I tried something new on Saturday night that I have been meaning to do for a while now – light stalking geosynchronous satellites.
There are around 6500 satellites orbiting the earth in Low Earth Orbit (LEO) (both functional and non-functional). These move relative to the stars and can be seen with the naked eye in the late evening when darkness allows our eyes to see sunlight reflecting off the orbiting satellites high above.
However, there are a lesser family of satellites in Geosynchronous or Geostationary orbit. These stay in fixed position relative to the earth’s surface. There are far fewer of these – around 560. They are much further out in space and so are almost permanently illuminated by the sun. These satellites are strung out along the only orbital plane possible for geostationary satellites, which is at a distance of 35,786 km directly above the earth’s equatorial line. At this specific orbital location, it is possible for satellites to keep pace with the earth’s rotational velocity and so remain stationary over the same point on the ground. In fact, they do drift slightly north and south, however this drift is periodically corrected with correcting boosts to ensure they stay in their geostationary orbit.
Typically if I am doing long exposure on DSO’s (Deep Space Objects), the telescope mount will track the stars in the opposite direction to the earth’s rotation, so that the stars stay stationary as points of light on the camera sensor and don’t form star trails. Occasionally however, an orbiting satellite or a geostationary satellite will pass through the frame and leave a single line of light streaking through the entire frame.
So what happens - if instead - you point the telescope at a location ~35,786 km above the earth’s equator where the highest geostationary satellites reside and then turn off the tracking motor on the mount that drives the telescope. Well, the telescope pointing direction then stays fixed and rotates with the earth, and the multitude of stars in the field of view will streak across the long exposure of your image. However, geostationary satellites in geosynchronous orbit also keep pace with the earth’s rotation, and so while the stars will streak by in a long exposure, the sunlight reflecting from geostationary satellites will accumulate in your image and appear as stationary points of light (if the exposure is long enough).
Voila ! It works.
This image from Saturday night shows 4 geostationary satellites captured in Monoceros:
USA 271 – A US department of Defense (DoD) space surveillance satellite for space satellite situational awareness.
USA 283 – USAF military communications relay station designed to keep military commanders in contact with government officials.
APSTAR 6D – A Chinese Ka-Band high through-put communications satellite.
APSTAR 6C – A Chinese C- Ka- and Ku-Band communications satellite with 45 transponders (TV broadcasts and cellular backhaul).
These all reside above the equator, just north of the Birds Head Peninsula in West Papua.