Wednesday, June 20, 2007

One more good ISS pass in Chicago

The last good pass of the International Space Station for Chicago in the near future will be tomorrow, June 22nd. You may be asking, why is that? To see the space station pass, you have to pass several criteria, each of which can derail easy viewing.

To first order:
1. The satellite must be above your local horizon.
2. The satellite must be sunlit while you are not.

On #1, a satellite's orbit around the Earth is a invariate thing--that is, it orbits the Earth and the ellipse it defines stays the same in an absolute reference. The Earth may rotate below it, but the object stays in the same fixed plane. Since the Earth rotates every 24 hours, you will roughly be under where the satellite orbits twice a day. The satellite also needs to be in the part of the orbit near you. Generally, this is not a problem, because 1. low earth orbit satellites have an orbital period of only 90 minutes and 2. you can see them several hundred miles away from their ground track. See this Java applet to see the Shuttle's visible ground track.

You also need the pass to occur when the satellite is lit by the sun, and have it dark where you are. This leads to satellites generally being most visible just after evening twilight and before dawn. In the summer, sunlight streaming over the pole can illuminate satellites for most of the night--many people remember staying up and watching the Perseids and seeing more satellites than meteors.

But because the satellite's orbit is fixed in space, and the Earth rotates around the Sun, at any one location the visibility factors come in and out of phase. The satellite's orbit hasn't really changed, but where the terminator is on Earth has. And tonight's ISS pass is the last good one for a few weeks.

So tomorrow, if it's clear, at 8:58 the ISS will start becoming visible in the WNW; pass well above Venus in the west; pass well above the moon, and reach a maximum altitude of 54 degrees at 9:01PM in the SW, and pass by Jupiter at 9:04PM in the SE.

Details at Heavens-Above