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Are the planets where we expect them to be

  1. Dec 18, 2009 #1
    I saw a film some time ago at NGC.
    It was told that the planets not are exactly there where expect them to be, when space probes arrives, but I did not got the point..
    What is wrong ?
  2. jcsd
  3. Dec 18, 2009 #2


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    Can you give the title of the film? I haven't heard of that problem, but I can tell you that the planets are there when probes arrive, otherwise the probes would miss, and we wouldn't have all the cool pictures we get from the probes on and around those planets.

    But perhaps the film was talking about planets being just a tiny bit out of their expected position; maybe enough difference to be measured, but not enough to make the probe miss? That seems pretty unlikely, since the margine for error on interplanetary probe missions is very small. I'd like to search for the qoute from the film itself, if you can remember the title (and if it's in English).
  4. Dec 18, 2009 #3
    It's long ago
    Yes it's only a tiny bit
  5. Dec 18, 2009 #4


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    More likely the course corrections the probes make are due to error in the probe's trajectory, not error in the expected position of the planet.
  6. Dec 18, 2009 #5

    D H

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    Since you cannot supply a source to determine what is truly being discussed, I suspect the problem is that the target planet is not where it is expected to be relative to the the incoming the space probe. Our imperfect modeling of the behavior solar system is a part of the total error. For example, there is about a 1 km uncertainty in Mars position over the last/next 10-20 years; errors increase moving forward or backward in time.

    Additional error arises from not quite knowing the state of the probe. The probes navigate by dead reckoning, with occasional Kalman updates based on Deep Space Network measurements. DSN range rate measurements are incredibly precise (~ 1 mm/sec). Measurements of range, are also quite good (~ 1 m). Things get worse normal to line of sight. Cross range position error (observable by using multiple antenna) is on the order of a kilometer. Transverse velocity is not directly observable.

    These are errors with respect to Earth -- and that brings us back to the uncertainties in the ephemerides.
  7. Dec 18, 2009 #6
  8. Dec 18, 2009 #7

    D H

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    I suspect its more along the mundane lines alluded to above. The Pioneer anomaly is observable only in the tiny handful of vehicles that have gone well beyond the planets; the flyby anomaly is a tiny possibly unexplained change in velocity during a planetary flyby. Neither of these pertains to the "planet not being where we expect them to be."

    Force due to solar radiation pressure is much smaller than thrust uncertainty. It is an effect, but not much of one, during transit. (Note: This is not quite the same as the Yarkovsky effect. The Yarkovsky effect is a second order effect that results from tumbling. We do not like our spacecraft to be tumbling.) Torque due to solar radiation pressure does make for small but persistent perturbative effects during long term operations. For this reason, the Mars Reconnaissance Observer underwent am eight day solar radiation pressure calibration at the end of the cruise phase to Mars.
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