2.3 Spacecraft operating history

The Pioneer 10 and 11 spacecraft spent about three decades in deep space. Most of what we know about the anomalous acceleration is based on data that was collected when the spacecraft were well into their second and third decades of operation. This raises an obvious question: what are the effects of aging and to what extent may aging be responsible for the anomalous acceleration?

In this section, we summarize our knowledge about the effects of aging on the spacecrafts’ subsystems.

2.3.1 Spacecraft physical configuration

The overall shape of the Pioneer 10 and 11 spacecraft is not expected to change significantly with age. The spacecraft is fundamentally a rigid body; other than the constant centrifugal force that arises as a result of the spacecraft’s rotation, there are no forces stretching, bending, or otherwise acting on the spacecraft structurally.

The Pioneer spacecraft had few moving parts. After initial boom deployment, the spacecrafts’ physical configurations remained largely unchanging, with a few notable exceptions.

Consumption of the fuel load on board resulted in small changes in the spacecrafts’ mass distribution during the large course correction maneuvers early in the Pioneer 10 and 11 missions. As the amount of fuel on board was small, and the fuel tank was situated near the spacecraft’s center-of-gravity, the effects of later attitude correction maneuvers, which consumed minuscule amounts of fuel, were likely negligible.

Some instruments had moving parts: notably, the Imaging Photopolarimeter (IPP) instrument had a telescope that was mounted on a scan platform, allowing it to be used for Jupiter imaging. Operating this instrument’s small moving parts, however, would have introduced only minute changes in the spacecraft’s mass distribution and thermal properties.

More notable was the spacecraft’s passive louver system. As discussed in Section 2.4, the louver system was designed to vent excess heat radiatively from the interior of the spacecraft. The state of the louver system can be determined as a function of the electronics platform temperatures. The position of the louver blades can significantly alter the thermal behavior of the spacecraft, by allowing a higher proportion of interior heat to escape through the louver system.

At large heliocentric distances (beyond ∼ 25 AU), the louver system is always closed, and the spacecraft’s physical configuration remains constant.

2.3.2 Changes in spacecraft mass

As we discussed in Section 2.2.1, the nominal launch mass of the Pioneer 10 and 11 spacecraft was ∼ 260 kg, of which ∼ 30 kg was propellant and pressurant.5 The spacecraft mass slowly decreased, primarily as a result of propellant usage. Additionally, small mass losses may occur due to fuel leaks, pressurant outgassing, He outgassing (α particles) from the RTGs and RHUs, and possibly, outgassing from the spacecraft batteries.

Pioneer 10 used only a moderate amount of propellant, as it performed no major trajectory correction maneuvers. If the propellant used amounted to one quarter of the propellant on board, this means that Pioneer 10’s mass would have decreased to ∼ 250 kg late in its mission (the 2002 JPL study used the nominal value of 251.883 kg). Further, it should be noted that most propellant usage occurred prior to Jupiter encounter; afterwards, Pioneer 10 only used minimal amounts of propellant for precession maneuvers, needed to keep its antenna aimed at the Earth.

Pioneer 11 performed major trajectory correction maneuvers en route to Jupiter and Saturn. The maneuvers were in order to allow it to follow a precisely calculated orbit that utilized a gravitational assist from Jupiter that was needed to set up the spacecraft for its encounter with Saturn. As a result, Pioneer 11 is believed to have used significantly more propellant than its twin, perhaps three quarters of the total available on board. The spacecraft’s mass, therefore, may have decreased to ∼ 232 kg following its encounter with Saturn (the mass used in the 2002 JPL study was somewhat higher, 239.73 kg [27Jump To The Next Citation Point]).

We note that these figures are crude estimates, as the actual amount of propellant on board is not telemetered. Sensors inside the propellant tank did offer temperature and pressure telemetry, but these sensors were not sufficiently reliable for a precise estimate of the remaining fuel on board.

The spacecraft can also lose mass due to outgassing. Two possible sources of outgassing are helium outgassing from the radioactive fuel on board in the radioisotope thermoelectric generators and radioisotope heater units, and chemical outgassing from the spacecraft’s battery. An upper limit of 18 g on helium outgassing can be established using the known physical properties of the 238Pu fuel (see Section 4), which is not significant. Similarly, the amount of gas that can escape from the spacecraft’s batteries is small, especially in view of the fact that the batteries performed nominally far longer than anticipated: under no circumstances can it exceed the battery mass, but in all likelihood, and especially in view of the fact that the batteries performed nominally throughout the mission, any outgassing is necessarily limited to a very small fraction of the ∼ 2.35 kg battery mass (see Section 4). Therefore, outgassing cannot have played a major role in the evolution of the spacecraft mass; furthermore, any mass loss due to outgassing is dwarfed by uncertainties in the mass of the remaining fuel inventory.

Could the spacecraft have gained mass, for instance, by collecting dust particles from the interplanetary medium? In situ measurements by the Pioneer spacecraft themselves provide an upper limit on the amount of dust encountered by the spacecraft. After passing Jupiter’s orbit, the dust flux measured by Pioneer 10 remained approximately constant, at 3 × 10−6 m−2s−1 particles [178]. The upper limit on particle sizes is 10−4 kg. Even assuming that all particles had masses near this upper limit, the total amount of mass that could have accumulated on the spacecraft over the course of 20 years would be no more than ∼ 1 kg. However, given that the spacecraft is moving through interplanetary space at a velocity of 10 km/s or higher, these assumptions on particle size would imply a dust density of ∼ 10−14 kg/m3, which is many orders of magnitude higher than more realistic estimates (see Section 4.3.5). Therefore, the actual dust mass accumulated on the spacecraft cannot be more than a few grams at the most; consequently, this mechanism for mass increase can also be safely ignored.

2.3.3 Instrumentation

It is not known how on-board instrumentation (i.e., telemetry sensors) respond to aging. What we know is that many sensors stopped providing usable readings when measured values (e.g., temperatures) dropped outside calibrated ranges [378379395397Jump To The Next Citation Point]. Other sensors continued to provide consistent readings, with no indication of sensor failure.

However, there were a few sensor anomalies that may be due to age-related sensor defects. Most notable among these are the anomalous readings from the propellant tank of Pioneer 10 (described in Section 2.3.6).

It is also unknown how on-board instrumentation responds when their supply voltage drops below the nominal level. Late in its mission, the electrical power subsystem on board Pioneer 10 no longer had sufficient power to maintain the nominal main bus voltage of 28 VDC. As this coincides with changes in physical sensor readings (e.g., drops in temperature), the extent to which those readings are affected by the drop in voltage is not readily evident.

2.3.4 Electrical system

Insofar as we can determine from telemetry, the electrical subsystems on board Pioneer 10 and 11 performed nominally throughout the missions, so long as sufficient electrical power was available from the RTGs. The on-board chemical batteries remained functional for many years; eventually, due to irreversible chemical changes and decreasing temperatures, the batteries ceased functioning.

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Figure 2.14: Changes in total RTG electrical output (in W) on board Pioneer 10 (left) and 11 (right), as computed using the missions’ on-board telemetry.

2.3.5 Radioisotope thermoelectrical generators

The effects of aging on the RTGs are complex. Internally, aging causes a degradation of the bimetallic thermocouples, contributing to their loss of efficiency and the decrease in RTG electrical power output. Externally, it has been conjectured [327] that the RTG exterior surfaces may have aged due to solar bleaching and impact by dust particles. The extent to which such degradation may have occurred (if at all) is unknown. The resulting fore-aft asymmetry may be a significant source of unaccounted-for acceleration in the approximately sunward direction.

2.3.6 Propulsion system

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Figure 2.15: Propulsion tank pressure (in pounds per square inch absolute; 1 psia = 6.895 kPa) on board Pioneer 10. The three intervals studied in [27Jump To The Next Citation Point] are marked by roman numerals and separated by vertical lines.

The propellant pressure sensor on board Pioneer 10 began to show anomalous behavior in June 1989. Propellant pressure, which remained steady up to this point, only decreasing slowly as a result of cooling and occasional propellant usage, suddenly began to show a sharp decay, dropping from over 300 psia down to about 150 psia by January 1992 (Figure 2.15View Image). At this time, the propellant pressure instantaneously increased to its pre-1989 value of ∼ 310 psia, after which it began dropping again. There were no corresponding changes or other anomalies in the observed propellant temperature and expellant temperature. Therefore, the most likely explanation for this anomaly is a sensor malfunction, not a real loss of fuel or propellant.

On December 18, 1975, subsequent to an attempted maneuver, and as a result of a stuck thruster valve, the spin of Pioneer 11 increased dramatically, from a spin rate of ∼ 5.5 revolutions per minute (rpm) to ∼ 7.7 rpm (Figure 2.16View Image, right panel). Fortunately, the thruster ceased firing before the spin rate increased to a value that would have threatened the spacecraft’s structural integrity or compromised its ability to carry out its mission.

2.3.7 Attitude control and spin

Encounter with the intense radiation environment in the vicinity of Jupiter damaged the star sensor on board Pioneer 10. As the sun sensors operate only up to a distance of ∼ 30 AU, Pioneer 10 had operated without a primary roll reference for many years before the end of its mission. (The rate of Pioneer 10’s spin was determined from measurements taken by its Imaging Photo-Polarimeter (IPP) instrument and other methods [349Jump To The Next Citation Point].)

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Figure 2.16: On-board spin rate measurements (in rpm) for Pioneer 10 (left) and Pioneer 11 (right). The sun sensor used on Pioneer 10 for spin determination was temporarily disabled between November 1983 and July 1985, and was turned off in May 1986, resulting in a ‘frozen’ value being telemetered that no longer reflected the actual spin rate of the spacecraft. Continuing spot measurements of the spin rate were made using the Imaging Photo-Polarimeter (IPP) until 1993. The anomalous increase in Pioneer 11’s spin rate early in the mission was due to a failed spin thruster. Continuing increases in the spin rate were due to maneuvers; when the spacecraft was undisturbed, its spin rate slowly decreased, as seen in Figure 2.17View Image.
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Figure 2.17: Zoomed plots of the spin rate of Pioneer 11. On the left, the interval examined in [27Jump To The Next Citation Point] is shown; maneuvers are clearly visible, resulting in discrete jumps in the spin rate. The figure on the right focuses on the first half of 1987; the decrease in the spin rate when the spacecraft was undisturbed is clearly evident.

The nominal spin rate of the Pioneer 10 and 11 spacecraft was 4.8 rpm. This spin rate was achieved by reducing the spacecraft’s initial rate of spin, provided by the launch vehicle, in successive stages, first by firing spin/despin thrusters, and then by extending the RTG and magnetometer booms. Later, spin could be precisely adjusted and corrected by the spin/despin thrusters.

On Pioneer 11, due to the spin thruster anomaly described in the previous section, the spacecraft’s spin remained at an abnormally high value. In fact, it further increased, presumably as a result of fuel leaks, all the way up to ∼ 8.4 rpm at the time of the last available telemetry data point, on February 11, 1994.

Meanwhile, Pioneer 10’s spin slowly decreased over time, probably due to a combination of effects that may include fuel leaks as well as the thermal recoil force and associated change in angular momentum.

The spin rates of Pioneer 10 and 11 are shown in Figure 2.16View Image. Spin was measured on board using one of several sensors, namely a star sensor and two sun sensors. The purpose of these sensors was to provide a roll reference pulse that could then be used to synchronize other equipment, including the IPP instrument and the navigational system.

The sun sensors required a minimum angle between the spacecraft’s spin axis and the spacecraft-Sun line. Further, they required that the spacecraft be within a certain distance of the Sun, in order for a reliable roll reference pulse to be generated. For these reasons, the sun sensors could not be used to provide a roll reference pulse once the spacecraft were more than ∼ 30 AU from the Sun.

There were no such limitations on the star sensor; however, the star sensor on board Pioneer 10 ceased functioning shortly after Jupiter encounter, probably due to radiation damage suffered while the spacecraft traversed the intense radiation environment in the gas giant’s vicinity.

As a result, Pioneer 10 lost its roll reference source when its distance from the Sun increased beyond ∼ 30 AU. Although the roll reference assembly continued to provide roll reference pulses at the last “frozen in” rate, this rate no longer matched the actual rate of revolution of the spacecraft.

Nevertheless, it was important to know the spin rate of the spacecraft with reasonable precision, in order to be able to carry out precession maneuvers reliably, and also because the spacecraft’s spin affected the spacecraft’s radio signal and the Doppler observable. For this reason, the IPP was reused as a surrogate star sensor, its images of the star field providing a reference that could then be used by the navigation team to compute the actual spin rate of the spacecraft on the Earth [349]. It was during the time when the IPP instrument was used to compute the rate of spin that a spin anomaly was detected. The spin-down rate of Pioneer 10 suddenly grew in 1990, and then eventually returned to its approximate previous value (Figure 2.16View Image, left panel.)

Very late in Pioneer 10’s mission, when the power on board was no longer sufficient to operate the IPP instrument, crude estimates of the spin rate were made using navigational data.

In contrast to the spin behavior of Pioneer 10, the spin rate of Pioneer 11 continued to increase after the initial jump following the thruster anomaly. However, a close look at detailed plots of the spin rate reveal a more intricate picture. It seems that Pioneer 11’s spin rate was actually decreasing between maneuvers, when the spacecraft was undisturbed; however, each precession maneuver increased the spacecraft’s spin rate by a notable amount. The rate of decrease between successive maneuvers was not constant, suggesting that fuel leaks played a more significant role in Pioneer 11’s spin behavior than in Pioneer 10’s.

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