BlueBird 6, the first Block 2 BlueBird satellite, appeared fainter than the earlier Block 1 spacecraft in an analysis of optical observations. Its mean apparent magnitude was 3.85, compared with 3.30 for Block 1 satellites, a difference of 0.55 magnitude. The reported standard deviations of the mean were plus or minus 0.10 for BlueBird 6 and plus or minus 0.07 for Block 1.
The same gap remained after the researchers adjusted the observations to a common distance of 1,000 kilometres. On that basis, BlueBird 6 had a mean magnitude of 4.32, compared with 3.77 for Block 1, again a difference of 0.55 magnitude. The adjustment provided a range-independent comparison between the two groups.
A larger spacecraft, but a weaker size effect
The result ran against the brightness expected from the satellite's larger size. The paper's size scaling predicted that BlueBird 6 would be 3.48 times as bright as the Block 1 spacecraft, equivalent to a 1.36-magnitude difference. Instead, the measured comparison put BlueBird 6 0.55 magnitude fainter. The authors describe the resulting shortfall from the size-scaled expectation as 1.91 magnitudes.
The analysis also expressed the difference per unit surface area. BlueBird 6's relative brightness was reported as 17% of Block 1, described in the paper as a 5.8-factor improvement in mitigation per unit area. The larger satellite therefore did not simply follow the brightness increase expected from its size.
Because the dataset is observational, the brightness gap is a comparison between the two spacecraft groups, not a causal test of the satellite design. The result shows that BlueBird 6 was fainter in these observations, but it does not establish which physical feature produced the difference.
What the observations covered
The table used 258 observations of BlueBird 6 and 546 observations of BlueBird 1 through 5. BlueBird 6 was compared with the Block 1 spacecraft in a nearly identical orbit, while later Block 2 satellites were left out because too few observations were available to produce robust brightness statistics.
Most of the measurements came from visual observers, who compared the satellites with nearby reference stars. A smaller electronic set from the Mini-MegaTORTORA system was recorded at 10 hertz and averaged into five-second means. The source says the visual magnitudes were within 0.1 of the Johnson V-band.
The researchers examined more than the overall averages. They studied magnitude distributions and looked at how brightness changed with a satellite's elevation, phase angle and beta angle. Viewing-geometry restrictions were used while examining those patterns.
Brightness changed with the viewing geometry
Both BlueBird 6 and the Block 1 group brightened as their elevation increased. Across the reported range from 20 to 80 degrees, the change was about 3 magnitudes for both groups. The elevation trend therefore appeared in both sets of observations alongside the difference between the spacecraft groups.
The two groups also showed the usual response to phase angle, and the fitted BlueBird 6 slope appeared steeper than the Block 1 slope. The paper does not give a numerical slope estimate or an interval for that comparison, so the result is best read as a descriptive pattern in the observations.
At larger beta angles, both groups became fainter. The fitted trend indicated about 2 magnitudes of dimming across the observed beta-angle span, while the Block 1 slope may have been larger. The study reports these trends without inferential tests, confidence intervals or formal uncertainty estimates for the model fits.
What the result does and does not establish
For BlueBird 6, the reported mean brightness exceeded two International Astronomical Union benchmark limits. It was 3.15 magnitudes above the research limit of magnitude 7.0, corresponding to a factor of 18, and 2.15 magnitudes above the aesthetic limit of magnitude 6.0, corresponding to a factor of 7.
The abstract makes a broader statement that all BlueBird satellites exceed the brightness limits recommended by the International Astronomical Union and will be fewer in number than other constellations. The detailed measurements supplied for the analysis, however, focus on BlueBird 6 and the Block 1 spacecraft, so the wider statement is presented as a paper-level conclusion rather than as a full set of satellite-by-satellite measurements.
The study does not establish that later Block 2 satellites share BlueBird 6's brightness statistics, because those later spacecraft were excluded from the comparison for lack of observations. Nor does the constellation-level statement show that having fewer satellites would offset the brightness of individual spacecraft.
The analysis reports descriptive means and fitted lines, but no inferential tests, confidence intervals or formal model-estimate uncertainties. It also combines mainly visual observations with a smaller electronic set, and the source notes only approximate equivalence to the Johnson V-band.
A preprint with data sources identified
The document is dated August 24, 2026, and the supplied metadata classifies it as a preprint. No funding source is reported in the supplied material.
The observations are available through the SCORE archive and the Mini-MegaTORTORA online database. The analysis software is available from the corresponding author, according to the supplied information.
Paper data and sources
Original title: BlueBird 6 is Fainter than Block 1 Satellites
Authors: Anthony Mallama, Richard E. Cole
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-24
DOI: Not available
Original paper · Full text