The Perils of Pioneer 11
By Henry Spencer
Around the time you are reading this, Pioneer 11 – over twenty years old and still limping slowly toward the stars – will finally fall silent. Its plutonium-heated power supplies have been steadily declining in output since they were made. In September 1995, the available power fell below that needed for continued scientific observations, and the spacecraft was officially retired. Sometime around now, power output will fall short of the minimum needed to operate the spacecraft, and the next time NASA’s Deep Space Network (DSN) searches for Pioneer 11′s signal, there will only be silence. Pioneer 11, possibly the luckiest spacecraft ever launched, will finally be dead.
Pioneers 10 and 11 were originally conceived in 1969 as small, simple, spin-stabilized spacecraft for humanity’s first venture into the outer Solar System. Unusual for a planetary mission, the project was run by the Ames Research Center rather than the Jet Propulsion Laboratory (JPL); Ames had previous experience with operating small spin-stabilized deep-space missions (namely Pioneers 6 to 9), and there was some feeling that it was good for JPL to have competition. The specific primary mission was a Jupiter flyby. As was normal at the time, two spacecraft were built and launched for the mission against the possibility that one might fail.
The chances of failure seemed especially high. The Pioneers would be the first spacecraft to fly through the asteroid belt; there was no significant chance of hitting a large asteroid, but hitting a few hundred grains of sand would be almost as bad, and nobody knew how many small particles might lurk undetected in the belt. The Pioneers would be the first deep-space missions powered entirely by radioisotope thermoelectric generators (RTGs), which had never been used for such lengthy missions, because sunlight would be too dim and Jupiter’s radiation belts could damage solar arrays. Providing maximum information about Jupiter’s radiation environment was a major goal of the mission, and that meant penetrating as deeply as possible into the radiation belts, whose exact intensity was unknown and whose effects on the spacecraft electronics were a major worry. Finally, constraints on the cost and mass of the spacecraft made on-board computers impossible. In 1969, radiation-hardened computers were heavy and expensive, so the Pioneers had to be controlled entirely from Earth, despite a round-trip communication delay that would exceed 90 minutes at Jupiter.
Pioneer 10 was launched in March 1972. Pioneer 11 followed in the next Jupiter launch window, in April 1973. Pioneer 11′s primary mission was to be the backup for Pioneer 10 – if Pioneer 10 failed before completing its Jupiter flyby, Pioneer 11 would repeat that flyby. If Pioneer 10 stayed healthy, something a little different could be done. If Pioneer 11 made a close flyby of Jupiter, the resulting gravity assist would throw it into a trajectory that would take it to Saturn five years later. The planetary alignment wasn’t very good. In fact, it could hardly have been worse. A good alignment, like Voyager 1′s, gives a flight time from Jupiter to Saturn of only about 18 months. But as a bonus to an already successful Jupiter mission, it was irresistible.
Pioneer 11′s perils started just after launch, when one of its RTG booms failed to extend properly. Frantic work by engineers at Ames and the DSN managed to extend it.
Both Pioneers survived the asteroid belt undamaged, despite early fears. In fact, they carried optical instruments to measure the density of small particles (by detecting light scattered from them as they passed the spacecraft), and the results showed a density much lower than even the more optimistic predictions.
Jupiter’s radiation belts, on the other hand, were every bit as strong as expected. As Pioneer 10 plunged deep into them (much deeper than the Voyagers later went), its radiation instruments came within a hairbreadth of their upper limits. The 92-minute round-trip delays prevented any sort of real-time response to electronics problems, but precalculated command sequences were sent at appropriate times to reset crucial systems regularly. The spacecraft’s electronic systems functioned rather erratically at times, but they never quit entirely. Ironically, the one major data loss was an item which might have anticipated one of the Voyagers’ most startling discoveries. The only close-up image of Io was lost when Pioneer 10′s imaging system acted up between reset commands.
Pioneer 10 emerged from the Jovian system battered and singed but intact, completing the primary mission.
Steering Pioneer 11 to Saturn did require a very close approach to Jupiter, about one-third the distance of Pioneer 10′s closest approach. This seemed to require a frighteningly deep dive into the radiation belts, but it could be done well away from Jupiter’s equator, where the belts are concentrated. A very high radiation-intensity peak had to be endured, although the total dose would be lower than Pioneer 10′s. The bonus was a look at Jupiter’s magnetosphere at high latitudes, probably the only one possible for many years, since the Voyager mission plans emphasized moon flybys and therefore stayed further out and closer to the equator. At closest encounter Pioneer 11 would be moving at nearly 50 km/s, still the all-time speed record for a spacecraft.
In the end, Pioneer 11′s experience at Jupiter was much like Pioneer 10′s, with erratic electronic behavior but no permanent damage. The biggest problem with the Pioneer 11 Jupiter flyby, as it happened, was a strike by diesel-generator operators at the DSN’s Australian tracking station, which almost disrupted communications for six hours near closest approach!
Pioneer 11 was on its way to Saturn, but it wasn’t out of the woods yet. For one thing, two of Pioneer 11′s instruments malfunctioned en route. Not long after the Jupiter encounter, occasional spurious commands started to appear. Several months of detective work finally identified the problem as misbehaving circuits in one meteorite-detection instrument, which was turned off permanently. More seriously, during the detective work, various other instruments had been turned off temporarily, and the plasma analyzer evidently got too cold during its shutdown because it refused to respond when commanded back on. This was a more serious loss, but fortunately it wasn’t permanent. Although it took two and a half years of thought and experimenting, eventually the damaged circuits were tricked into responding, and the plasma analyzer was brought back into operation in time for the 1979 Saturn encounter.
While all this electronic troubleshooting was happening, a major debate was underway: Where should Pioneer be aimed? For the Jupiter encounter there had been little choice due to the need to reach Saturn and the need to limit radiation exposure. Beyond Saturn there were no specific future plans, and indeed no specific need for the spacecraft to survive, so there were many options. Imaging of the moons was unimportant now, because the Voyagers – bigger spacecraft with much better camera systems – had already swung past Jupiter and were en route to Saturn, less than two years behind Pioneer.
A number of trajectories were proposed, including one that involved flying through the Cassini Division in the rings. It’s just as well that option wasn’t pursued, because we now know that the Cassini Division is not a gap in the rings, just a thin spot. Gradually, two major proposals emerged.
The more daring trajectory would take Pioneer inside the rings, with closest approach about halfway between the cloud tops and the innermost edge of the main rings. This was a relatively dangerous area, because there was known to be at least one faint ring inside the main ones. The estimated probability of spacecraft survival ranged from over 99% to under 1%, depending on whose ring-density model you believed. The payoff was a unique opportunity to observe Saturn and its magnetosphere up close, using an old spacecraft whose useful life was nearly over anyway. However, actually losing the spacecraft at the ring-plane crossing would considerably reduce the data return. After a long debate, the principal investigators who ran Pioneer’s instruments voted 11 to 1 in favor of this “inside” mission.
The more conservative “outside” plan specified two ring-plane crossings, both well outside the visible rings. The chosen distances for the crossings matched the flyby distance needed for Voyager 2 if it were to reach Uranus. The Voyager planners, given a unique and irreplaceable opportunity to visit two more planets, badly wanted to know if that distance presented any risks to their spacecraft. Such a flyby was also much safer for Pioneer, assuring Saturn data return after ring-plane crossing and also providing for a continued mission on into deep space. The final decision was made at NASA Headquarters: Using Pioneer as a pathfinder for the Voyager Uranus-Neptune mission was more important than getting maximum return from the Pioneer flyby alone. Pioneer would take the relatively safe “outside” trajectory.
There was also considerable freedom to pick the exact time of the Saturn encounter, and this encounter would be timed so that the spacecraft would be in sight of two DSN stations during the most critical times. On the morning of September 1, 1979, the Ames controllers tensed as Pioneer crossed Saturn’s ring plane … and then gradually relaxed as the signal kept on coming, showing that Pioneer had survived. (Well, I oversimplify, since they were actually hearing about it 86 minutes after the real event. But that was Earth’s view of the situation.) The busiest hour of the flyby – closest approach below the rings – was underway and the spacecraft had survived.
A few minutes later, safely past the ring plane, on the cautious, conservatively-planned “outside” trajectory, Pioneer 11 almost hit one of Saturn’s moons.
Without warning, the outputs of several of Pioneer’s radiation detectors suddenly plunged to nearly zero, held roughly steady for eight seconds, and then snapped back up to their former values. For the same eight seconds, the magnetometer recorded major disturbances in Saturn’s magnetic field. Pioneer had streaked through the magnetic “wake” of a moon roughly 200 km across, at a distance of no more than a few thousand kilometres – the closest it had come to any large object since leaving Earth. Later, the villain was tentatively identified as a moon discovered the previous day from Pioneer’s imaging, and suspected from earlier observations by Earth-based telescopes. After the Voyager flybys, it became clear that there are two similarly-sized moons (now named Epimetheus and Janus) in the same orbit, and there is still some uncertainty about just which one was the object of Pioneer’s near-miss.
The remainder of the Saturn encounter, and indeed the remainder of Pioneer 11′s life, was less eventful. It returned a lot of useful data from Saturn, although this was somewhat eclipsed by the far greater returns from the Voyager flybys that followed. After that, Pioneer 11 headed out into open space, slightly behind its older companion Pioneer 10 and the newer but faster Voyagers. In 1990, it left the Solar System (by an admittedly arbitrary definition). A slow trickle of data on the solar wind, the solar magnetic field, and cosmic rays continued to come back to Earth. A few years ago, Pioneer 11 started having trouble pointing its antenna accurately at Earth, but otherwise its instruments and equipment are still in good shape. Only its power sources are fading.
There had been hope that Pioneer 11 might reach the heliopause, where the Sun’s atmosphere and fields end and the Galaxy’s begin, before it died. Due to how the planetary encounters worked out, Pioneer 11 happened to be heading out in the best direction, straight into the galactic wind. Alas, the heliopause is further away than was once thought, and probably only the Voyagers will survive to reach it. (Pioneer 10 is expected to last until 1999, but it’s heading out in about the worst direction, down the Sun’s tail. The Voyagers will last 20 to 30 more years if nothing breaks.) After one of the most exciting lives ever led by a planetary probe, Pioneer 11 has died quietly in retirement.
Henry Spencer is an amateur space historian
and one of the founding members of the Canadian Space Society (CSS).
This article was first published in the January/February 1997 edition of the Canadian Space Gazette.
Mirrored from the now defunct GeoCities site