Satellite flare

Satellite flare, also known as satellite glint, is a satellite pass visible to the naked eye as a brief, bright "flare". It is caused by the reflection toward the Earth below of sunlight incident on satellite surfaces such as solar panels and antennas (e.g., synthetic aperture radar).

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  • Top: a simulated animation of a typical Iridium flare
  • Middle: Both images show a flare of an Iridium satellite. Comet Holmes can be seen in the right image, slightly above the tree branch.
  • Bottom: Video of a flare

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.[1]

Controlled satellites
Flaring from reflection of the Sun

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
Double flare - Iridium 6 and its replacement, #51, both flare in a 21-second exposure.

While the first-generation Iridium satellites were still controlled, their flares could be predicted.[2] 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.[3]

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.

From 2017 to 2019, a new generation of Iridium satellites was deployed which does not exhibit the characteristic flares. As the first generation were replaced and retired, flares became less frequent. The last of the first-generation satellites was deorbited on 27 December 2019, ending predictable Iridium flares.[4][1]

Non-Iridium flares
A COSMO-SkyMed flare
A MetOp-A flare

There are many controlled satellites in addition to Iridium satellites, which can also flare, but most flares of these satellites do not exceed magnitude −2, and are therefore often overlooked.

MetOp-A, B, and C, however, can produce predictable flares up to −5 magnitude. Four COSMO-SkyMed satellites can produce flares up to −4 magnitude, and lasting much longer than the Iridium flares.

The International Space Station (ISS) is known to cause bright ISS flares.[5][6][7][8]

Planned low-orbit satellite constellations such as Starlink are a concern for the astronomical community because of light pollution.[9][10][11]

Uncontrolled satellites

When the satellite's orientation goes out of control, it becomes possible to predict only a trajectory of its pass, at any point of which it can flare up. These satellites are also described as "tumbling". This category includes a lot of rotating rocket bodies, some failed Iridium satellites, ALOS satellite (which can produce flashes up to −10 mag), etc. The most important and valuable information about tumbling satellites is a period of flashes. It can vary from 0.3–0.5 seconds (rapidly rotating objects) to a minute or more (slowly rotating objects). Other important characteristics are the amplitude of changes in brightness and period of repetition of these changes.

Humanity Star was a passive satellite which is primarily designed to produce satellite flares.

Observation
Main article: List of satellite pass predictors

While satellites may be seen by chance, there are websites and mobile apps which provide location specific information as to when and where in the sky a satellite flare may be seen (for controlled satellites), or trajectory of a tumbling satellite's pass (for uncontrolled satellites) in the sky.

See also

References
  1. "Catch the Iridium". Catch the Iridium. Retrieved 6 May 2020.
  2. Peat, Chris (15 May 2018). "The end of Iridium flares?". Heavens-Above. Munich. Retrieved 2 November 2018.
  3. "Iridium Flare Satellite Tracker". Apollo Satellite Communications. Retrieved 6 November 2019.
  4. Carter, Jamie (14 November 2017). "Iridium flares to cease by end of 2018". BBC Sky at Night Magazine. Retrieved 26 May 2018.
  5. "Watch International Space Station Flybys All Night Long". Sky & Telescope. 15 May 2019. Retrieved 6 May 2020.
  6. Dickinson, David (14 May 2019). "The International Space Station Rides High Through the May Sky". Universe Today. Retrieved 6 May 2020.
  7. July 2008, Joe Rao 11. "Spot the Space Station". Space.com. Retrieved 6 May 2020.
  8. "Spot The Station". spotthestation.nasa.gov. Retrieved 6 May 2020.
  9. "After SpaceX Starlink Launch, a Fear of Satellites That Outnumber All Visible Stars". The New York Times. Retrieved 3 June 2019.
  10. "The unexpected brightness of new satellites could ruin the night sky". The Economist. Retrieved 3 June 2019.
  11. "SpaceX's Starlink Could Change The Night Sky Forever, And Astronomers Are Not Happy". Forbes. Retrieved 3 June 2019.
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