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Time Dilation and space missions

  1. Jan 17, 2008 #1
    I have read many NASA history series books on early space missions and none of them mention time dilation. In fact none mention albert Einstein or general or special relativity.
    Even books like "orders of magnitude" and others which show timelines of important discoveries relating to flight there is no mention. Did NASA need to factor time dilation into there missions for example voyager, or pioneer, or viking. They make no mention of it in the books. Was it classified information at the time or something, or was it just figured to be of no interest of publication?
     
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  3. Jan 17, 2008 #2
    To be honest I'm not completely sure on the answer to this one. I would think that relativity would probably factor into the equations used for many, if not all, of the missions you mentioned. But the effects would be so minor they wouldn't really be noteworthy in the context of popular science. It definitely wouldn't be "classified" information. At the time these missions took place, relativity was a well known theory, with many predictions already tested and proven accurate.
     
  4. Jan 17, 2008 #3

    russ_watters

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    You can calculate it and see for yourself, but time dilation is not a relevant issue with speeds as slow as what we can achieve.
     
  5. Jan 17, 2008 #4
    That may be true, but what about gravitational fields.
     
  6. Jan 17, 2008 #5

    russ_watters

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    The impact of gravitational fields is also negligible for anything that doesn't require atomic clock-level time precision.
     
  7. Jan 17, 2008 #6
    According to Carl Sagan it is necessary to calculate it into our satelites computer systems.
     
  8. Jan 17, 2008 #7
    I'm pretty sure some calculations for space flight dynamics for certain missions have required relativistic corrections for propagation times, but they're rare and are also extremely small - we're talking about clock cycle times. It matters when the round-trip propagation time is what you're using for a range estimate, but not when you're just wondering when a vehicle will be visible or anything like that.
     
  9. Jan 17, 2008 #8

    chroot

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    We need to consider relativity for things like GPS, because they involve precise timing and the coordination of signals travelling at the speed of light.

    Human bodies will probably never be able to go fast enough to make time dilation a concern for anyone, unless you're terribly concerned about your friends aging a couple of nanoseconds more than you.

    Time dilation has absolutely no impact on space travel with current technology, and probably will not have any meaningful impact on any kind of space travel that anyone will invent in the next couple of centuries. Missions to other planets literally use nothing more than the Newtonian model of gravitation, because it's entirely sufficient.

    - Warren
     
  10. Jan 17, 2008 #9

    D H

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    Just to elaborate a bit -- we do not need to consider relativity to model the orbits of the GPS satellites. Newtonian mechanics does a fine job of predicting the orbits of the GPS satellites. We only need to consider relativity for its effect on timing, and then only for the GPS satellites.

    Each GPS satellite broadcasts information that includes a time stamp. The location determination algorithms used by GPS receivers are extremely sensitive to errors in the broadcast timing information. To achieve an accuracy of 5-10 meters, the timing errors must be no more than a few nanoseconds/second or so. Note well: That error corresponds to about 1/20 of a second per year! No normal clock can achieve this kind of accuracy. To satisfy this accuracy requirement, each GPS satellite is equipped with an atomic clock.

    The combined relativistic effects (general and special) on timing for the GPS satellites amount to 38 nanoseconds/second, which exceeds the error tolerance by a factor of ten. The GPS system must account for the relativistic effects. That 38 nanoseconds/second time difference amounts to just over one second per year. An astronaut orbiting for years at GPS altitude would not notice the effect.
     
  11. Jan 17, 2008 #10
    Don't we need to have accurate timing to relay signals between probes and us in deep space for example voyager. Is it just that we are satisfied with the level of inaccuracy because the probe may not end up exactly where we want it, but close enough?
     
  12. Jan 17, 2008 #11

    D H

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    The only satellites of which I am aware that carry atomic clocks are the GPS satellites. Most other satellites carry run-of-the-mill quartz clocks, pretty much like the one on your computer. The inherent errors in these clocks vastly overwhelm the errors that result from ignoring relativistic time dilation effects.

    Similarly, the errors that result from not knowing a vehicle's state perfectly vastly overwhelm the errors that result from ignoring non-Newtonian gravitational effects. The Deep Space Network that tracks things like Voyager can measure velocity fairly precisely but not position. We on the ground don't know exactly where those probes are. Even the satellites themselves don't know exactly where they are. The absolute navigation sensors on the vehicle are only so good. A satellite approaching another planet has to use different sensors to look for the planet before it does its final burns.

    Bottom line: There is no reason to address errors induced by ignoring relativistic effects because these errors are very tiny compared to other errors, even after we compensate for those other errors to the best of our ability.
     
  13. Jan 17, 2008 #12
    No not at all as said before that fastest space ship we have sent into space is much too slow to worry about relativistic effects.
    Also, deep space probes do not use relays we signal direct, but even that would not matter.
    We only need to know how fast our signals travel (light speed) and account for typical Doppler effects for orbit observations and tracking.
    The accuracy required for GPS positioning of which set of photons are arriving at your location from which satellites based on the time here and in each satellite requires a whole new level of time accuracy.
    When you read up on GPS and how it works, you should see it is not the same as calculating or observing simple departing orbits for voyager etc.
     
  14. Jan 17, 2008 #13
    Again you are ignoring that I am talking about the gravitational fields. I understand that it is this that is concern with GPS sat not speed. Anyways I think you guys have made it clear.

    I'm still left feeling that compared to GPS sats voyager must have been affected much more by gravitational effects than GPS sats.

    Why not design a space probe that we can track? Is it too complicated?
     
  15. Jan 17, 2008 #14
    I believe that General Relativistic effects, i.e. gravitational effects, are too small to be significant. If a signal were to be sent in a path that went very close to the Sun, then okay, it would matter in that case (as in the early confirmations of GR where radar signals were reflected off Venus when it was just setting - or maybe rising - over the Sun's horizon).

    I work at the lab that supports the MESSENGER mission that just did a fly-by of Mercury this past Monday. I'll ask around to see if its orbit takes it close enough to the Sun for GR effects to be significant for any reason. After all, we all know that Mercury's orbit requires GR for accurate predictions, so maybe MESSENGER's does, too.
     
  16. Jan 17, 2008 #15

    pervect

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  17. Jan 17, 2008 #16

    chroot

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    Why would we need any kind of precise timing between the Earth and, say, the Voyager probe? Voyager did not carry any sophisticated clocks, nor is it in any way involved with any kind of time measurements.

    - Warren
     
  18. Jan 17, 2008 #17
    I guess it wouldn't unless we needed to know exactly where it was.
     
  19. Jan 17, 2008 #18

    chroot

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    How exactly do we need to know its position? We can bounce radar off it and know were it is down to an accuracy of probably a couple of hundred miles, even when it's at the very edge of the solar system. How useful would it be to know where it was down to, say, a centimeter? What would we do with that information?

    - Warren
     
  20. Jan 17, 2008 #19
    I just didn't know that it could stay on coarse without our help. So it senses its position and makes its corrections all on its own.
     
  21. Jan 17, 2008 #20

    chroot

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    No, it doesn't really sense its position OR make corrections on its own. Voyager, at least, is not going anywhere specific, nor does it have any fuel left to do any maneuvering.

    Other, newer probes are directed to their destinations by careful timing of rocket burns, but we're still talking precisions on the order of minutes, not nanoseconds. Relativity just isn't significant inside the solar system.

    - Warren
     
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