This image of the NASA mission control shows the Mercator screen at the centre. I'm probably wrong about this (feel free to shoot me down) but the Mercator up there appears to render what look like 3 different orbit tracks.

Why are there 3 orbit tracks? Does Mission control track multiple craft in adjacent orbit? Are these something like an nominal, sub-nominal, and supra-nominal orbit? What is the typical margin between these orbits?


As @David said, those are the current and the two next orbits of the same spacecraft. On this photo you can see the ISS superimposed on one of the orbits, and the orbit numbers (4, 5, 6) to the left.
Mission control screen

As @Gerrit said, if the spacecraft isn't doing any maneuvering, the next orbit can be predicted with 100% accuracy. The only information you need is the orbit's period. As the Earth rotates beneath the spacecraft, the orbit's period determines the distance between the ground tracks.

(edited because my initial answer was incorrect)

  • $\begingroup$ I can understand previous, and current. How far in advance can variations to the next orbit be made? What guides that decision? Does mission control just (for want of a better phrase) play it by the ear? $\endgroup$ – Everyone Apr 30 '14 at 14:35
  • $\begingroup$ Orbits are rather predictable. $\endgroup$ – gerrit Apr 30 '14 at 14:40
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    $\begingroup$ @Hobbes with 100% accuracy is a little misleading. We are good, but not that good. For example we can predict where it'll be to the nearest meter, centimetre, maybe even millimetre, but not micrometer. We also can't 100% guarantee it wont be knocked off course by a collision of some sort. This is mostly semantics, but it's misleading to say any exact future prediction is 100% accurate. $\endgroup$ – ThePlanMan Apr 30 '14 at 19:30
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    $\begingroup$ @MichaelKjörling of course predicting to the nearest micrometer wouldn't make you 100% accurate, I was making a point. Predicting to the nearest 1.61619926 × 10^-35m would be 100% accurate though! $\endgroup$ – ThePlanMan Apr 30 '14 at 21:43
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    $\begingroup$ @FraserOfSmeg But by then, which spacecraft atom is it that you are tracking? :) $\endgroup$ – a CVn Apr 30 '14 at 21:45

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