Wednesday, October 30, 2013

Geostationary Satellites!

Space is far from empty.  Especially in the Earth's neighborhood there are lots of objects.  Rare is a twilight while out observing from a dark site when you do not spot a dozen or more satellites moving across the sky.  Fortunately it is not as bad as the movie "Gravity" when a space station or habitable pod is just a few dozen miles away, but in the near-earth environment there are lots of both naked-eye satellites, as well as fainter ones that show up in telescopic or binocular views.

Some of the easiest to photograph, especially from a dark site, are the geostationary satellites.  Mostly communication or weather stations, they orbit precisely every 24 hours over the equator, so an observer on the ground will see these points of light fixed in the sky.  NOT fixed with respect to the stars, but fixed with respect to the ground.  All you need to image them is a tripod and camera and you can spot them because they are fixed with respect to the trailing stars as the earth rotates.  Because of parallax, from northern latitudes they appear to be located south of the celestial equator - from Tucson's 32 degrees latitude, they appear at about -5.5 degrees declination.  A few weeks ago, before our recent Midwest trip, I went out towards the base of Kitt Peak to image some of these  Shown at left here is the result of 50 (!) co-added exposures with a DSLR and 50mm lens running 90 second exposures, pointed a little east (about 10 degrees east of due south).  I had in-camera noise reduction turned on to minimize hot pixels that would confuse identification, so 50X90s=75minutes of exposure over a period of 2.5 hours.  The bright points are the geostationaries - there were over 25 in the full frame, this one is cropped about 25%.  The dashes are the stars that trailed through the frame.  Besides the point like satellites, there are also a large number of trails NOT in the same direction as the stars.  These are geosynchronous satellites - likely rocket boosters or inoperative satellites that have been moved out of the crowded geostationary positions.  They also orbit every 24 hours, but have a slight inclination, so they do not orbit over a single point. 

Many of these geosynchronous orbits trace out a figure 8 shape as seen from the Earths surface if it could be followed for a full 24 hours.  At left in this frame is a close-up of the satellites, taken with my 70-200 zoom (set to 90mm focal length), again, for the full 2.5 hour period.  This one is highly cropped of the right part of the field above, and in it you can see how well aligned the geostationary ones are to the earth as they don't budge much.  The one not in the plane of the others traced out a looping curve in the 2.5 hours of coverage...

The last exposure shown here is one of the single frames with the 50mm lens, cropped pretty tightly from the left side of the first frame above.  Besides the line of geostationaries, there are a trio of geosynchronous satellites visible, one of which (the bottom) that is glinting sunlight to my position.  I picked the observation period (a couple weeks after equinox) that was favorable for catching glints like this, and many of them caught the sun, brightening a lot before they went into the Earth's shadow.  I was able to spot several of them in binoculars, and I'm pretty sure I could see them naked eye.  Certainly the one below left would have been easily visible to the naked-eye during the 10 minutes it was flaring.  Unfortunately, they move so slowly relative to the stars that they are hard to identify only from their appearance, unless you have the advantage of watching the camera display as the frames are coming in.  Interestingly, over heavily populated areas, like the US, they can position and use them with .1 degrees angular separation, which at the geostationary distance of  22,000 miles (36,000km) corresponds to a real separation of about 44 miles (73km).

I'd like to be able to identify some of these, but haven't yet as I need to figure out how to measure the east-west alignment more accurately.  Since these were about 10 degrees east of due south, and we're at about 112 degrees longitude, they are mostly about 102 degrees west longitude.  I'll continue to observe them occasionally, and I encourage you to do the same - it's easy as there are no tracking issues!

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