A very bright iridium satellite flare in the night sky. Satellite flare is the phenomenon caused by the reflective surfaces on satellites (such as antennas or solar panels) reflecting sunlight directly onto the Earth below and appearing as a brief, bright "flare". With highly reflecting triple antennas, the Iridium communication satellites create the brightest flares in the night sky, some bright enough to illuminate night landscape for a short time. They create predictable and quickly moving illuminated spots of about 10 km diameter. To an observer this looks like a very bright flare in the sky with duration of a few seconds. However this flashing has been also a problem for serious deep sky astronomical study, as the flares occasionally disturb observations. When not flaring, the satellites are often visible crossing the night very faintly.