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Anomalies in spacecraft velocities
As old as Voyage...
post Feb 29 2008, 07:06 PM
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Very interesting story on SPACE.com regarding currently unexplained anomalies in the velocities of spacecraft after their Earth flybys.

http://www.space.com/scienceastronomy/0802...ft-anomaly.html

Perhaps the twisting of spacetime induced by Earth's rotation could be gently altering the spacecraft's velocity.

http://www.voanews.com/english/archive/200...FTOKEN=51791561

In any case its pretty cool to have an unexplained force meddling with our best laid plans.



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nprev
post Feb 29 2008, 07:57 PM
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Mmm...don't forget that the gravitational constant G is the least precisely known of all the physical constants, and it's a direct coefficient for many trajectory calculations. I suspect that part of the answer lies in this uncertainty.

If anyone were to undertake a serious investigation of these anomalies, I'd suggest a very precisely balanced spin-stabilized vehicle (with operational mass known to the highest degree possible), minimal outgassing, and almost nothing ancillary but the basic bus functions needed to keep it stable & relay data that would do a high-perigee (to negate drag effects) Molinya style Earth orbit with supporting tracking.

Think we're really getting very deep into the weeds here; statistical uncertainties seem like the most probable causes for these observed effects.


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centsworth_II
post Feb 29 2008, 08:00 PM
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According to one of Emily Lakdawalla's Planetary Society Blog entries,
at least some relativistic effects are already accounted for in spaceflight.

Accounting for general relativity at Mercury
"It turns out that general relativity is routinely accounted for in
spacecraft navigation.... the NASA navigation software developed
at JPL....incorporates the Ted Moyer formulations for navigation
which includes mathematical expressions that describe the effects
of general relativity."
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nprev
post Feb 29 2008, 08:29 PM
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That's pretty much a validation of how far down you have to go in order to account for all the possible effects. Mercury's orbital velocity is way below any significant fraction of the speed of light; we're probably talking about mm/sec variations in acceleration at most, but of course this is cumulative.


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mchan
post Mar 1 2008, 06:21 AM
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Emily's article describes comparison between pure Newtonian calculations and that which includes general relativity effects for the first flyby:

"The flyby altitude at Mercury increased by about 10 km, but the closest approach time changed by about 13 seconds, which corresponds to about 65 km."

The relativistic effects is quoted as 0.5e-6 mm/s^2. Carried over a long period of time it makes a big difference.
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nprev
post Mar 1 2008, 06:58 AM
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Wow, did I ever overestimate!!! Same point applies, though; such things do add up, and presumably so does error in planetary mass estimates, the uncertainty in the value of G, drag from planetary atmospheres (however tenuous), drag effects from the extended solar atmosphere & localized gas concentrations such as those distributed along planetary orbits, and a whole lot of patently trivial effects that considered systemically do have a measurable impact.

Sorry to be an iconoclast, but the only way that the "Pioneer Effect" would have been surprising to me is if it wasn't vectored towards the Solar System's center of mass. I'm far from convinced that we need to invoke modifications of fundamental physics to explain it.


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remcook
post Mar 1 2008, 03:54 PM
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"The researchers looked at six deep-space probes Galileo I and II to Jupiter[...]" blink.gif Two flybys perhaps...
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nprev
post Mar 1 2008, 05:50 PM
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The author obviously meant the Voyagers...but the editor or somebody in the chain definitely should've caught that (this is space.com, after all!!!) Nice find, Rem. smile.gif


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Greg Hullender
post Mar 1 2008, 07:01 PM
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QUOTE (remcook @ Mar 1 2008, 07:54 AM) *
"The researchers looked at six deep-space probes Galileo I and II to Jupiter[...]" blink.gif Two flybys perhaps...


Yes, Galileo was a VEEGA, so it had two different Earth flybys. They're looking at anomolies in probes that did Earth gravitational assists. Talking about anomolies around 13 mm/s when the expected error was only 0.1 mm/sec.

Messenger was the only one where they didn't see anything. They're suggesting maybe because it was the most symmetrical of the group, but who knows. It was also the only one trying to lose, not gain, velocity.

I'm with nprev, though; highly unlikely this is new physics. More likely it's some factor no one thought to include because it was thought to be "too small to measure." Maybe something involving light pressure on the probe -- complicated by the way the probe is colored. Who knows? Be interesting to see what they finid though.

--Greg
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Floyd
post Mar 1 2008, 10:31 PM
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QUOTE (nprev @ Feb 29 2008, 02:57 PM) *
...would do a high-perigee (to negate drag effects) Molinya style Earth orbit with supporting tracking.

The effect seemed to be associated with unbound orbits. Did not seem to show up in simple orbit orbits--only fly by's or for craft unbound to the sun--ie leaving the solar system.


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Greg Hullender
post Mar 2 2008, 04:41 AM
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QUOTE (Floyd @ Mar 1 2008, 02:31 PM) *
The effect seemed to be associated with unbound orbits. Did not seem to show up in simple orbit orbits--only fly by's or for craft unbound to the sun--ie leaving the solar system.

Not qute. Galileo was in elliptical solar orbit at each of its Earth flybys. It was hyperbolic with respect to Earth, but not the Sun.

Remains to be seen what that should make any difference, of course.

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edstrick
post Mar 2 2008, 10:31 AM
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There is a sort of "weirdness" to the whole thing that makes me very suspicious that something's been missed that should be "Oh Damn" obvious, something almost as obvious but missed as the nt/lb force units screwup on Mars Climate Orbiter.
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scalbers
post Mar 2 2008, 04:56 PM
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QUOTE (centsworth_II @ Feb 29 2008, 08:00 PM) *
According to one of Emily Lakdawalla's Planetary Society Blog entries,
at least some relativistic effects are already accounted for in spaceflight.

Accounting for general relativity at Mercury
"It turns out that general relativity is routinely accounted for in
spacecraft navigation.... the NASA navigation software developed
at JPL....incorporates the Ted Moyer formulations for navigation
which includes mathematical expressions that describe the effects
of general relativity."


I can also vouch for this about General Relativity. Back in the '70s when I worked briefly at JPL I wrote a numerical integration program to calculate planetary positions and conjunctions (as a hobby). This was based on information from DE96 (the latest JPL ephemeris at the time) and some other references. Like the procedure used in DE96, I included in my program explicit terms that correct for GR and utilized them for all the planets out to Saturn. This was the basis for my Sky and Telescope article in March 1979.

EDIT: Some additional comments to this discussion.

1) I think Mercury has more of an effect partly due to its eccentric orbit, thus the precession of its perihelion is more noticeable compared to the case of a circular orbit.

2) The uncertainty of the Gravitational Constant is much greater than the uncertainty of the spacecraft orbits. The corresponding masses are also uncertain, but the combination of G and M still can be stated with higher precision.

3) I wonder if the JPL ephemerides would/should account for frame dragging (as noted in post #1), or something simple like forces generated by Earth's magnetic field?


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