Anatomy of a Near Miss
I wanted to revisit the previous post, describing a near miss of ISS I witnessed back in August. Recall that both my brother and I saw the same 2 satellites cross in the sky at the same time.
First, let’s just take a look at the geometry that allows satellites to be seen in the first place. If you’re going to see a satellite from the ground, it needs to be lit (by the sun) and you need to be in shadow (night). Here’s a picture of the geometry from my location on Aug. 29, 2011.
Orbit Geometry and Earth Shadow for an ISS pass on Aug. 29, 2011
The red cylinder off to the right is the shadow, the Sun is off to the left. A little closer look allows us to see what’s happening. The two satellites (ISS in the blue, SL rocket body in the red) have orbits that are above the shadow. Note that Friday Harbor and Seattle are both in darkness.