Satellite flare, also known as satellite glint, is a brief and bright "flare" in visibility of an satellite. It is caused by the temporary direct reflection of light from the surface of a spacecraft in orbit, such as from its solar panels or antennas (e.g., synthetic aperture radar), toward an observer. As a form of light pollution it can negatively affect ground-based astronomy, stargazing, and indigenous people. Many satellites flare with magnitudes bright enough to see with the unaided eye, i.e. brighter than magnitude +6.5. Smaller magnitude numbers are brighter, so negative magnitudes are brighter than positive magnitudes, i.e. the scale is reverse logarithmic (see apparent magnitude). The Iridium constellation was one of the first anthropogenic sources of near-space light pollution to draw criticism. Larger satellite constellations, like OneWeb and Starlink, have received increased criticism. Scientific and policy analyses have raised questions about which regulatory bodies hold jurisdiction over human actions that obscure starlight in ways that affect astronomy, stargazers, and indigenous communities.
Controlled satellites
The time and place of the satellite's flare can be predicted only when the satellite is controlled, and its orientation in space is known. In this case it is possible to predict the exact time of the flare, its place in the sky, the brightness and duration.
Iridium flares
The first generation of the Iridium constellation launched a total of 95 telecommunication satellites in low Earth orbit which were known to cause Iridium flares, the brightest flares of all orbiting satellites, starting in 1997. From 2017 to 2019 they were replaced with a new generation that does not produce flares, with the first generation completely deorbited by 27 December 2019. While the first-generation Iridium satellites were still controlled, their flares could be predicted. These Iridium communication satellites had three polished door-sized antennas, 120° apart and at 40° angles with the main bus. The forward antenna faced the direction the satellite is traveling. Occasionally, an antenna reflects sunlight directly down at Earth, creating a predictable and quickly moving illuminated spot on the surface below of about 10 km (6 mi) diameter. To an observer this looks like a bright flash, or flare in the sky, with a duration of a few seconds. Ranging up to −9.5 magnitude, some of the flares were so bright that they could be seen in the daytime. This flashing caused some annoyance to astronomers, as the flares occasionally disturbed observations. As the Iridium constellation consisted of 66 working satellites, Iridium flares were visible quite often (2 to 4 times per night). Flares of brightness −5 magnitude occurred 3 to 4 times per week, and −8 magnitude were visible 3 to 5 times per month for stationary observers. Flares could also occur from solar panels, but they were not as bright (up to −3.5 magnitude). Such flares lasted about twice as long as those from the main mission antennas (MMA), because the so-called "mirror angle" for the solar panels was twice that for the MMAs. There were also rare cases of flares from MMAs and solar panels, or two MMAs (front and either right or left) of one satellite in a single pass. The flares were bright enough to be seen at night in big cities where light pollution usually prevents most stellar observation. When not flaring, the satellites were often visible crossing the night sky at a typical magnitude of 6, similar to a dim star.
Mega-constellations
Low-orbit satellite constellations such as Starlink are a concern for astronomers, stargazers, and indigenous communities because of light pollution. In February 2020, the Russian Academy of Sciences said it would send a letter to the United Nations complaining that Starlink's satellites will damage "30-40% of astronomical images." In addition to their impact on ground-based astronomy, the ongoing expansion of mega-constellations has the potential to harm spaced-based telescope observations. Numerous satellite operators have criticized SpaceX for attempting to overwhelm the FCC with paperwork as a means to gain approval to launch 42,000 satellites, which has raised questions about which aspects of space law pertain to light pollution from satellites. SpaceX and Elon Musk have asserted in meetings with the National Academy of Sciences and in FCC filings that "SpaceX is committed to reducing satellite brightness to allow enjoyment of the skies and not thwart scientific discovery" and that its objectives are (1) to "make the satellites generally invisible to the naked eye within a week of launch", and (2) to "minimize Starlink's impact on astronomy by darkening satellites so they do not saturate observatory detectors." There are more than 10,400 Starlink satellites in orbit as of June 2026. Starlink satellites can flare repeatedly in an isolated area of the sky, typically directly above the sun (below the horizon) as they transit the highest latitude of their orbits. This phenomenon is most obvious when satellites are low over the horizon, and is due to the large number of Starlink satellites that are orbiting the Earth, predominantly at ~53° orbital inclination. The flares from Starlink have been misidentified as UFOs by airline pilots due to their unusual repetitive nature, which is visually analogous to a car's headlights at night getting brighter (then dimmer) as it rounds a turn. At cruising altitudes for commercial aircraft at night, pilots looking toward the setting or rising sun can often see the "racetrack" reflections from multiple satellites in criss-crossing orbits. Satellite constellations also present challenges to Indigenous astronomy and traditional practices that rely on the night sky, such as Polynesian wayfinding.
Other satellite flares
Many other controlled satellites also flare to magnitudes visible to the naked eye, i.e. larger than +6.5. MetOp-B and C, however, can produce predictable flares up to −5 magnitude (MetOp-A is no longer controlled) . Four COSMO-SkyMed satellites can produce flares up to -3 magnitude, and lasting much longer than the Iridium flares. The Terrasar X and Tandem X also can produce predictable flares up to -3 magnitude. The International Space Station (ISS) is known to cause bright ISS flares.
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