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.
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:
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?
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.
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'?
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.
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?
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.
I think the measurement aspect is only part of the interest. The other is there might be previously unknown physics at work. We don't get too many chances to experiment on things in space, so surprises mean there's an understanding gap.
At one point, the motion of Mercury didn't fit into the rules of mechanics, which we thought we understood. It turns out we didn't, and general relativity filled that gap.
It's far more likely the explanation is a mundane force not considered before, than new physics. But we have to look.
It means our understanding of physics is incomplete. The answer to the anomaly could have wide ranging impact across everything we do (as pretty much everything we do is ruled by physics). Or maybe it's something very simple. We don't know until we figure it out.
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?
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.
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.
For a body that's nearly spherical like the Earth you do a multipole expansion. Most of the gravity comes from a monopole (just a point mass at the center of gravity). Then you calculate (or measure) the contribution to the gravitational field from a quadrupole moment, and then an octupole moment and so on.
https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_p...