Two iridium satellites flare in the night sky over a telescope. 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.