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According to The Pioneer Missions, my emphasis,

The Pioneer 11 Mission ended on 30 September 1995, when the last transmission from the spacecraft was received. There have been no communications with Pioneer 11 since. The Earth's motion has carried it out of the view of the spacecraft antenna. The spacecraft cannot be maneuvered to point back at the Earth. It is not known whether the spacecraft is still transmitting a signal.

If I am interpreting NASA's depiction of the relative positions of distant spacecraft correctly, it appears that Pioneer 11 is roughly in the plane of the solar system ecliptic:

Image source

Given that Pioneer 11 was launched (presumably on a heliocentric orbit) directly toward Jupiter, and assuming for a moment that we didn't deliberately tell it to aim its antenna away from Earth, I don't understand why the Earth's motion would have carried it out of the view of the spacecraft's antenna. Even if the Earth's revolution around the Sun would periodically degrade the signal received, possibly to the point of providing insufficient signal-to-noise ratio to decode the signal, it would seem like the signal strength should peak once a year, with an additional lower peak once a year.

Hobbes' claim of a 0.7 AU beamwidth at Earth even for Voyager (based on -3 dB received power and a 0.32° beam angle (from center to half power)), and wider for Pioneer, would seem to be plenty wide enough that precise aiming would not be required; being in the general ballpark should be enough to be able to establish communications with the spacecraft at least once a year.

We do of course have good reason to believe that the transmitter is no longer working (also here), but that is not what NASA claims in the quote at the top of this question.

What is the basis for NASA's statement that the Earth's motion has carried Earth out of the view of Pioneer 11's antenna?

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  • $\begingroup$ The Pioneer spacecraft use hydrazine thrusters for attitude control; I can't find a definitive answer, but I would guess they ran out of propellant, so could no longer keep the antenna pointed. $\endgroup$ – Russell Borogove Mar 11 '16 at 22:33
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    $\begingroup$ @RussellBorogove Loss of attitude control would have been my guess too, had not the NASA statement been seemingly highly specific in that it's the Earth's motion that caused this, as opposed to loss of spacecraft attitude control. If your guess was correct, would it not have been easier to just state that the spacecraft ran out of fuel and thus can't maintain aim towards Earth? (No need to get into all of the gory details in a summary meant for the general public, but it could certainly give enough detail to be clear about what the issue is.) $\endgroup$ – a CVn Mar 11 '16 at 22:36
  • $\begingroup$ Yeah. Also, this preso suggests that Pioneer 11 still had significant pressure in propellant tanks up through 1996. It also mentions that the sun direction sensor doesn't work beyond 30 AU out, so maybe that's the issue. vttoth.com/DOCUMENTS/TELANAL.pdf $\endgroup$ – Russell Borogove Mar 11 '16 at 22:44
  • $\begingroup$ I think it's best to call it a 9 year old archived article or popular mission summary webpage, and not call it a "NASA statement" which makes it sound official. This NASA page on Lagrange points should point to Journey to the Far Side of the Sun (1969) and not point to The Man from Planet X (1951) but I wouldn't call that a "NASA statement" either. It's possible that it should say "Pioneer's motion" or "its motion" "has carried earth out of..." but then you have to say "apparent angular motion" and the writer may have been under a word count li $\endgroup$ – uhoh Mar 12 '16 at 2:48
  • $\begingroup$ My beamwidth comment appeared in this question originally: space.stackexchange.com/questions/14284/… . I haven't found a beam angle for the Pioneer antennas yet. $\endgroup$ – Hobbes Mar 12 '16 at 8:41
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Looking at the trajectories of the Pioneers, the wording "The Earth's motion has carried it out of the view of the spacecraft antenna" seems to be a poor choice of words.

enter image description here

you can see that in the long term, Pioneer has to rotate to keep Earth in view. Once it's out of fuel, it can't do this maneuver any more. For Pioneer 11's last transmission, it's possible the antenna beam just grazed one segment of Earth's orbit. As Earth continued on its orbit, it'd move out of the antenna's field of view for most of the year. The following year, Pioneer's changed position has moved the antenna's field of view away from Earth's orbit entirely.

The Pioneers' high gain antenna has a beam width of 3° (gain at the edges is -10 dB relative to the center).

For Voyager:

The transmit boresight gain of the HGA was 36 dB at S-band and 47 dB at X-band. The half-power half-width of the antenna beam was 0.32 degrees at X-band and 1.1 degrees at S-band.

A halving in power translates to a 3 dB loss.

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