Why was radio contact with Pioneer lost earlier than with Voyager?

Question

Why was radio contact lost with the Pioneer probes much earlier than with the Voyager probes? The Voyagers were launched only 4 years after the Pioneers, yet the Voyagers are projected to last into the 2020s, while the Pioneers stopped transmitting in 1995 and 2003 respectively. Both were powered by RTGs as well.

Answer 1

Pioneers

Several factors lead to the loss of signal.

1. Radiated power: Pioneer 10's broadcast power is particularly low. 8 W at ~2.2 GHz ①
2. Very narrow beam; the antenna gain is +65dB, which means 1/(10^6.5) the broadcast surface area... which means a pretty narrow cone. (I don't know the math for the actual beam angle.)
3. Input power decreases - the RTGs still produce plenty of energy, but the corroding of the thermocouples results in progressively less input power to run the transmitters. (Corroded thermocouples are less efficient at converting heat to electricity.)
4. Continued drag on the spacecraft. ② This is relevant because Pioneer cannot continue to reorient indefinitely - fuel is a finite resource.
5. Budget cuts.

The combination of these factors leads to the cumulative failure.

Primary is that Pioneer is not producing intelligible data due to weak signal, so it was no longer cost effective to track Pioneer 10. Pioneer 11 shutdown was primarily due to power loss. ③

In looking at how the decision works - at the range they are at, the signal is very weak, and antenna accuracy (to keep Earth in the center of the antenna lobe) is a function of rotational control over attitude. Even so, the required dish size to get data to or from Pioneer 10 was a minimum 64m dish to have the required signal strength. As it goes further out, the minimum antennae to receive its signal increases. At present, only a handful of scopes can communicate with Pioneer 10 at all, and the data is so weak that it's unreliable.

The Pioneers used gas thrusters as their reaction control ④, which provides a fundamental limit to their pointing accuracy.

The drag of the interplanetary medium (IPM) and the solar wind are also issues. ② because the medium is not actually uniform, this results in slight differentials across the spacecraft, which, given the decades, are able to slow the spin and alter the angle. Keeping in mind that the IPM includes chunks of dust and even small pebbles, and the Pioneer velocities make a pebble impact comparable to a small caliber firearm hit, one at the right spot could wobble the spacecraft out of communication.

The exact cause of Pioneer 11 failure isn't actually known, but it's presumed to be power failure (as the telemetry showed decreasing power on a decay curve), possibly coupled with an impact event and/or short circuit. If it were knocked off axis, it might still be broadcasting but have earth outside the antenna's broadcast lobe. The knocked off axis hypothesis isn't likely, as the location is still on the predicted course.

The expectation of Pioneer 10 to continue to degrade in power consumption contributed in the public explanations of termination of the Pioneer 10 mission, equally as much as the expense of the 65m, 100m, and Arecibo antenna time, and the ability to spend that money on missions returning more data.

Voyagers

Voyager, on the other hand, has better thermocouples, younger thermocouples, more power produced, more power (18 w)⑤, and while only a 48 dB gain, it's in a better frequency noise-wise (~ 8 GHz), it's still sending useful data from instruments, and had much higher surplus power at launch.

Voyager power losses have been a shallower curve than the pioneer program; more of the initial power available is still available. Further, the original power was much higher.

The instrumentation for Voyager included several more experiments that are not of use in the extended mission; they can be turned off with no loss of data gain. This allows more of the available power to be used on the ones that are relevant.

The Voyager missions as originally conceived of also included a longer operational period - more than 10 years was expected, versus Pioneer's 2 years.

To recap:

• Voyagers 1 & 2
  • Better frequency (less noise).
  • Bigger dish & better aim.
  • More power in ship (both at start and at present)
  • More working instruments
  • working instruments on mission goals for current mission
  • not as far along the loss of power curve ⑥
  • still have significant fuel reserves so can stay pointed.
  • Designed for a longer operational period.
• Pioneer 11
  • Power failure monitored resulted in expected failure to be able to compute and communicate about the time signal was lost
  • power failure curve precluded useful instrumentation for extended mission.
• Pioneer 10
  • Instrumentation not suited for extended mission.
  • Signal strength and accuracy not conducive to the transmission of data.
  • Fuel reserves nearly depleted
  • probability of failure made continued contact not worth the money.

References
① http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1972-012A
② http://arxiv.org/pdf/1307.0537.pdf
③ http://nineplanets.org/spacecraft.html
④ http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900009039.pdf
⑤ http://www.uhf-satcom.com/misc/datasheet/dh2va.pdf
⑥ http://voyager.jpl.nasa.gov/spacecraft/spacecraftlife.html

Answer 2

Interplanetary communication is mainly dependent on signal strength (for transmission) and antenna size (for reception).

The Pioneers use a 9-foot antenna and an 8-watt transmitter. The Voyagers use a 12-foot antenna and a 20-watt transmitter, allowing a substantially stronger signal to be received on Earth.

Answer 3

In addition to a better transmitter, the Voyagers have better power reserves: their RTGs supplied 470 W at launch, while the Pioneer RTGs supplied 160 W at launch. So the Voyager RTGs will take much longer to decay to a point where they can't power the spacecraft.

NASA seems to think RTG decay is the primary reason we can't receive Pioneer 10 any more:

Pioneer 10 made valuable scientific investigations in the outer regions of our solar system until the end of its science mission on 31 March 1997. The Pioneer 10 weak signal continued to be tracked by the DSN as part of an advanced concept study of communication technology in support of NASA's future interstellar probe mission. The power source on Pioneer 10 finally degraded to the point where the signal to Earth dropped below the threshold for detection in 2003.

We're not sure about Pioneer 11's fate:

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.