Re: ISS altitude

Date: Thu Dec 04 2003 - 12:36:23 EST

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    Andrew Gray wrote:
    "I do try to make a practice of observing ISS when I can, and dragging people to see it; it has an inherent gosh-wow feature."
    Same here. Besides, it's nice to see what $50 billion looks like...
    "...observed an undocking, or the two moving apart? It strikes me that a bright dot separating into two would be an interesting sight..."
    Orbital rendezvous is too slow to be visually interesting: a noticeable separation for a ground-based observer (assuming no optical aid) would be about ten minutes of arc. At typical distances for the ISS, that's a linear separation of 15km or so. To see a significant (25%?) change in this distance during, say, one minute, the two spacecraft would need to have a relative speed of about 200 kph. Typical speeds at these distances are more like 10 kph. So usually the best you can hope for is to see a Progress trailing or following ISS at some unchanging distance.
    On the other hand, perspective effects can sometimes create the *illusion* of rapid relative motion. When two objects are orbiting in close formation, if one is below the other ("below" meaning closer to the Earth here), then as they pass over your location, you will see them lined up only when the line between them points straight at you with apparent motion before and after. This can create the visual illusion of relative motion when the objects have zero (or low) relative velocity between them.
    Frank E. Reed
    75% Mystic, Connecticut
    25% Chicago, Illinois
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