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
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 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. 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. Such flares lasted about twice as long as those from the main mission antennas, 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 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.
Non-Iridium flares
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 is known to cause bright ISS flares. Planned low-orbit satellite constellations such as Starlink are a concern for the astronomical community because of light pollution.
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, etc. The most important and valuable information about tumbling satellites is a period of flashes. It can vary from 0.3–0.5 seconds to a minute or more. 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
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, or trajectory of a tumbling satellite's pass in the sky.
