# The Pioneer Anomaly

1. Jul 9, 2009

### Bjarne

Several space probes was by gravity assist manoeuvre gaining between 4 and 13 mm velocity per second. Are this values average based on acceleration through time, or are these pure deceleration per second?

2. Jul 9, 2009

### sylas

I'm not quite sure what distinction you are making there.

The Pioneer anomaly is based on the measured velocity of the spacecraft as they are coasting, and the velocity is measured by the Doppler shift of communications.

During the close flyby of other planets, its not so easy to measure the changing velocity, so it has nothing to do with the gravity assist maneuvers that were used to boost velocity relative to the Sun as it passed by those planets. Also, too close to the Sun the force given by solar radiation is sufficiently strong to make make isolating the anomaly difficult.

However, during the long stages coasting in empty space, we should expect to see Doppler signals reflecting the motion of the telescopes on Earth, including both rotation and revolution around the Sun, and also the slowing of the spacecraft as it moves out from the Sun.

Careful analysis of the signal, after removing all the effects of Earth's own motions and the expected deceleration from the Sun, show that the space probe is moving very slightly slower than expected, as if there is a tiny additional acceleration back towards the Sun. The magnitude of this apparent additional acceleration on Pioneer 10 was around about 8*10-10 ms-2. This is tiny, but it is definitely there in the data.

To give an idea of just how small this additional acceleration is, one possible explanation is an imbalance of the heat being radiated from the spacecraft. 60 watts additional energy radiated out away from the Sun would be sufficient. Since the craft radiates about 2000W in total, a comparatively small directional bias in the radiation might work. So too would a slow gas leak. Either one is a possible cause of the excess. However, it has not been possible to identify such an extra impulse, and so other alternatives are also considered, including changes in fundamental physics. It remains a mystery.

Cheers -- sylas

3. Jul 9, 2009

### Nabeshin

I think you're referring to this anomaly, rather than the pioneer anomaly (related? maybe.)

http://en.wikipedia.org/wiki/Flyby_anomaly

4. Jul 9, 2009

### Bjarne

Yes that's right, flyby anomaly...
For instance Near experienced an anomalous velocity increase of 13.46 mm/s after its Earth encounter.

It's a pretty strong acceleration, million times stronger than the force decelerating Piooner 10 and 11.

I was just wondering why the big difference.

5. Jul 9, 2009

### sylas

Ah! Yes, I've heard of this anomaly also, but have not looked into it as much. I think the short answer is that no-one knows for sure, at present.

Cheers -- sylas

6. Jul 9, 2009

### D H

Staff Emeritus

7. Sep 3, 2009

### Widdekind

For the Pioneer Anomaly, what about Ram Pressure from diffuse dust & gas ? According to Wikipedia, Pioneer-10 is speeding along at around 12.5 km s-1, masses 258 kg, and looks like its cross section is roughly 4 m2. This, attributing the deceleration to Ram Pressure merely implies a diffuse dust density of:
$$\rho = \frac{m \; a}{A \; v^{2}} \approx 3 \times 10^{-16} \; kg \; m^{-3}$$​
which — if the diffuse density is exclusively gas — is equivalent to about $$n \approx 2 \times 10^{11} \; m^{-3}$$, or $$n \approx 2 \times 10^{5} \; cm^{-3}$$. Are such densities completely implausible ?

Could this have something to do w/ the "Termination Shock" of the Heliosphere ? Compare:

Last edited: Sep 3, 2009
8. Sep 3, 2009

### ideasrule

According to Wikipedia' "Interplanetary medium" article, the interplanetary medium's density is 5 particles per cm^3 near Earth's orbit and decreases with increasing distance from the Sun. So yes, 2*10^5 per cm^3 is implausible.

The only spacecraft that has passed the termination shock is Voyager 1, with Voyager 2 soon to follow. Pioneers 1 and 2, which have long become non-functional, are far, far behind.

9. Sep 4, 2009

### Widdekind

Thanks for the link — but what is mass density of dust, from the Interplanetary Dust Cloud ?? All that matters is the mass density — a few comparatively gargantuan dust grains could do the work of billions of plasma particles.

According to Interplanetary Dust, by Eberhard Grün, pp. 325-327, IPD is surprisingly dense, even far from the Sun:

http://img215.imageshack.us/img215/6128/ipdpg325.th.tif [Broken]
http://img215.imageshack.us/img215/1189/ipdpg327.th.tif [Broken]

Last edited by a moderator: May 4, 2017
10. Sep 4, 2009

### Widdekind

According to the same cited source (IPD), pg. 331, Interstellar Dust is "raining" (my word) through our Solar System, from Ecliptic Longitude 73 degrees, Ecliptic Latitude -5 degree:
According to this calculator, those coordinates correspond to Galactic Longitude 182.57318855, Galactic Latitude: -16.63240332.

180 degrees Galactic Longitude points away from the Galactic Core, so the ISD stream stems from "Rimward" (vs. "Coreward") of our Solar System.

Perhaps the "hail" of ISD could be slowing the spaceprobes ??

Last edited by a moderator: May 4, 2017