In Part 1 of explaining imaging satellite operations, we assumed a low earth orbit (LEO) satellite is one of the better options for getting great “Earth-selfies” with a camera on board. We also learned the range of LEOs is pretty big: 99 to 1200 miles. But we also know closer is mainly better (see previous post to understand why), so naturally, why not choose the closest possible orbit of 99 miles?
The big problem is the atmosphere. Again, Wikipedia tells us there are gasses between 50 and 313 miles from Earth’s surface. These gasses tend to slow a satellite down (drag). If a satellite experiences too much drag, it will eventually slow down too much. A satellite that’s attempting to orbit between 50 to 313 miles will experience significant drag and slow down a lot.
Such drag is also called aerobraking (but only by those who need to feel important using big words). If a satellite slows down too much, it will eventually be unable to complete an orbit, re-enter the atmosphere, and hit the Earth. This can result in unpleasant consequences for those around the satellite’s impact point if the re-entry is unintended and uncontrolled (just ask the Aussies: http://www.theaustralian.com.au/news/nasa-satellite-crashes-to-earth/story-e6frg6n6-1226145421345).
So, back to the question: how close to the Earth’s surface can we get a satellite to orbit the Earth without encountering atmospheric drag? And Wikipedia answers, “313 miles.” Easy, right? That’s not to say satellites can’t orbit the Earth using less than that number, but these would have to have to be considered disposable satellites.
Assuming the people running these camera sensors on the satellites want the best, most-detailed pictures, but don’t want the satellite to hit the Earth, then around 313 miles would be optimal for their satellite orbits.
But wait, there’s more…