Flyby anomaly | Hacker News Reader

Flyby anomaly

en.wikipedia.org

153 pointsmmastrac21 comments

Even more fascinating is the list of unsolved problems linked in this article:

https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_p...

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

https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_p...

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-...

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?

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...

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?

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

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.

They thought they had eliminated radiation as a possible cause of the Pioneer anomaly, but hadn't.

Double checking electromagnetic forces is an excellent idea.

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.

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.

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

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?

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... which was solved after 20 years of digging and LOTS of speculative ideas that did not work out).

The relationship of the anomaly with the various variables are still being debated

Note, this is an important anomaly not because of its magnitude, but rather, its relation to the measurement error.

The Pioneer anomaly was solved by precise calculation of the heat radiance of spacecraft.

But of course, it might end up being a trivial explanation (something on the measurement equipment, for example)

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.

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