The Newest and Largest Starlink Satellites Are Also the Faintest

SpaceX launched their first batch of second-generation Starlink satellites on February 27th. These spacecraft are called “Mini,” but they are only small in comparison to the full-size satellites that will come later. The 116 square meters of surface area make them more than four times the size of the first-generation spacecraft.

The Minis’ large dimensions were an immediate concern for professional and amateur astronomers alike because area usually translates to brightness. However, SpaceX changed their physical design and concept of operations (conops) in order to mitigate their brightness. The company developed a highly reflective dielectric mirror film and a low-reflectivity black paint, which are applied to several parts of the spacecraft body. The mirror-like surface reflects sunlight into space instead of scattering it toward observers on the ground. In addition, the solar panels can be oriented so that observers do not see their sunlit sides.

The brightness mitigation plan sounded promising but measurements were needed to determine its effectiveness. So, a group of satellite observers began recording magnitudes. Scott Harrington recorded the first data point visually on March 14th. He has since obtained 125 additional magnitudes from his dark-sky location in Arkansas. Meanwhile, Andreas Hornig developed software to process video observations. He derived 108 magnitude measurements recorded from Macedonia on the night of April 12th alone. In all, we have acquired 506 brightness measurements for our study.

SpaceX launched three additional batches of 21 or more Mini satellites in April, May, and June. These spacecraft ascend from low, orbit-insertion heights toward their eventual altitude at 560-km (350 mi). Until May, we were observing Mini satellites at all heights without knowing whether they were operating for brightness mitigation. Then Richard Cole in the UK noticed that some spacecraft had leveled off at 480 km. He reasoned that these satellites might already be in mitigation mode and suggested that we prioritize them.

We found that the Minis at that height were several magnitudes fainter than those at other altitudes. SpaceX sent us a message on May 16th confirming that Richard was correct. Now that we could distinguish between mitigated and unmitigated spacecraft, we began to characterize the brightness of each group, prioritizing measurements for those satellites that were already operational.

Observed brightness indicates how severely satellites impact celestial observations. The average magnitude for mitigated Mini spacecraft in our database is 7.1, just below the limit set by astronomers’ recommended guidelines. So, most of them are invisible to the unaided eye and do not interfere greatly with research.

Next, we adjusted the observed magnitudes to a uniform distance of 1,000 km from the observer. This adjustment compensates for brightness variations due to actual distances, which ranged from 301 km for a satellite that was falling out of orbit to 1,448 km for one at high altitude. Magnitude standardization allowed us to compare the intrinsic brightness of mitigated and unmitigated spacecraft. The averages derived from distance-adjusted magnitudes are 7.9 and 5.1, respectively. Their difference indicates that satellites operating for brightness mitigation are dimmed by a factor of 12.

In summary, the design changes and brightness-mitigation operations that SpaceX introduced for Generation 2 satellites effectively reduces their brightness. Minis are actually fainter than all three models of the smaller Gen 1 spacecraft. However, the full-size Gen 2 satellites may present a greater challenge.

More Starlink satellite magnitudes are needed to further characterize and to monitor the brightness of Mini and full-sized Gen 2 satellites. Visual observers can find instructions in this paper, while videographers can learn about camera techniques here. A detailed account of the research described in this article is available on the astronomy preprint arXiv.

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