On any clear night, a dozen or so artificial satellites can be seen moving slowly across the sky. Looking like slow-moving stars, sometimes as bright as magnitude +2, they take about 10 minutes to travel from horizon to horizon. Satellites are most prominent in the first and last hours of the night, since they are then still clear of the Earth’s shadow. Sometimes they fade suddenly and disappear as the satellite enters the shadow. Generally, Russian satellites tend to orbit south-north or north-south, as they are launched from reasonably high-latitude sites.
Some satellites move together in groups of two or three – possibly giving rise to UFO reports! A few, such as the Hubble Space Telescope and the International Space Station, are distinctly brighter than the rest. The Iridium communication satellites can produce spectacular flashes (popularly called “flares”) when the Sun glints off their shiny surfaces. They briefly shine at magnitude –8 and can even be seen in daylight.
There are several areas in which the amateur can play an important role, ranging from just observing a satellite (noting its optical characteristics such as magnitude, whether it appeared steady or was tumbling), to the specialized art of determining the position of a satellite against the celestial background so that an accurate orbit can be calculated.
Approximately 350 satellites have no orbital data from official sources as they are classified military launches. A handful of amateur space detectives have located, identified and keep regular tabs on such objects. In the last few years it has become possible to observe the superficial structure of the larger satellites using simple, cheap webcams or low-noise commercial surveillance cameras.
South Africa has always been a location of importance to the satellite tracking community on account of it being in the southern hemisphere and also under the flight path of many satellites during the first few hours in orbit.
Further information is available from the Shallow Sky Section.
How to Visually Observe Satellites by Greg Roberts, available as a free PDF download, is an introduction to observing and studying satellites. Written by one of the world’s leading satellite trackers, it covers the basic concepts and even gives advice on the equipment and techniques required for automated satellite tracking.