Time Dilation and space missions

[W3pcq]

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?

 

[CJames]

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.

 

[russ_watters]

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.

 

[W3pcq]

That may be true, but what about gravitational fields.

 

[russ_watters]

The impact of gravitational fields is also negligible for anything that doesn't require atomic clock-level time precision.

 

[W3pcq]

According to Carl Sagan it is necessary to calculate it into our satelites computer systems.

 

[belliott4488]

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.

 

[chroot]

We need to consider relativity for things like GPS, because they involve precise timing and the coordination of signals traveling 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

 

[D H]

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

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.

 

[W3pcq]

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?

 

[D H]

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.

 

[RandallB]

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?

No not at all as said before that fastest spaceship 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.

 

[W3pcq]

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?

 

[belliott4488]

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.

 

[pervect]

GR effects on time (gravitational time dilation) are not very large in human terms, but are easily measurable with modern atomic clocks. See for instance :

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/gratim.html

in particular the Scout Rocket experiment and the Harvard tower experiment.

 

[chroot]

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

 

[W3pcq]

I guess it wouldn't unless we needed to know exactly where it was.

 

[chroot]

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

 

[W3pcq]

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.

 

[chroot]

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

 

[W3pcq]

Thanks for all of the replies. I think my curiosity is about satisfied. I just don't like being confused.

 

[sysreset]

Any idea how much time dilation a probe descending Jupiter's atmosphere would experience? Or a probe descending to the surface of the sun?

 

[yuiop]

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?

I think you are referring to the famous Pioneer 10 and 11 spacecraft missions where a small anomalous acceleration towards the sun was detected. The data has been analysed taking GTR into account and the discrepancy remains unaccounted for. A grant has recently been given to analyse the data more fully. The most likely candidate for the discepancy is that radiators that keep its power reactor cool are mounted on the side of the spacecraft away away from the sun and the radiated heat caused an acceleration towards the sun. As far as I know even this explanation has been discounted.

One curiosity is that the Voyager spacecraft did not show this anomaly but they were not spin stabilised like the Pioneer spacecraft and it was not possible to track the Voyager spacecraft accurately enough for the anomaly to show up.

Suggestions that "new physics" is involved is countered by the argument that if gravity did not work the way we think it does in the solar system then the anomaly would show up in the orbits of the outer planets. However, the planets have a near constant distance from the sun give or take the eccentricty of their orbits, while the spacecraft have a large radial velocity. Maybe that makes a difference? There have some suggestions to launch a probe to investigate this anomaly more fully, but they are taken too seriously as the gut feeling is that GTR is not wrong and the discrepancy is probably an as yet unidentified systematic effect.

Some links on the subject:

http://arxiv.org/PS_cache/gr-qc/pdf/9903/9903024v2.pdf

http://www.xs4all.nl/~carlkop/gravnew.html

"One research team is seeking support to investigate flight data from the early stages of the Pioneer 10 and 11 missions, in a bid to understand why the craft are now travelling
slightly more slowly than mission planners envisaged. A second, far more ambitious proposal would launch a dedicated space mission to study the effect,which has left the craft hundreds of kilometres closer to the Sun than had been anticipated." http://www.generationcp.org/vw/Download/ARM_2004/bios_news.pdf

 

[yuiop]

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

Although Voyager was not going anywhere in particular, corrections were made while it was in radio contact to get it into the correct position to get a slingshot boost from planets to get out of the solar system as quickly and efficiently as possible. Corrections to its orientation were also made to keep its radio receiving and transmitting antenae facing towards the Earth for efficient communications. The Voyager spacecraft required more corrections than the spin stabilised Pioneer spacecraft , so they were less useful for measuring acceleration due to gravity. I personally think that if it was possible to track a spacecraft to nanosecond / centimeter precision it would be very useful to check our understanding of GTR, dark matter and dark energy. Gravity Probe B showed that it is possible to conduct extremely accurate space missions that can detect and analyse some of the effects and predictions of GTR.

 

[belliott4488]

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

To be able to maintain contact with a satellite we must be able to predict its future locations. To do that we need precise determination of its position and velocity. The more accurate the determination, the better and longer the predictions. Many satellites uses transponders for tracking purposes, so errors in timing translate to errors in estimates of position.

I have never worked with satellites as deep in space as the Voyagers, however, so I don't know the level of precision required for them. No satellite I ever supported required GR corrections, although it was discussed - and subsequently dismissed - for one mission that had very tight requirements for estimation of orbital state vectors.

 

[chroot]

Again, the word "precisely" is loaded. How much precision is necessary for communication with antennae with unavoidably wide radiation patterns? Feet? Miles? Hundreds of miles?

- Warren

 

[belliott4488]

Again, the word "precisely" is loaded. How much precision is necessary for communication with antennae with unavoidably wide radiation patterns? Feet? Miles? Hundreds of miles?

- Warren

I might be missing your point, but what does the beam width have to do with this? The pointing angles of the antenna are not much use for determining angular position, especially for deep space vehicles; maybe that's what you mean? In any case, the range estimates come from timing information, like for GPS. If you're talking about a vehicle that's moving tens of km/sec, then millisecond errors translate to tens of meters in range error. Maybe that matters, maybe not - depends on what you want to do with the information.

 

[chroot]

That's rather my point. Relativity would be important if you wanted to measure the spacecraft 's position to a precision of perhaps a centimeter or less. On the other hand, normal communications will work just fine even if your position estimate is hundreds of miles off (at least when the spacecraft is at the edge of the solar system). There's no need to be able to measure Voyager's position so precisely, so it did not include the hardware to make such measurements possible.

- Warren

 

[RandallB]

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?

They can and do track all space probes with great accuracy using classical “Newtonian” orbital math. And they can adjust those calculations with relativistic effects and know very well that the change is insignificant because relativistic effects from speed acceleration and gravitation are insignificant. Including no need to have incredible precision for time on the probe. Accurate measurements separated by large distance observation separations – like 6 months apart – is how they gain accuracy with triangulations and Doppler effects.

What you seem to be ignoring is that a GPS requires much more than just tracking the simple orbit of GPS sats which can be done without relativity.
GPS requires detailed tracking of time stamped groups of photons (radio signals), they do travel at relativistic speeds and require accurate time stamps. Getting that additional detail beyond orbital tracking is highly sensitive to small gravitational effects, as well as SR effects, on time.

Do you really understand what is involved in a GPS system?

 

[gonegahgah]

The world was in awe when a plane sent around the world showed a different time for its atomic clock than the one left behind.

So I think the more interesting question to ask is why have we do we not have proud announcements about how space ships returning to Earth have verified the effects of relativity on clocks? They certainly travel much further than a plane traveling once around the Earth.

Here is another puzzle to me so it may be an opportunity for someone to explain it to me. GR says clocks will tick faster at lesser gravities (ie. higher up). Yet pendulum clocks do the opposite and tick slower at lesser gravities. Don't pendulum clocks count as clocks?

 

[Doc Al]

Here is another puzzle to me so it may be an opportunity for someone to explain it to me. GR says clocks will tick faster at lesser gravities (ie. higher up). Yet pendulum clocks do the opposite and tick slower at lesser gravities. Don't pendulum clocks count as clocks?

Take away the Earth and the pendulum clock wouldn't work at all. You must consider the Earth itself as a key part of a pendulum or pendulum clock.

 

[Janus]

Here is another puzzle to me so it may be an opportunity for someone to explain it to me. GR says clocks will tick faster at lesser gravities (ie. higher up). Yet pendulum clocks do the opposite and tick slower at lesser gravities. Don't pendulum clocks count as clocks?

Well for one thing, the swing of a pendulum clock depends on the gravitational "force" and the Gravitational time dilation depends on the gravitational "potential". Gravitational force varies by the inverse of the square[i/] of the distance from the center of the gravity field and gravitational potential varies by the inverse of the distance.

Thus it is possible create a situation where the gravitational force felt by two pendulum clocks are equal, but they are at different gravitational potentials. For example, one clock sitting on the surface of the Earth and the other sitting on the surface of a body with 4 times the mass of the Earth and twice the radius. The two clocks would feel the same gravitational force but would be at different gravitatonal potentials, and the second clock would run faster (as seen by a distant observer) than the first.

 

[gonegahgah]

Thanks Doc. Yep. At zero g time ceases to exist for a pendulum clock - if not for us - because there is nothing to pull the pendulum down.

My prophecy is that "aging time" has something to do with sub-atomic spin (whereas pendulums rock back and forth) but that is personal conjecture and neither here nor there.
In that respect if something is at 2 from the centre then it would be 1/2 for potential and 1/4 for force, and at 3 from the centre it would be 1/3 for potential and 1/9 for force regardless of the size or density of the planet.

Is that incorrect? Otherwise can you help me with the difference?

It is also my understanding, though you were probably just providing a simplified model, that the closer you get to a large gravitational body, the less attraction is experienced down (although small by proportion) and more is experienced sideways and cancels out. So that, close to the surface of the Earth the mass directly below is a direct attraction down but mass off to the sides below you has an attraction with a component sideways and proportionally less down. This is in the same way that once you drop below the surface then you become attracted upwards towards the mass above you cancelling out some of your attraction to mass below. Is this correct?
Hi Janus. Surely gravitational potential varies "by the inverse [not the square acknowledged] of the distance" "from the centre of the gravity field" as well.

 

[gonegahgah]

Thanks Doc. Yep. At zero g time ceases to exist for a pendulum clock - if not for us - because there is nothing to pull the pendulum down.

My prophecy is that "aging time" has something to do with sub-atomic spin (whereas pendulums rock back and forth) but that is personal conjecture and neither here nor there.

Hi Janus. (Rewritten; accidentally deleted) Doesn't gravitational potential vary "by the inverse [not squared acknowledged] of the distance" "from the center of the gravity" also.

In that respect if something is at 2 from the centre then it would be 1/2 for potential and 1/4 for force, and at 3 from the centre it would be 1/3 for potential and 1/9 for force regardless of the size or density of the planet.

Is that incorrect? Otherwise can you help me with the difference?

It is also my understanding, though you were probably just providing a simplified model, that the closer you get to a large gravitational body, the less attraction is experienced down (although small by proportion) and more is experienced sideways and cancels out. So that, close to the surface of the Earth the mass directly below is a direct attraction down but mass off to the sides below you has an attraction with a component sideways and vector-wise less down. This is in the same way that once you drop below the surface then you become attracted upwards towards the mass above you cancelling out some of your attraction to mass below. Is this correct?