Planning for Saturn

The dilemma for the Saturn-encounter planners lay in chosing the most appropriate

., Pioneer 11 was able to 'double back' on a

By passing under Jupiter's south polar region, trajectory steeply inclined to the ecliptic.

., Pioneer 11 was able to 'double back' on a

1 September 1379

After Jupiter, Pioneer 11 flew high above the ecliptic on a five-year cruise to Saturn, on the far side of the Solar System.

1 September 1379

After Jupiter, Pioneer 11 flew high above the ecliptic on a five-year cruise to Saturn, on the far side of the Solar System.

trajectory for the spacecraft to fly through the system. Pioneer 11 had accomplished its primary mission at Jupiter, so Saturn, as C.F. Hall explained, was ''gravy'', and because it was not constrained by the need to undertake a slingshot for another target the scientists were free to accept a degree of risk in pursuing specific scientific objectives.5

As before, JPL wished to explore the region through which it hoped to send one of its own spacecraft on the Grand Tour, and this slingshot called for crossing the ring plane at a planetocentric distance of about 2.9 radii, which was just beyond the 'A' ring, and then closing to about 1.4 radii of the planet. This time, however, Ames was willing to risk damage to the spacecraft by crossing the ring plane at 1.15 radii, then passing within 1.06 radii of Saturn, which would skim a mere 9,000 kilometres above the cloud tops and would provide a very accurate map of the gravitational field in order to investigate its internal structure. If, as seemed likely, Saturn possessed a magnetic field, then charting the inner magnetosphere in this way would yield information which the later JPL missions - which would be required to keep their distance from the planet - would not be able to supply. An option of flying through the apparently safe gap of Cassini's Division satisfied no one. The debate therefore centred upon the relative dangers of the inner 'D' ring versus the outer 'E' ring.6 Of course, matching the uncertainties experienced by Pioneer 10 when penetrating the Jovian magnetosphere, the trailblazing nature of this flight meant that key decisions had to be made in the absence of 'hard' data because no one really knew what the ring-plane environment was like - despite more than three centuries of telescopic study. The nub of the issue was the mean sizes of the 'particles' in the various parts of the ring system. A flake of ice less than 1 millimetre across would be unlikely to do serious damage to this particular spacecraft, even striking at 112,000 kilometres per hour. A piece of ice larger than 10 millimetres, however, would almost certainly be crippling. By any reasonable theory, the particle population should be inversely proportional to size - that is, most of the material should be at the small end of the size range. A large object would be more likely to disable the spacecraft, but there should be fewer such particles and they should be widely spaced, so there ought to be a reasonable prospect of the spacecraft passing between them. The real danger came from particles ranging in size from 1 to 10 millimetres, in essence from something the size of a grain of sand to the size of a pea, because these would be fairly numerous and fairly densely packed.

Another aspect of the decision involved Titan, Saturn's largest satellite, which would be on the far side of its orbit during Pioneer 11's flight through the Saturnian system. The scientific objectives would be:

• to measure the temperature of Titan's atmosphere;

• to accurately determine the satellite's mass;

• to obtain spin-scan images of its visible surface; and

• to improve information on its orbit and ephemeris.

The 'inner' option would produce a very close fly-by of Titan on the way out, but if Pioneer 11 was disabled in crossing the ring plane this opportunity would be preempted. The fly-by of the 'outer' option would not be as close, but at least the spacecraft stood a more reasonable chance of surviving to report its observations of this fascinating moon. On 8 November 1977 C.F. Hall formally recommended aiming for the 'French Division' between the 'B' and 'C' rings, but several weeks later Thomas Young, the Director of Planetary Programs in Washington, decided in favour of the 'outer' option because it was ''essential'' to determine whether a spacecraft could survive passage through the ring plane on the trajectory required for the Grand Tour. Irrespective of whatever happened to Pioneer 11, the 'outer' option would yield invaluable insight: if it was lost, the Grand Tour would have to be reassessed, and perhaps even abandoned; if it survived its passage through the ring plane, the Grand Tour would be able to proceed with a reasonable expectation of making a successful slingshot at Saturn.

In July 1978, Pioneer 11 fired its thrusters to refine its trajectory to pursue the 'outer' option. Planning for this historic second encounter was complicated by the fact that Saturn would be at superior conjunction on 11 September 1979. With the spacecraft on the far side of the Sun, the solar corona would degrade the radio signal. Indeed, if a solar storm blasted plasma across the line of sight, the signal might become unreadable. As the angular separation would diminish by about 1 degree per day, it was deemed best to use this manoeuvre to advance the encounter by several days, to 1 September. Hence, Pioneer 11 would be able report particles and fields data during the approach and throughout the fly-by, but only for about a week on the way out before its signal was lost. Furthermore, to provide a degree of redundancy in the receipt of the crucial data at the ring-plane crossing, this event was scheduled to occur when two of the Deep Space Network antennas had a line of sight.

To terrestrial observers in September 1979, Saturn's rings were inclined at a mere 2 degrees, displaying the illuminated southern face, and they were 'closing' towards edge-on in 1980. Pioneer 11, however, was approaching the planet from north of the ecliptic, and from its perspective the ring system was inclined 6 degrees and showing the non-illuminated northern face. Thus, as the spacecraft approached, it would view the system in silhouette, with the most opaque sections of the rings appearing dark. It would see the sunlit face only after crossing the ring plane. As the fly-by loomed, there was every expectation that the data would revolutionise our understanding of the ringed planet. As a particles and fields platform, Pioneer 11's scientific objectives at Saturn were similar to those undertaken at Jupiter, namely:

• to map the planet's magnetic field (presuming that it had one) and determine its intensity, direction and structure;

• to determine the energy spectra of the electrons and protons along the spacecraft's trajectory through the system;

• to map the interaction of the Saturnian magnetosphere with the solar wind;

• to determine the structure of the planet's upper atmosphere where molecules were expected to be electrically charged to form an ionosphere;

• to measure the temperature of Saturn's atmosphere;

• to map the thermal structure of the atmosphere by infrared observations coupled with radio occultation data;

• to obtain spin-scan images of the planet and the ring system;

• to make polarimetry measurements of the planet, the ring system and some of the satellites;

• to determine how the spacecraft's trajectory was perturbed as a result of flying close to the planet and its major satellites, in order to refine estimates of their masses;

• to obtain information to enable the orbits and ephemerides of the planet and its major satellites to be refined; and

• to investigate the ring-plane environment to determine whether it would be safe to fly a Voyager spacecraft through it.

As Saturn is twice as far away from Earth as Jupiter, the spacecraft was able to transmit at only one-quarter the data rate that had been possible at Jupiter - that is, a mere 512 bits per second. With solar interference to contend with, only the largest Deep Space Network antennas were capable of receiving the signal.

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