Space Probes Too Slow - Scientists Ask "Why?"

Rudolf writes "Newsweek has an article this week, available here, about NASA calculating that space probes, such as Pioneer 10, 11, and Ulysses, are slowing down more than they should. A team of astronomers and physicists couldn't figure it out, so they published their findings in Physical Review Letters to generate discussion. Several possible causes of the slowing have been discussed, but nothing that completely solves the puzzle. Anyone care to rethink gravity and time?" Update: 09/29 09:00 by H :Thanks to Mark for his link to the original citation.

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Article and comments from the web

[kaip]

For those interested, the original articles and articles that cite (and comment) it can be found from the web: gr-qc/9808081

Warped space maybe?

[BrianH]

Physics majors are welcome to correct me here (I'm a bit outside of my field), but I seem to remember reading an article a while back discussing the possibility that gravity warps time/space (Scientific American or Popular Science...I don't remember which). The theory held that large gravitational wells, and especially large rotating gravitational wells, literally pull at the fabric of time/space and stretch it out near the center of the affected area. To prove/disprove the theory, the scientists involved were going to put a satellite into orbit and set a high mass object into a rapid spin. If the theory was correct, spacetime distortion would be detectable around the outer edges of the rotating test object.

Could this be what we're seeing, only on a much larger scale? Perhaps the Sun, with its massive gravity well, has caused time/space to stretch within our solar system, and what we're seeing here is the effect of the probes re-entering "normal" space. From our perspective within the "stretched" area, it would appear that the craft was slowing down.

I would love it if someone could provide more info on this theory, and fill me in on whether or not it could possibly apply here.

I thought this was solved (links)

[teraflop user]

I first saw this story in New Scientist a year ago. I think it was reported in Slashdot at the time.

A month later New Scientist published this story, suggesting that the slowing was due to the reaction from heat radiated from the probes RTG power plant.

They still appear to be arguing over whether this effect is big enough. Measurements involving heat are notoriously difficult, as the cold fusion debacle showed.

'Extra' gravity, dark matter?

[Kartoffel]

One of the discrepancies that drives the dark matter theories is that the universe appears to contain much more mass than we can see. Dark matter is just a theoretical substance that has not been proven to exist; many people would say that it's a fake thing made up to explain weird phenomena (like the ether theory 100 years ago).

Now we've got additional symptoms of the universe being more massive than it ought to be. The probes are slowing down faster than expected, as if there was 'dark matter' collected near the sun.

From these *observations* I propose 2 hypotheses---

1. If dark matter is real, it must be concentrated more densely near the sun and less densely farther away. Otherwise the distribution of dark matter would not slow the probes. This makes sense because dark matter, being massive and subject to normal physical laws, would tend to collect near stars and other massive objects. In any case, why heck can't we see it?

2. Like 'ether', dark matter is not real at all. There is an unkown phenomena manifesting itself here. If so, the unknown force(s) could very well be the same ones that caused the observations that led people to propose dark matter in the first place.

How would _you_ solve the Dirac Equation?

Re:'Extra' gravity, dark matter?

[rde]

1. If dark matter is real, it must be concentrated more densely near the sun and less densely farther away. Otherwise the distribution of dark matter would not slow the probes.
I don't think so. If the dark matter were concentrated near the sun, we'd automatically include it in our calculations; most likely in Newton's gravitational constant.
General relativity answered the problems Newton's theory had with Mercury's orbit, and does a damn fine job with all predictions gravitational around the sun. If this is the case, dark matter has to exert a force other than gravity. And if that's the case, the Pioneers' acceleration away from the sun should increase as it moves out of the dark matter's influence.
Of course, I could be talking bollocks.

We've known this forever

[Anonymous Coward]

The universe simply doesn't exist
outside the borders of our solar system.

The probes are slowing down because
they are literally hitting the "wall";

Our solar system system was created
by a diety that didn't want to go
through all the fuss that comes with
creating a complete universe.

Breaking through the borders of our
solar system will let us reach the
realm of the gods.

SEND ME ALL YOUR MONEY AND GET
A SEAT ON MY SPACESHIP TO JESUS!