2.1 The Pioneer 10 and 11 missions

The Pioneer missions were the first to cross the asteroid belt, perform in situ observations of the interplanetary medium in the outer solar system, and close-up observations of the gas giant Jupiter. Their mission design was characterized by simplicity: a powerful launch vehicle placed the spacecraft on an hyperbolic trajectory aimed directly at Jupiter, which the spacecraft were expected to reach approximately 21 months after launch.

At the distance of Jupiter, operating a spacecraft using solar panels is no longer practical (certainly not at the level of technology that was available to the designers of the Pioneer missions in the late 1960s.) For this reason, nuclear power was chosen as the means to provide electrical power to the spacecraft, in the form of 238Pu powered radioisotope thermoelectric generators (RTGs). As even this was relatively new technology at the time the missions were designed, the power subsystem was suitably over-engineered, the design requirement being a completely functional spacecraft capable of performing all planned science observations with only three (out of four) RTGs operating.

Such conservative engineering characterized the entire design of these spacecraft and their missions, and it was likely responsible for the two spacecrafts’ exceptional longevity, and their ability to deliver science results that far exceeded the expectations of their designers. The original plan envisioned a primary mission of 600 – 900 days in duration. Nevertheless, following its encounter with Jupiter, Pioneer 10 remained functional for over 30 years; meanwhile Pioneer 11, though not as long lived as its sister craft, successfully navigated a path across the solar system for another encounter with Saturn, offering the first close-up observations of the ringed planet.

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Figure 2.1: Trajectories of Pioneer 10 and 11 during their primary missions in the solar system (from [126Jump To The Next Citation Point]). The time ticks shown along the trajectories and planetary orbits represent the distance traveled during each year.
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Figure 2.2: Ecliptic pole view of the Pioneer 10 and Pioneer 11 trajectories during major parts of their extended missions. Pioneer 10 is traveling in a direction almost opposite to the galactic center, while Pioneer 11 is heading approximately in the shortest direction to the heliopause. The direction of the solar system’s motion in the galaxy is approximately towards the top. (From [27Jump To The Next Citation Point].)

After the Jupiter and Saturn (for Pioneer 11) encounters (see Figure 2.1View Image), the craft followed escape hyperbolic orbits near the plane of the ecliptic on opposite sides of the solar system, continuing their extended missions [126]. (See Figure 2.2View Image.) The spacecraft explored the outer regions of the solar system, studying energetic particles from the Sun (solar wind), and cosmic rays entering our portion of the Milky Way. Major milestones of the two Pioneer projects are shown in Table 2.1.

Table 2.1: Major milestones of the Pioneer 10 and 11 projects.
Event Pioneer 10 Pioneer 11
Launch March 3, 1972 March 6, 1973
Jupiter encounter December 3, 1973 December 4, 1974
Saturn encounter N/A September 1, 1979
Last telemetry received April 27, 2002 September 30, 1995

The Pioneers were excellent vehicles for the purposes of precision celestial mechanics experiments [24Jump To The Next Citation Point27Jump To The Next Citation Point32194255256257259260262263274390Jump To The Next Citation Point391Jump To The Next Citation Point392Jump To The Next Citation Point393Jump To The Next Citation Point]. This was due to a combination of many factors, including the presence of a coherent mode transceiver on board, their attitude control (spin-stabilized, with a minimum number of attitude correction maneuvers using thrusters), power design (the RTGs being on extended booms aided the stability of craft and also reduced thermal effects on the craft; see Figure 2.3View Image), and precise Doppler tracking (with the accuracy of post-fit Doppler residuals at the level of mHz). The exceptional “built-in” acceleration sensitivity of the Pioneer 10 and 11 spacecraft naturally allowed them to reach a level of accuracy of ∼ 10–10 m/s2. The result was one of the most precise spacecraft navigations in deep space to date [269].

2.1.1 Pioneer 10 mission details

Pioneer 10 was launched on 2 March 1972 (3 March 1972 at 01:49 Universal Coordinated Time) from Cape Canaveral on top of an Atlas/Centaur/TE364-4 launch vehicle [190]. The launch marked the first use of the Atlas-Centaur as a three-stage launch vehicle.

The launch vehicle configuration was an Atlas launch vehicle equipped with a Centaur D upper stage and a TE364-4 solid-fuel third stage that provided additional thrust and also supplied the initial spin of the spacecraft. The third stage was required to accelerate Pioneer 10 to the speed of 14.39 km/s, needed for the flight to Jupiter.

After a powered flight of approximately 14 minutes, the spacecraft was separated from its launch vehicle; its initial spin ∼ 60 revolutions per minute (rpm) was reduced by thrusters, and then reduced further when the magnetometer and RTG booms were extended [283145]. The spacecraft was then oriented to ensure that its high-gain antenna pointed towards the Earth. Thus, the initial cruise phase from the Earth to Jupiter began.

The first interplanetary cruise phase of Pioneer 10 took approximately 21 months. During this time, Pioneer 10 successfully crossed the asteroid belt, demonstrating for the first time that this region of the solar system is safe for spacecraft to travel through.

Pioneer 10 arrived at Jupiter in late November, 1973 [162]. Its closest approach to the red giant occurred on 4 December 1973, at 02:25 UTC. It performed the first ever close-up observations of the gas giant, before continuing its journey out of the solar system on an hyperbolic escape trajectory (Figure 2.2View Image). During the planetary encounter, Pioneer 10 took several photographs of the planet and its moons, measured Jupiter’s magnetic fields, and observed the planet’s radiation belts. Radiation in the Jovian environment, potentially damaging to the spacecraft’s electronics, was a concern to the mission designers. However, Pioneer 10 survived the planetary encounter without significant damage, although its star sensor became inoperative shortly afterwards [349Jump To The Next Citation Point], a likely result of excessive radiation exposure near Jupiter.

The encounter with Jupiter changed Pioneer 10’s trajectory as was planned by JPL navigators [404]. As a result, Pioneer 10 was now on an hyperbolic escape trajectory that took it to ever more distant parts of the solar system. Originally, signal loss was expected before Pioneer 10 reached twice the heliocentric distance of Jupiter (the downlink telecommunication power margin was 6 dB at the time of Jupiter encounter); however, continuing upgrades to the facilities of the Deep Space Network (DSN) permitted tracking of Pioneer 10 until the official termination of Pioneer 10’s science mission in 1997 and even beyond.

Pioneer 10 continued to make valuable scientific investigations until its science mission ended on March 31, 1997. After this date, Pioneer 10’s weak signal was tracked by the NASA’s DSN as part of an advanced concept study of communication technology in support of NASA’s future interstellar probe mission. Pioneer 10 eventually became the first man-made object to leave the solar system.

During one of the last attempts to contact Pioneer 10, in April 2001, at first no signal was detected; however, the spacecraft’s signal did appear once it detected a signal from the Earth and its radio system switched to coherent mode. From this, it was concluded that the on-board transmitter frequency reference (temperature controlled crystal oscillator) failed, possibly due to the combined effects of aging, the extreme cold environment of deep space, and a drop in the main bus voltage due to the depletion of the spacecraft’s RTG power source. This failure had no impact on the ability to obtain precision Doppler measurements from Pioneer 10.

On March 2, 2002 NASA’s DSN made another contact with Pioneer 10 and confirmed that the spacecraft was still operational thirty years after its launch on March 3, 1972 (UT). The uplink signal was transmitted on March 1 from the DSN’s Goldstone, California facility and a downlink response was received twenty-two hours later by the 70-meter antenna at Madrid, Spain. At this time the spacecraft was 11.9 billion kilometers from Earth at about 79.9 AU from the Sun and heading outward into interstellar space in the general direction of Aldebaran at a distance of about 68 light years from the Earth, and a travel time of two million years. The last telemetry data point was obtained from Pioneer 10 on 27 April 2002 when the craft was 80 AU from the Sun.

The last signal from Pioneer 10 was received on Earth on 23 January 2003, when NASA’s DSN received a very weak signal from the venerable spacecraft from the distance of ∼ 82.1 AU from the Sun. The previous three contacts had very faint signals with no telemetry received. At that time, NASA engineers reported that Pioneer 10’s RTG has decayed to the point where it may not have enough power to send additional transmissions to Earth. Consequently, the DSN did not detect a signal during a contact attempt on 7 February 2003. Thus, after more than 30 years in space, the Pioneer 10 spacecraft sent its last signal to Earth.

The final attempt to contact Pioneer 10 took place on the 34th anniversary of its launch, on 3 – 5 March 2006 [397Jump To The Next Citation Point]. At that time, the spacecraft was 90.08 AU from the Sun, moving at 12.08 km/s. The round-trip light time (i.e., time needed for a DSN radio signal to reach Pioneer 10 and return back to the Earth) was approximately 24 h 56 m, so the same antenna, DSS-14 at Goldstone, CA, was used for the track. Unfortunately, no signal was received. Given the age of the spacecraft’s power source, it was clear that there was no longer sufficient electrical power on board to operate the transmitter [378].

2.1.2 Pioneer 11 mission details

Pioneer 11 followed its older sister approximately one year later. It was launched on 5 April 1973 (on April 6, 1973 at 02:11 UTC), also on top of an Atlas/Centaur/TE364-4 launch vehicle. The second stage used for Pioneer 11 was a Centaur D-1A, while the third stage was a TE364-4 solid fuel vehicle.

After safe passage through the asteroid belt on 19 April 1974, Pioneer 11’s thrusters were fired to add another ∼ 65 m/s to the spacecraft’s velocity. This adjusted the aiming point at Jupiter to 43,000 km above the cloud tops. The close approach also allowed the spacecraft to be accelerated by Jupiter to a velocity of 48.06 km/s, so that it would be carried across the solar system some 2.4 billion km to Saturn.

Early in its mission, Pioneer 11 suffered a propulsion system anomaly that caused the spin rate of the spacecraft to increase significantly (see Figure 2.16View Image). Fortunately, the spin rate was not high enough to endanger the spacecraft or compromise its mission objectives.

Pioneer 11’s first interplanetary cruise phase lasted approximately 20 months. During this time, a major trajectory correction maneuver was performed, aiming Pioneer 11 for a precision encounter with Jupiter. Pioneer 11’s closest approach to Jupiter occurred on 2 December 1974 at 17:22 UTC. This encounter provided the necessary gravity assist to alter Pioneer 11’s trajectory for a planned encounter with Saturn (see Figure 2.1View Image).

The second interplanetary cruise phase of Pioneer 11’s mission took it across the solar system. Initially, Pioneer 11’s heliocentric distance was actually decreasing as it followed an hyperbolic trajectory taking the spacecraft more than 1 AU above the plane of the ecliptic. This phase of the mission culminated in a successful encounter with Saturn. Pioneer 11’s closest approach to the ringed planet occurred on 1 September 1979, at 16:31 UTC. Still fully operational, Pioneer 11 was able to make close-up observations of the ringed planet.

After this second planetary encounter, Pioneer 11 continued to escape the solar system on an hyperbolic escape trajectory, and remained operational for many years. Pioneer 11 explored the outer regions of our solar system, studying the solar wind and cosmic rays.

The spacecraft sent its last coherent Doppler data on October 1, 1990 while at 31.7 AU from the Sun2. In October 1990 a microwave relay switch failed on board Pioneer 11, in its communications subsystem. The most notable consequence of this failure is that it was no longer possible to operate this spacecraft’s radio system in coherent mode, which is required for precision Doppler observations. Therefore, after this event, precision Doppler data was no longer produced by the Pioneer 11 spacecraft.

The spacecraft continued to provide science observations until the end of its mission in 1995. In September 1995, Pioneer 11 was at a distance of 6.5 billion km from Earth. At that distance, it takes over 6 hours for the radio signal to reach Earth. However, by September 1995, Pioneer 11 could no longer make any scientific observations as its power supply was nearly depleted. On 30 September 1995, routine daily mission operations were stopped. Intermittent contact continued until November 1995, at which time the last communication with Pioneer 11 took place. There has been no communication with Pioneer 11 since. The Earth’s motion has carried our planet out of the view of the spacecraft antenna.

2.1.3 Pioneer 10 and 11 project documentation

Up until 2005, very little documentation on the Pioneer spacecraft was available to researchers. Indeed, around this time much of the Pioneer archival material stored at NASA’s Ames Research Center was scheduled for destruction due to budget constraints.

The growing interest in the Pioneer anomaly helped to initiate an effort at the NASA Ames Research Center to recover the entire archive of the Pioneer Project documents for the period from 1966 to 2003 (see details in [379397Jump To The Next Citation Point]). This massive archive contains all Pioneer 10 and 11 project documents discussing the spacecraft and mission design, fabrication of various components, results of various tests performed during fabrication, assembly, pre-launch, as well as calibrations performed on the vehicles; and also administrative documents including quarterly reports, memoranda, etc. Most of the maneuver records, spin rate data, significant events of the craft, etc., have also been identified.

A complete set of Pioneer-related documentation is listed in the Bibliography. Here, we mention some of the more significant pieces of documentation that are essential to understanding the Pioneer 10 and 11 spacecraft and their anomalous accelerations:

The on-going study of the Pioneer anomaly would not be possible without these resources, much of which was preserved only because of the labors of dedicated individuals.

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