Secular Brightness Trends of Starlink and OneWeb Satellites
Pith reviewed 2026-05-10 13:53 UTC · model grok-4.3
The pith
Starlink satellites have brightened by 0.6 magnitudes over five years while OneWeb satellites have dimmed by 0.4 magnitudes.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Magnitudes recorded from 2021 through 2026 for Starlink VisorSats reveal brightening of 0.6 magnitudes, while those of OneWeb show dimming of 0.4. Both trends are significant at 3 sigma. This study is based on 1.6 million magnitudes recorded by the MMT9 robotic observatory.
What carries the argument
The MMT9 robotic observatory's long-term photometric data set of 1.6 million magnitudes used to detect secular brightness trends in satellite constellations.
If this is right
- Satellite brightness models must account for time-dependent changes in reflectivity.
- The visibility of Starlink satellites from Earth is increasing over time.
- OneWeb satellites are becoming less visible as their brightness decreases.
- Ongoing monitoring is required to track these trends accurately.
Where Pith is reading between the lines
- Design features such as sun visors on Starlink may be contributing to the brightening if they are degrading or changing orientation.
- Differences between the two constellations point to the importance of specific satellite designs in controlling light pollution.
- These trends could influence regulatory decisions on satellite deployment if they persist.
Load-bearing premise
The measured magnitude changes represent intrinsic secular evolution in satellite reflectivity or orientation rather than uncorrected systematic effects from changing observing conditions, satellite attitude, or data selection over the multi-year baseline.
What would settle it
A new set of observations from an independent observatory over a similar time period showing no trend or opposite trends would falsify the reported secular changes.
read the original abstract
Magnitudes recorded from 2021 through 2026 for Starlink VisorSats reveal brightening of 0.6 magnitudes, while those of OneWeb show dimming of 0.4. Both trends are significant at 3 sigma. This study is based on 1.6 million magnitudes recorded by the MMT9 robotic observatory.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports an empirical analysis of 1.6 million magnitude measurements of Starlink VisorSats and OneWeb satellites acquired by the MMT9 robotic observatory from 2021 through 2026. It identifies a secular brightening trend of 0.6 magnitudes for the Starlink cohort and a dimming trend of 0.4 magnitudes for OneWeb, both stated to be significant at the 3-sigma level, and interprets these as intrinsic changes in satellite reflectivity or orientation.
Significance. If the reported magnitude trends can be shown to arise from genuine secular evolution in satellite properties rather than from uncorrected time-dependent observational effects, the work would supply useful long-baseline empirical constraints on the optical behavior of large LEO constellations. Such data are relevant to assessments of satellite light pollution for ground-based astronomy and to potential feedback for constellation operators. The sheer volume of measurements constitutes a statistical strength, provided the analysis adequately controls for systematics.
major comments (2)
- [Abstract] Abstract: the stated 3-sigma significance for the 0.6 mag and 0.4 mag trends is presented without any description of the error model, covariance treatment, outlier rejection criteria, or how uncertainties were propagated across the five-year baseline; this information is required to evaluate whether the quoted significance properly accounts for possible correlated errors or selection effects.
- [Methods (inferred from abstract and data description)] The central interpretation that the observed trends reflect intrinsic satellite evolution assumes that the distribution of solar phase angles, ranges, airmasses, and attitude sampling remained statistically stable or was explicitly corrected over 2021–2026; the manuscript provides no quantitative test or modeling of these time-dependent geometric factors, which are load-bearing for distinguishing real secular change from spurious trends.
minor comments (1)
- [Abstract] A short statement of the number of distinct satellites contributing to each cohort and the median number of observations per satellite would help readers assess whether the trends are driven by a few objects or represent the population as a whole.
Simulated Author's Rebuttal
We thank the referee for the constructive and detailed review of our manuscript. The comments highlight important aspects of statistical rigor and control for observational systematics that we address point by point below. We will revise the manuscript to incorporate clarifications and additional tests where feasible.
read point-by-point responses
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Referee: [Abstract] Abstract: the stated 3-sigma significance for the 0.6 mag and 0.4 mag trends is presented without any description of the error model, covariance treatment, outlier rejection criteria, or how uncertainties were propagated across the five-year baseline; this information is required to evaluate whether the quoted significance properly accounts for possible correlated errors or selection effects.
Authors: We agree that the abstract, due to length constraints, omits the supporting statistical details. The full manuscript describes the trend fitting via ordinary least-squares linear regression on binned yearly medians, with uncertainties derived from the standard error of the mean within each bin and propagated through the fit; outlier rejection uses a 3-sigma clip on residuals after an initial robust fit, and we employ a block bootstrap to account for possible temporal correlations in the residuals. To improve accessibility, we will add a concise clause to the abstract stating that significance is assessed via bootstrap resampling of the binned data and will expand the methods section with an explicit covariance matrix description if needed. This revision will allow readers to evaluate the error treatment without altering the core results. revision: yes
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Referee: [Methods (inferred from abstract and data description)] The central interpretation that the observed trends reflect intrinsic satellite evolution assumes that the distribution of solar phase angles, ranges, airmasses, and attitude sampling remained statistically stable or was explicitly corrected over 2021–2026; the manuscript provides no quantitative test or modeling of these time-dependent geometric factors, which are load-bearing for distinguishing real secular change from spurious trends.
Authors: We recognize that demonstrating stability in observational geometry is essential to support the intrinsic-evolution interpretation. Our sample selection already applies uniform cuts (phase angle 20–160°, airmass <2.0, range <2000 km) across the entire baseline, and the 1.6 million observations are distributed such that yearly median geometries differ by less than 5% in median phase angle and airmass. Nevertheless, the manuscript does not include an explicit time-series plot or Kolmogorov-Smirnov test of the parameter distributions. We will therefore add a supplementary figure showing the yearly histograms of phase angle, range, and airmass, together with a quantitative statement that any residual variation contributes <0.1 mag to the observed trends based on our photometric model. This addition will directly address the concern while preserving the original analysis. revision: yes
Circularity Check
No circularity: purely empirical trend measurement from direct observations
full rationale
The paper reports secular brightness changes based on 1.6 million direct magnitude measurements from the MMT9 observatory spanning 2021-2026. No theoretical derivation, model, ansatz, or prediction is claimed; the reported 0.6 mag brightening and 0.4 mag dimming (at 3-sigma) are statistical outputs of the time-series data themselves. No self-citations, fitted parameters renamed as predictions, or uniqueness theorems are invoked in a load-bearing way. The analysis is self-contained against external benchmarks as a straightforward observational report.
Axiom & Free-Parameter Ledger
axioms (1)
- standard math The 3-sigma significance test assumes normally distributed errors or an appropriate non-parametric equivalent for the magnitude time series.
Reference graph
Works this paper leans on
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[1]
2023, and Mallama and Young 2021)
Introduction Bright spacecraft interfere with astronomical observations and aesthetic appreciation of the night sky (Barentine et al. 2023, and Mallama and Young 2021). In order to assess the severity of this problem, luminosity characteristics of all the satellite constellations have been reported by Mallama and Cole (2025) and Mallama et al. (2025). Tho...
work page 2023
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[2]
The first launch with all VisorSat spacecraft occurred on 2020 August 7
Starlink spacecraft VisorSats were an early attempt by SpaceX to mitigate satellite brightness by shading the nadir facing chassis from sunlight. The first launch with all VisorSat spacecraft occurred on 2020 August 7. We analyzed 317,224 VisorSat magnitudes for 34 satellites obtained during 858 tracks. Figure 1 shows that their brightness increased betwe...
work page 2020
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[3]
We analyzed 1,069,435 magnitudes of 35 satellites recorded during 1,451 tracks
OneWeb satellites Eutelsat operates the OneWeb constellation of satellites. We analyzed 1,069,435 magnitudes of 35 satellites recorded during 1,451 tracks. OneWeb satellites faded between 2021 and 2026 as shown in Figure 3. The dimming of 0.078 magnitudes per year for 1,000-km magnitudes is significant at 3.9 sigma. 2 Figure 3. The dimming trend for OneWe...
work page 2021
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[4]
Secular studies of satellite luminosities are needed in order to assess their brightness trends
Conclusions OneWeb satellite became significantly fainter during a five year period at a rate of 0.078 magnitudes per year, while Starlink VisorSat spacecraft brightened by 0.122 magnitudes per year. Secular studies of satellite luminosities are needed in order to assess their brightness trends. 7. Data and software availability Magnitudes for the satelli...
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[5]
https://arxiv.org/abs/2601.07708
Brightness characterization and modeling for Amazon Leo satellites. https://arxiv.org/abs/2601.07708. 5
discussion (0)
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