A favourite pastime of ours while we were on holiday in Spain was to gaze at the night sky. There were no city lights nearby, no clouds and no moon to illuminate the sky. This meant that satellites roaming the sky were a very common sight. However, occasionally a satellite would illuminate brightly and then fade. What caused this?
鈥� This usually occurs around an hour before sunrise or after sunset, when the sun is not visible from the ground, but is still visible from the low Earth orbit of many satellites.
Sunlight reflects off a surface on the satellite, such as a solar panel, causing the bright flash of light visible from the ground. The light fades quickly as the satellite moves on and no longer reflects the sunlight to the observer. Effectively, the satellite acts as a moving mirror, tracing a path of reflected sunlight along the ground.
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Kin Yan Chew, Middletown, Connecticut, US
鈥� In the 1960s I operated the Royal Observatory Edinburgh鈥檚 satellite kinetheodolite 鈥� a device for tracking airborne or orbiting objects 鈥� and the Smithsonian Astrophysical Observatory鈥檚 20-inch aperture f/1 Baker-Nunn satellite-tracking Schmidt cameras.
Of the hundreds of satellites which I observed and photographed, many changed in brightness as they crossed the sky. This is caused by the changing aspect of the satellite during its orbit, as seen by a fixed observer, and in some cases its tumbling or rotating motion, presenting different faces that reflect sunlight. The effect is particularly pronounced with those satellites that have large flat surfaces. Satellites also disappear into the Earth鈥檚 shadow in the evening (for a satellite moving west to east), and appear out of the shadow in the morning.
Today I am a member of the local astronomy society in Guernsey. With many thousands of satellites now in Earth orbit, I typically see about 30 on clear nights, many changing in brightness. Of special interest are the Iridium telecommunications satellites, whose highly reflective antennas produce extremely bright, directional flares lasting a few seconds, some of which can be seen in daylight.
Predictions of when you can see these and other satellites can be found at .
David Le Conte, Castel, Guernsey, Channel Islands
鈥� In the mid-1960s I worked on a communications project known as 鈥淪pace Junk鈥�, which involved bouncing microwave signals off of the growing numbers of satellites and their final-stage rocket bodies then in low Earth orbit. In order to accurately point the dishes at the satellites at night, we used an optical tracker to augment and improve the theoretical predictions. Consequently I became very familiar with the visual characteristics of these various pieces of junk.
鈥淭he satellite acts as a moving mirror, tracing a path of reflected sunlight along the ground鈥�
We observed that while some objects had a fairly constant brightness, others varied strongly and some exhibited a regular flashing pattern. We found that there was a precise correlation between the type of object and the observed visual signature.
The actual satellites, which in general had a fairly regular shape, had a fairly constant brightness, while the rocket bodies, which generally are cylindrical, exhibited a flashing pattern since they are not stabilised and usually finish up tumbling end over end. The tumbling rate can vary widely from seconds to minutes.
These days there are thousands of visible objects in low Earth orbit and each one will have its own visual pattern.
Richard Harris, Malvern, Worcestershire, UK
鈥� One of my 鈥渢hings to do before I die鈥� includes making an Iridium telephone call at the same time as an Iridium satellite flare is overhead. There would be something poetic about seeing the satellite being used to relay my call whilst whispering sweet nothings to my fianc茅e. She, however, prefers flowers.
Barry Hahn, London, UK