
Flyby anomaly - mmastrac
https://en.wikipedia.org/wiki/Flyby_anomaly
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Mahn
Even more fascinating is the list of unsolved problems linked in this article:

[https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_p...](https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_physics)

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ck2
Amazing list. Surprisingly short list at the end of stuff people solved:

[https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_p...](https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_physics#Problems_solved_in_recent_decades)

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pierrec
The article states that a dark matter halo around the earth is considered a
plausible explanation for the anomaly. Combine this with the possible "hairy"
configuration of dark matter around the earth [1], and it fits in rather well
with the inconsistent way in which the anomaly manifests itself: probes are
affected differently depending on how close they fly by the root of a dark
matter hair.

[1]: [http://www.nasa.gov/feature/jpl/earth-might-have-hairy-
dark-...](http://www.nasa.gov/feature/jpl/earth-might-have-hairy-dark-matter)

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sudhirj
Wouldn't the shape of the oceans (tides), or the elevation of the part of the
earth passed over be enough to cause this amount of change?

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jandrewrogers
No, there are already satellites and other sensors that continuously measure
slight anomalies and variations in the gravity field. They can account for
that. The best known are the GRACE satellites:

[https://en.wikipedia.org/wiki/Gravity_Recovery_and_Climate_E...](https://en.wikipedia.org/wiki/Gravity_Recovery_and_Climate_Experiment)

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tbabb
I take it the influence of Earth's magnetic field is accounted for, including
adjustments for relativity and any induced eddy currents in the spacecraft?

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dingo_bat
If agencies like NASA and ESA consider this an anomaly, they must have
eliminated obvious stuff like eddy currents and relativity.

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Udo
Not really. The problem is there are many unknown factors. Consulting the list
of likely explanation candidates, I think so far they haven't even ruled out
whether it's a signal artifact as opposed to an actual speedup.

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xupybd
Wow, that seems like a pretty major anomaly. I'd imagine there are more than a
few frustrated scientists at NASA trying to figure that one out.

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jacquesm
Please forgive the naivety of my question, if these space craft move at 4 to
16 km/second why is a few mm/second discrepancy (6 orders of magnitude
difference) a 'major anomaly'?

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IanCal
Perfectly sensible question. I think the important thing here is to consider
the size of the anomaly not relative to the speed these craft are travelling
but the precision that we can predict & measure their expected speed.

So we're left in a position where there's a difference between the prediction
and the measurement, when the measurement is highly accurate. That means
something is out, and that might be extremely important for long range
flights.

I'm not too sure how to work some of these things out, but it's possible that
a few mm/second difference really adds up when you're on an enormous trip. If
you're out by 10mm/second here, then your next flyby will leave you even
further out, then the next, etc. until you crash into a planet. Cassini was
out by under 1 mm/s, but its flight path would have been 2 billion miles.
These things travel for years, and can't make too many corrections.

More info on the crazy journey it took:
[http://www.theplanetstoday.com/cassini_flight_path.html](http://www.theplanetstoday.com/cassini_flight_path.html)

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jacquesm
Thank you for the answer.

Another one of those questions: doesn't the acceleration of such a fly-by
depend in large amount on the masses of the various objects, and if that's
correct could this indicate an error in the mass of one of the objects (say,
Cassini having a mass (not a weight) 1/1000000 less than we think it does)?
And also, could it be indicative of a number of factors all being off by a
fraction of the resulting discrepancy, or even two factors where one is a
negative and the other is a slightly different positive?

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musgravepeter
The mass of the "test particle" Cassini will cancel out of orbit equation but
will matter for drag, radiation pressure etc. If it's outside the error bars
they gave for mass, then this is conceivable.

OTOH this is a pretty conservative bunch.

It may "just" be something in the details (much like the Pioneer anomoly:
[http://www.planetary.org/blogs/bruce-
betts/3459.html?referre...](http://www.planetary.org/blogs/bruce-
betts/3459.html?referrer=https://www.google.ca/) which was solved after 20
years of digging and LOTS of speculative ideas that did not work out).

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bitL
Somehow related question that was puzzling me since forever - how do you
calculate cumulative gravity (including expected speed of light limit on the
spread of gravitational waves) at a given point around a planet, i.e. not
around a single point with a mass, but an irregular large body?

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tremguy
Didn't completely understand what you mean with the gravitational waves and
speed of light but for calculating gravitational force in a point you
basically need to have a function describing the form of the object(and the
density if the object doesn't have uniform density) and then do a
(weighted)integral regarding the force excerted by each infinitesimal point in
the object.

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bitL
I meant your satellite and planet are moving, hence gravity is a bit behind
your actual location due to a speed limit on gravitational waves propagation,
i.e. each part of the planet is contributing gravitational force at different
times from the past. Something like when you make a photo of a nebula roughly
700ly away and the top of nebula is 698ly away from you whereas the bottom is
705ly - what you get in the picture are pixels from different times, not a
single time snapshot of the nebula.

