Secular Brightness Trends of Starlink and OneWeb Satellites

Based on 1.6 million magnitude measurements from the MMT9 robotic observatory between 2021 and 2026, this study reveals statistically significant secular trends of 0.6-magnitude brightening for Starlink VisorSats and 0.4-magnitude dimming for OneWeb satellites.

The Gist

Imagine the night sky as a giant, quiet library. For centuries, astronomers have been the librarians, carefully reading the faint whispers of distant stars. But recently, a new kind of "patron" has entered the library: thousands of man-made satellites zooming across the ceiling, leaving bright trails that distract from the books.

This paper is a report card on how two of the biggest groups of these space travelers—Starlink (owned by SpaceX) and OneWeb (owned by Eutelsat)—have changed their "flashlight brightness" over the last five years.

Here is the story of what happened, told in simple terms:

The Big Picture: A Five-Year Watch

The author, Anthony Mallama, acted like a very patient security guard. Using a robotic telescope in Russia (nicknamed MMT9), he took 1.6 million snapshots of these satellites between 2021 and 2026. He wanted to know: Are these satellites getting brighter, dimmer, or staying the same as they age?

Think of it like watching a car's headlights over five years. Do they get brighter as the engine warms up, or do they get dimmer as the bulbs wear out?

The Two Main Characters

1. Starlink: The "VisorSat" and the "V1.5"

Starlink tried a few different designs to hide their lights from astronomers.

• The VisorSat (The Early Model): These were the first satellites equipped with little "sunshades" (visors) to block sunlight from hitting the shiny parts of the satellite.
  • The Result: Surprisingly, these sunshaded satellites got brighter over time. It's as if someone turned up the volume on a radio that was supposed to be quiet. They got about 0.6 "steps" brighter over five years.
• The V1.5 (The Later Model): SpaceX realized the sunshades blocked their own laser internet signals, so they removed them.
  • The Result: These satellites also seemed to get a little brighter, but the change was so small and shaky that the scientists couldn't say for sure if it was real or just a fluke.

2. OneWeb: The Fading Star

OneWeb is a different company with a different fleet of satellites.

• The Result: These satellites did the opposite of Starlink. They got dimmer over time. It's like a candle slowly burning down. They faded by about 0.4 "steps" over five years.

The Mystery: Why?

This is where the plot gets interesting. Usually, when things get old in space, they get dirty or damaged by cosmic rays (like a car getting rusted by salt air). You might expect them to get dimmer.

• OneWeb's Dimming: The author checked if this was just "old age." He compared brand-new OneWeb satellites (launched in 2024) with older ones. They were almost the same brightness. This suggests the fading isn't because the satellites are breaking down. It's a mystery! Maybe the surface material is changing in a way we don't understand yet.
• Starlink's Brightening: Why are the VisorSats getting brighter? The author asked the companies (SpaceX and Eutelsat), but they didn't have an answer. It's like asking a car manufacturer why a specific model's headlights are getting brighter after five years, and they shrug and say, "We don't know."

The Takeaway

The most important lesson from this paper is that satellites are not static objects. They change.

• Some get brighter (Starlink VisorSats).
• Some get dimmer (OneWeb).
• Some stay the same.

If astronomers want to keep the night sky clear for looking at the universe, they can't just take a picture of a satellite once and assume it will look the same forever. They need to keep watching, year after year, to see how these "space cars" evolve.

In short: The night sky is full of moving lights that are changing their brightness in unexpected ways, and we still don't fully understand why. We need to keep our eyes on them to protect our view of the stars.

Technical Summary

1. Problem Statement

The proliferation of Low Earth Orbit (LEO) satellite constellations poses a significant threat to ground-based astronomy and the aesthetic quality of the night sky. While initial studies have characterized the luminosity of these satellites shortly after launch, there is a critical lack of data regarding long-term secular trends (changes over multiple years) in satellite brightness. It is unknown whether satellite surfaces degrade, darken, or brighten over time due to space weathering, material aging, or operational changes, which is essential for accurately modeling their impact on astronomical observations.

2. Methodology

The study utilizes a massive dataset collected by the MMT9 robotic observatory in Russia (43.65°N, 41.43°E).

• Data Volume: The analysis covers 1.6 million photometric measurements recorded between 2021 and 2026.
• Observation Technique:
  • The MMT9 records data at a frequency of 10 Hz across sky tracks.
  • Data was filtered to include only satellites at their operational altitudes.
  • Standardization: To ensure uniformity, apparent magnitudes were adjusted to a standard distance of 1,000 km using the inverse square law of light.
  • Temporal Binning: Data points were aggregated into 0.1-year time segments.
  • Phase Function: Corrections for phase angle were intentionally omitted to avoid introducing additional uncertainty; the study focuses on the 1,000-km standardized magnitudes.
• Statistical Analysis: Linear least-squares fits were applied to determine brightness trends (slopes), with statistical significance assessed via sigma ($\sigma$) levels.

3. Key Contributions

• Long-Term Monitoring: This is one of the first studies to report multi-year (5-year) brightness trends for operational satellite constellations, moving beyond "snapshot" assessments taken immediately post-launch.
• Differentiation of Constellations: The study isolates specific satellite generations, distinguishing between Starlink VisorSats (early mitigation attempt), Starlink V1.5, and OneWeb satellites.
• Investigation of Aging: The paper specifically tests the hypothesis that "space weathering" or material aging causes brightness changes by comparing "early" vs. "later" launched OneWeb satellites.

4. Results

The study reveals divergent brightness trends between the two major constellations, both statistically significant at the $3\sigma$ level or higher.

A. Starlink Constellation

• VisorSats (Early Generation):
  • Trend: Significant brightening.
  • Rate: $-0.122$ magnitudes per year (negative slope indicates brightening in astronomical magnitude scale).
  • Significance: $3.1\sigma$.
  • Context: These satellites featured sunshades (visors) to block sunlight from the chassis. The trend suggests these surfaces may be degrading or becoming more reflective over time.
• Starlink V1.5:
  • Trend: Slight brightening ($-0.067$ mag/year).
  • Significance: Not statistically significant ($1.3\sigma$).
  • Context: Visors were removed from V1.5 to accommodate laser inter-satellite links.

B. OneWeb Constellation

• Trend: Significant dimming (fading).
• Rate: $+0.078$ magnitudes per year.
• Significance: $3.9\sigma$ (highly significant).
• Cause Analysis: The study investigated whether this dimming was due to material aging or space weathering by comparing "early" launches (pre-2022) with "later" launches (Oct 2024).
  • Finding: The brightness of early and later satellites was nearly identical in the 2025–2026 period.
  • Conclusion: The fading is not caused by the aging of materials or space weathering, as newer satellites are already as dim as older ones. The root cause remains unknown, and communications with Eutelsat did not resolve the issue.

Summary of Trends (Table 1 Data)

Constellation	Type	Slope (mag/yr)	Sigma
Starlink	VisorSat	$-0.122$ (Brightening)	$3.1$
Starlink	V1.5	$-0.067$ (Brightening)	$1.3$ (NS)
OneWeb	All	$+0.078$ (Dimming)	$3.9$

(Note: NS = Not Significant)

5. Significance and Implications

• Dynamic Modeling: The findings demonstrate that satellite brightness is not static. Current models used by astronomers to predict satellite trails may be inaccurate if they rely on initial launch brightness data rather than secular trends.
• Mitigation Effectiveness: The brightening of VisorSats suggests that early mitigation strategies (visors) may degrade over time, potentially increasing the interference problem rather than solving it permanently.
• Unknown Mechanisms: The discovery that OneWeb satellites are dimming significantly, yet not due to aging, points to a potential design change, operational mode shift, or an uncharacterized physical phenomenon in the space environment.
• Policy and Planning: The need for "secular studies" is highlighted as a requirement for the International Astronomical Union (IAU) and regulatory bodies to accurately assess the long-term impact of mega-constellations on the "Dark and Quiet Sky."

Conclusion: The paper concludes that Starlink VisorSats are becoming brighter while OneWeb satellites are becoming fainter over a five-year period. These secular trends are statistically robust and necessitate continuous monitoring to update astronomical impact models. The specific physical causes for these trends remain unresolved.