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    Re: Satellite positions
    From: Frank Reed
    Date: 2013 May 1, 11:59 -0700

    Bill, you wrote:
    "I was rather amazed that the viewing windows for the ISS and others were
    so brief."

    Yes, the ISS is in a relatively low orbit which means it crosses the sky in just a few minutes, orbiting the Earth once in 90 minutes. This is an "economic" choice. The station's orbit is always decaying since it's low enough to face significant drag from the very thin upper atmosphere. The station's orbital altitude decreases about 15 feet per orbit or about 250 feet per day. That means it must be periodically re-boosted (usually these days by docked Progress spacecraft). There's a nice graph of that here:
    http://www.heavens-above.com/IssHeight.aspx.
    But in exchange for the "cost" of all that re-boosting, since it's so low, it is relatively easy to reach. Crew and cargo deliveries are correspondingly less expensive. But if any of various possible causes ever leave the ISS unmanned for as little as eighteen months, that big beast is going to make one helluva fireworks show when it comes tumbling out of the sky.

    At the other end of the spectrum, Vanguard 1 is still up there along with several other early US satellites and their upper-stage boosters. They were intentionally placed in much higher orbits than the early Soviet satellites which decayed within weeks or months after launch. You can still see Vanguard 1, now 55 years after its launch, using a backyard telescope. Nothing from the first four or five years of the Soviet space program is still up there. There's one interesting descendant of the early Soviet space program in orbit right now. It's called "Bion-M1" : http://en.wikipedia.org/wiki/Bion_(satellite). This is a brand new spacecraft carrying some gerbils and geckos for basic biology experiments, but the basic capsule is nearly identical to the Vostok space capsules which carried the first cosmonauts into orbit over 50 years ago. Here's a tracking link: http://www.heavens-above.com/SatInfo.aspx?satid=39130.

    -FER
    PS: A bit of orbital mechanics fun. The ISS orbits the Earth once in 90 minutes. Now suppose I have a small asteroid, let's say a mile in diameter. The orbital period for a surface-skimming satellite around that asteroid --or any planet, moon, asteroid, meteoroid or other "normal" solid object-- would also be about 90 minutes (probably closer to two hours but barely any difference). Mean density matters, too, and you can work that out if you enjoy the physics of it, but for a quick "ballpark" estimate of orbital speeds, all you need to know is "90 minutes for low orbit". For higher orbits, you can throw in Kepler's Third Law (distance cubed proportional to period squared), and for escape orbits, just multiply the circular orbit speed by sqrt(2). That's a lot of orbital info for just a small amount of math.

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