Satellite flare, also known as satellite glint or iridium flare, is the visible phenomenon caused by the reflective surfaces of passing satellites (such as antennas, SAR or solar panels), reflecting sunlight toward the Earth below and appearing as a brief, bright "flare".
The first generation of the Iridium constellation with 66 active telecommunication satellites in low Earth orbit were known to cause the brightest flares of all orbiting satellites. Throughout 2017 and 2018 they are being replaced with a new generation that does not produce flares, with the first generation expected to be completely phased out by late 2018.
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.
If a first-generation Iridium satellite is still controlled, its flares can be predicted. The Iridium communication satellites have three polished door-sized antennas, 120° apart and at 40° angles with the main bus. The forward antenna faces 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 (6.2 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 are so bright that they can be seen in the daytime. This flashing has caused some annoyance to astronomers, as the flares occasionally disturb observations.
As the Iridium constellation consists of 66 working satellites, Iridium flares are visible quite often (2–4 times per night). Flares of brightness −5 magnitude occur 3–4 times per week; −8 magnitude may be visible 3–5 times per month for stationary observers.
Flares may also occur from solar panels, but they are not as bright (up to −3.5 magnitude). Such flares last about twice as long as those from the main mission antennas (MMA), because the so-called "mirror angle" for the solar panels is twice that for the MMAs. There are 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 can be bright enough to be seen at night in big cities where light pollution usually prevents most stellar observation. When not flaring, the satellites are often visible crossing the night sky at a typical magnitude of 6, similar to a dim star.
Throughout 2017 and 2018, a new generation of Iridium satellites is being deployed which does not exhibit the characteristic flares. As the first generation have been replaced and retired, flares have become less frequent. The last of the first-generation satellites are expected to be deorbited in late 2018, ending predictable Iridium flares.
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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.
Most flares from other satellites are almost indistinguishable from dim Iridium flares.
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.
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.
- Peat, Chris (2018-05-15). "The end of Iridium flares?". Heavens-Above. Munich. Retrieved 2018-11-02.
- "Iridium Flare Satellite Tracker". Apollo Satellite Communications. Retrieved 2019-11-06.
- Carter, Jamie (2017-11-14). "Iridium flares to cease by end of 2018". BBC Sky at Night Magazine. Retrieved 2018-05-26.
|Wikimedia Commons has media related to Satellite flare.|
.net /projects /previsat / – PreviSat software which calculates satellite flares (Iridium, MetOp, SkyMed)
.heavens-above .com /IridiumFlares .aspx – schedule of upcoming Iridium flares
.sourceforge .net – IridiumFlares prediction software (Java application)
.org /iridium .html – "Catch a Flaring Iridium" at Visual Satellite Observer
.fifthstarlabs .com – SkyGuide
.youtube .com /watch?v=ePBE7pHzAdA Iridium 25 flare near the Moon