thousand, we are talking about millions of satellites. Some are just tiny dots, but others are massive mirrors or giant antennas designed to be very bright.

The paper asks: What happens to our view of the stars if the sky is filled with these moving lights?

2. The Three Ways Satellites Ruin the View

The author identifies three main ways satellites mess up astronomy, using some great analogies:

A. The "Streaks" (Direct Trails)

The Analogy: Imagine you are taking a long-exposure photograph of a quiet, dark forest at night. Suddenly, a swarm of fireflies flies across your camera lens. Instead of seeing the trees clearly, your photo is covered in bright streaks of light.
The Reality: When a satellite passes in front of a telescope, it leaves a bright line (a trail) across the image.

The Impact: If there are only a few satellites, you can just cut out the streaks. But if there are one million satellites, the sky becomes a "spaghetti bowl" of streaks. The paper predicts that for large telescopes, up to 30% of the sky in a single photo could be covered in these streaks, making the data useless.

B. The "Haze" (Scattered Light)

The Analogy: Think of a streetlamp on a foggy night. Even if you aren't looking directly at the bulb, the whole sky glows with a hazy orange light because the fog scatters the light.
The Reality: Satellites reflect sunlight. Even when they aren't directly in your telescope's view, their light hits the Earth's atmosphere and scatters, creating a general "glow" that brightens the whole sky.

The Impact: This is like turning up the brightness on your TV while trying to watch a movie. The contrast is lost. The faintest, most distant objects in the universe disappear into the background glow.
The Worst Case: The paper looks at a company called Reflect Orbital, which plans to launch satellites that act as giant mirrors to light up cities at night. If they launch 50,000 of these, the night sky could become 3 to 4 times brighter than it is now. A "pristine" dark sky would turn into a "semi-suburban" sky, making deep-space observation impossible.

C. The "Ghosting" (Diffuse Light)

The Analogy: Imagine a room full of people whispering. Individually, you can't hear them. But together, the collective whisper creates a background noise that drowns out a quiet conversation.
The Reality: Some satellites are too faint to be seen as streaks, but their combined light adds up. This creates a "diffuse" background noise that interferes with sensitive instruments, especially those that analyze light (spectrographs).

3. The "Saturation" Trap

There is a special danger for the Vera C. Rubin Observatory, a massive new telescope designed to scan the entire sky.

The Analogy: Imagine a very sensitive microphone. If a normal person speaks, it records perfectly. But if a jet engine flies overhead, the microphone "blows out" (saturates). It doesn't just record the jet; the sound waves bounce around the room, creating "ghost" echoes that ruin the recording for everyone else in the room.
The Reality: If a bright satellite crosses the Rubin telescope's view, it doesn't just leave one streak. It creates a cascade of "ghost trails" across the entire image, ruining the whole picture. The paper warns that if satellites are brighter than a certain limit (magnitude 7), even a moderate number of them could make this telescope useless for its main job.

4. The Verdict: How Many is Too Many?

The author ran simulations to see what happens with different numbers of satellites:

The "Safe" Zone (60,000 satellites): If all satellites are kept very dim (fainter than the recommended limit), the damage is manageable. It's annoying, like a few smudges on a window, but you can still see through.
The "Mega" Zone (1,000,000 satellites): Even if they are dim, there are so many that the sky is covered in streaks. You lose about 10–20% of your data. It's like trying to read a book while someone is constantly tapping on the page.
The "Bright" Zone (Reflect Orbital / AST SpaceMobile): This is the disaster scenario.
- 5,000 giant mirrors: The sky glows 20–30% brighter.
- 50,000 giant mirrors: The sky becomes 300% brighter. The night sky would look like a cloudy, light-polluted city sky. The "dark time" needed for deep astronomy would effectively vanish.

5. The Conclusion

The paper concludes that the current plans to launch over 1.7 million satellites, many of which are very bright, would be devastating for astronomy.

The Solution: We must keep satellites dim (fainter than magnitude 7) and limit the total number to under 100,000 if we want to keep the night sky usable for science.
The Warning: If we don't regulate this, the next generation of astronomers might find that the universe has become too bright to see. The "dark sky" we take for granted could become a thing of the past, replaced by a sky full of artificial stars.

In short: We are building a ceiling of lights that might block our view of the stars forever. We need to dim the lights and count them carefully, or we lose our window to the universe.

1. Problem Statement

The rapid proliferation of Low Earth Orbit (LEO) satellite constellations poses a critical threat to ground-based optical astronomy. While previous studies focused primarily on direct satellite trails (streaks crossing images), this paper addresses the cumulative and often overlooked impacts of:

Diffuse light pollution: Light from satellites too faint to be detected as individual trails but bright enough to raise the sky background.
Scattered light pollution: Atmospheric scattering (Rayleigh and Mie) of sunlight reflected by satellites, which increases the overall sky brightness.
Saturation effects: The impact of very bright satellites on sensitive detectors (e.g., the Rubin Observatory LSST), where a single bright trail can cause "ghost trails" via electronic cross-talk, ruining a significant portion of the field of view (FoV).

The study evaluates proposed constellations ranging from current deployments (~60,000 satellites) to "mega-constellations" (1 million+ satellites) and extremely bright reflector concepts (e.g., Reflect Orbital, AST SpaceMobile).

2. Methodology

The author employs a numerical modeling approach combining atmospheric physics with orbital mechanics:

Simulation Framework: The study uses an expanded Python implementation of the SatConAnalytic package (based on Bassa et al., 2022).
Atmospheric Scattering Model: Adapted from moonlight sky-brightness calculations (Krisciunas & Schaefer 1991), the model calculates scattered light using:
- Rayleigh scattering: Caused by air molecules.
- Mie scattering: Caused by atmospheric aerosols.
- The model is validated against observations of moonlight and stellar background light.
Diffuse Light Calculation: Integrates the flux from satellites below the detection threshold of specific instruments to quantify their contribution to the diffuse sky background.
Orbital Mechanics: Satellites are modeled using Keplerian motion across various shells (altitudes 328–2100 km). The simulation discards satellites in Earth's shadow or below the horizon.
Instruments Modeled:
- FORS2 (VLT): A traditional 6×6 arcmin imager (300s exposure, $V \sim 25.8$ limit). Represents standard large-telescope imaging.
- LSST (Rubin Observatory): A 3.5° wide-field camera (15s exposure, $V \sim 24.9$ limit). Represents instruments highly sensitive to saturation and cross-talk.
Scenarios Evaluated:
- BHG2022: ~64,500 satellites (current/next-gen Starlink, OneWeb, Kuiper) at $V_{550km} = 7$ .
- SXODC: 1 million satellites (SpaceX Orbital Data Center) at $V_{550km} = 7, 6, \text{and } 5$ .
- AST SpaceMobile: 243 and 3,000 "BlueBird" satellites ( $V_{550km} \approx 2$ ).
- Reflect Orbital: 36, 5,000, and 50,000 mirror-like satellites ( $V_{550km} \approx -4.3$ ).

3. Key Contributions

Quantification of Scattered/Diffuse Light: Unlike previous works focusing on trails, this paper rigorously quantifies how satellite constellations elevate the entire sky background via atmospheric scattering, even when trails are not visible.
Saturation Threshold Analysis: It demonstrates that for saturating instruments like LSST, the "effective magnitude" ( $V_{eff}$ ) is the critical metric. Satellites brighter than $V_{550km} = 7$ cause disproportionate FoV losses due to electronic cross-talk, not just the physical trail width.
Cumulative Impact Modeling: The study establishes that the total number of satellites is a cumulative parameter; even if individual satellites meet brightness limits, the aggregate population can degrade observing conditions.
Scenario-Specific Projections: It provides specific forecasts for 2027, 2030, and 2035 based on proposed commercial constellations, moving beyond theoretical limits to real-world deployment plans.

4. Key Results

A. Manageable Constellations (~60,000 satellites, $V_{550km} \ge 7$ )

Impact: Negligible to marginal.
Sky Brightness: Contributes only $\sim 10^{-4}$ of the natural dark sky.
FoV Loss: < 1% for traditional imagers; manageable for LSST.
Conclusion: If all satellites adhere to the $V_{550km} \ge 7$ limit, a population of ~60,000 is sustainable.

B. Mega-Constellations (1 Million satellites)

Impact: Severe degradation of observing conditions.
Trails: Even at $V_{550km} = 7$ , the sheer number of satellites results in pervasive trails. At zenith, FoV losses reach 6–15% (rising to 28% near constellation cusps).
Brightness Sensitivity: If these satellites are slightly brighter ( $V_{550km} = 6$ $V_{550 k m} = 6$ or $5$), the impact becomes disastrous.
- At $V_{550km} = 5$ , LSST observations would be lost almost entirely (>30% loss).
- Thousands of satellites would become visible to the naked eye, altering the appearance of the night sky.

C. Very Bright Satellites (AST SpaceMobile, $V_{550km} \approx 2$ )

Impact: Disproportionate damage to saturating instruments.
FoV Loss: Even with only 3,000 satellites, LSST FoV losses rise to 1–6% (vs. $10^{-5}$ if they were $V=7$ ).
Diffuse Light: Contributes ~0.2–0.4% to the sky background, approaching the 1% limit for prime observatories.

D. Extremely Bright Satellites (Reflect Orbital, $V_{550km} \approx -4$ )

Impact: Catastrophic for astronomy.
Sky Brightness:
- 5,000 satellites: Increases scattered sky background by 20–30% ( $V \approx 21.7$ MpSA).
- 50,000 satellites: Increases background by 200–300% ( $V \approx 20.5$ MpSA), pushing a prime site to Bortle Class 4.5 ("semi-suburban"), rendering deep-sky observations impossible.
FoV Loss: A single 50,000-satellite constellation would cause 30–100% FoV loss for LSST, effectively making the telescope useless for most of the night.
Visibility: These satellites would appear brighter than Venus or the full Moon, visible even in light-polluted areas.

5. Significance and Conclusions

The paper concludes that the total population and individual brightness are the two critical parameters determining the survivability of optical astronomy.

The "100,000" Limit: Even if all satellites strictly adhere to the $V_{550km} \ge 7$ limit, the total population must remain below ~100,000 to ensure FoV losses do not exceed typical technical downtime (3%).
The "7" Limit is Critical: Maintaining $V_{550km} \ge 7$ is essential not just for naked-eye visibility, but specifically to prevent detector saturation in instruments like LSST.
Current Proposals are Unsustainable: The currently proposed global constellation population exceeds 1.7 million objects, many of which are brighter than $V=7$ . The paper asserts that without drastic mitigation (reducing numbers and dimming satellites), these deployments will substantially degrade and, in some cases, destroy the capability of major observatories to conduct deep-sky surveys and spectroscopy.

The study emphasizes that the effect is cumulative; the astronomical community cannot simply accept "a few" bright constellations without considering the aggregate impact of all proposed systems.