June 1982:the Voyager 2 Saturn flyby

Voyager 2 at Saturn

As 1980 drew to a close, life at the Jet Propulsion Laboratory returned to normal. The teams of visiting scientists returned home to begin the detailed analysis of the volumes of Voyager 1 data returned from Saturn. The excitement of the first flyby was over, the planning for the Voyager 2 encounter was just beginning.

On the basis of Voyager 1’s results it was decided to make significant alterations to the plans for Voyager 2’s journey through the Saturnian system. Voyager I had provided a "broad brush" coverage of the system, while Voyager 2 would be commanded to concentrate on specific targets in an attempt to better understand the discoveries of its twin.

This replanning was made possible by the flexibility designed into the spacecraft nearly a decade before. During the planning stage it was realised that discoveries made by the first spacecraft might require major changes to the encounter operations of the second. An eight month gap between encounters was therefore built into the flight plan. To provide this flexibility and reliability three of Voyager's systems use doubly redun dant, reprogrammable digital computers. These are the Command Control Subsystem (CCS), the Articulation and Attitude Control System (AACS) and the Flight Data System (FDS).

The CCS is the brain of the Voyager spacecraft. It can Issue commands to the spacecraft subsystems from its memory, decode commands from Earth to update its memory and pass those commands to other subsystems. The AACS controls attitude and points the scan platform on which the science instruments are mounted. The FDS controls the scientific instruments and arranges all of the science and engineering data for transmission to Earth. It also controls the data storage subsystem, an eight track tape recorder capable of storing over 500 megabits of data.

Voyager 2’s trajectory past Saturn was determined by the need to keep the spacecraft on course for Uranus (approval of the extension of the mission to Uranus had been received in November) but the complex series of experiments could be changed in line with Voyager’s new objectives. These changes were derived by JPL engineers and the mission’s principal science investigators. New encounter sequences were then prepared for Voyager's CCS. Before these could be transmitted to the spacecraft they were run through complex computer simulations to ensure maximum scientific return and spacecraft safety. When Voyager 2 reached Saturn the round trip radio time would have increased to nearly 3 hours and if a software problem were to arise useful observing time would be lost while ground control attempted to rectify it.

The main changes to the Voyager 2 science objectives may be summarised as:

1. Voyager 2 would concentrate on studies of the complex ring system, searching for evidence of small moons which might explain the proliferation of ringlets.

2. Additional studies of the braided F ring would be made including stereo imaging and an occultation experiment using the star Beta Tauri.

3. Attempts would be made to view the B ring exactly edge-on to determine if the “spokes” observed by Voyager 1 were "levitated” above the ring plane.

4. Special studies of the two eccentric rings observed in the B ring and in Cassini's division.

5. Reduced emphasis of imaging of Titan but a search for aerosols in Titan’s atmosphere would be carried out. This had been impossible for Voyager 1 because of a failure in its photopolarimeter.

6. A detailed look at some of the small moons discovered bv Voyager 1 and from Earth. Of special interest were the small moons found sharing orbits with the classical satellites, e.g. Dione B and the leading and trailing "’Trojan” moonlcts of Tethys.

7. A study of electrostatic discharges discovered by Voyager 1. These were found to have a period of 10 hr 10 min, similar to the period of any satellites embedded in the B ring. 'These discharges may also be related to the B ring spokes.

8. A systematic search on Saturn’s unlit side between equa¬ torial and polar latitudes for aurorae.

Voyager 2’s observations of Saturn's rings would be aided by three factors. Firstly, the spacecraft cameras were more sensitive than those of its predecessor. Secondly. Voyager 2 would pass closer to the rings and, thirdly, the rings would be better illuminated as the Saturnian year continued towards Northern Summer. Additionally. Voyager 2 would be on the illuminated side of the rings at its point of closest approach.

Closing in on Saturn

Although some of the details of the near encounter sequences of Voyager 2 had been changed, and the spacecraft was flying a different trajectory, the general shape of the flyby was the same as for Voyager 1.

Encounter operations began on 5 June 1981 with the observatory phase. Beginning 82 days before encounter and lasting eight weeks, this period of long distance observations allowed a second, short-term, history of the planet to be recorded.

The observatory phase began with a 43-hour session of near continuous photography covering four planet rotations. Throughout the remainder of the observatory phase Saturn was imaged every 72 degrees of its rotation (so five photographs covered a complete rotation). These pictures allowed the assembly of a colour movie showing the approach. The spacecraft’s UV spectrometer scanned across the entire Saturn system mapping the intensity of the emissions from Titan’s orbit. As with Voyager 1, radio emissions were sampled daily with the planetary radio astronomy experiment and frequent scans were made by the plasma wave instruments. Also during this phase numerous calibrations of the scientific instruments were made to ensure the best possible scientific data during the near encounter.

On 31 July, 26 days before closest approach, the narrow angle camera could no longer capture the entire planet in a single frame. The imaging was switched to two-by-two mosaics, using four frames to build up each complete picture. At this point Voyager 2 was a little less than 25 million kilometres from Saturn and had entered the 12-day Far Encounter 1 phase. During this phase, the observations were continued as the detail visible on the planet increased day by day.

By 11 August the range to Saturn had reduced such that two by two mosaics no longer provided full coverage of the planet. The Far Encounter 2 phase had begun. At this time Voyager 2 was 14.5 million km from closest approach and all of its instruments were gathering data on the planet, satellites and radiation environment. During this period, a series of photographs of the' B ring were taken by the narrow angle camera as more details of the elusive spokes were sought. Spoke imaging took place over a period of about 30 hours with the Voyager camera panning left to right to take the four images necessary to ensure complete coverage of the ring system. Each four image sequence lasted about 16 minutes.

On 18 August a final trajectory correction manoeuvre was made to refine Voyager 2’s aim point. This was necessary because it had been estimated that an error of a mere 20 seconds at encounter could compromise some of the scientific results. In the end Voyager 2 arrived only 66 km off its aim point and 3.1 seconds ahead of schedule. Early in the Far Encounter Phase Voyager 2 made its closest approach to lapetus, Hyperion and Titan. Just 18 hours before closest approach the spacecraft began its near encounter.

Close Encounter

The close encounter operations were more complex than those of Voyager 1 because it had been instructed to study specific targets, requiring extensive use of the movable scan platform carrying the scientific instruments. On the last day of the Far Encounter 2 phase there had been 33 different observations and the near encounter would involve rapid switching between a number of targets.

On 22 August, while still 3V5 days from Saturn, Voyager 2 passed the mysterious, low density moon Iapetus. Iapetus has long been known to have hemispheres of widely differing reflectivity (or albedo). The closest distance was a little over 900,000 km but even this was 1.5 million km nearer than Voyager 1. By monitoring the effect of Iapetus’ weak gravita¬ tional field on the trajectory of Voyager 2 the satellite’s mass, and hence its density, could be determined. This showed Iapetus to have a density of only 1.1 gm/cc. This very low figure suggests a composition of 35 per cent rock, 55 per cent water ice and about 10 per cent methane ice.

The next moon to come under intensive scrutiny was Hyperion, a small body which Voyager 1 had hardly glimpsed. As Voyager 2 imaged Hyperion the changing orientation of the satellite caused it to appear in a variety of forms. Before its true shape could be determined the satellite was described as shaped like a peanut, a tuna-fish can and a hockey puck. Eventually the semi-official description of hamburger-shaped evolved with dimensions of about 210 by 360 km. Of great interest was the discovery that the long axis of Hyperion points out of the plane of its orbit - it had been expected that the moon would be gravity gradient stabilised to point directly towards Saturn. To have taken up this peculiar attitude Hyperion must have been struck by another body, probably within the last 100 million years.

As Voyager 2 sped towards Saturn it took a look at Titan, although the range was over 600,000 km, a hundred times further than its predecessors flypast. Even at this range the cameras showed that the polar hood observed by Voyager 1 had changed. It was now more of a collar surrounding the pole. Voyager passed by Titan about 18 hours before closest approach to Saturn and shortly after it formally entered its near encounter phase.

As the spacecraft closed in on the planet it continued to photograph the rings and details on the planet itself. The search for moonlets embedded in the rings continued in vain. Four hours and 20 minutes from closest approach the newly discovered moon 1980S26, known as Dione B, was glimpsed at a range of 318,000 km. Shortly afterwards the first of two planned occultation experiments began. The structure of Saturn’s rings was to be investigated with unprecedented accuracy.

Probing the Rings

The occultation of the star Delta Scorpii was one of Voyager 2’s most important ring observations. For 2 hours and 20 minutes the spacecraft's photopolarimeter was aimed so that the star was viewed through the ring system. As the star blinked on and off Voyagqr was'able to count the number of ringlets and measure the size of particles down to about 500 m in size. The star, magnitude 2.34, was chosen because of its brightness and because its occultation by the rings took place in Saturn’s shadow, minimising the interference from scattered sunlight. By ingenious computer processing the results from this experiment were reconstructed in the form of “photographs” showing the fine structure within the 70 km-wide F ring strand. Resolution was at least 10 times higher than with the best optical images.

During the occultation experiment over 700,000 data points were recorded at a rate of about 100 per second. Initial analysis suggested that resolution could be as high as 100 m in places, and that the thickness of the outer edge of the A ring was less than 200 m.

A search was made for fluctuations in ring particle density since spiral density waves within the rings had been proposed to account for the complex structure. Wave features were found with peak to peak distances ranging from 100 km to the 100 m resolution limit.

The Inner Moons

During the occultation experiment Voyager 2 had passed Dione with a maximum possible resolution of 12 km, some four times lower than Voyager 1 because of the increased distance. Soon after, Mimas was passed at a distance of about 310,000 km and Saturn closest approach was less than an hour away.

During this hour Voyager made its closest approach to three of the newly discovered small Saturnian moons 1980S25 (trailing Tethys), 1980S28 (orbiting outside the A ring) and 1980S26 ( orbiting outside the F ring). Both 1980S25 and I980S28 were imaged at resolutions of about 5 to 10 km before Voyager 2 skimmed 100.000 km above the cloud tops of Saturn.

Closest approach occurred at 8.21 JPL local time, as Voyager 2 swept down on the rings from above their illuminated side. As seen from Earth Voyager 2 proceeded around the right hand side of the planet, the gravitational attraction of Saturn turning the trajectory through almost 90 degrees and swinging the craft onto its course for distant Uranus.

Nine minutes after closest approach Voyager passed I980S27, another small moonlel. before its rendezvous with Enceladus. Voyager 1 had passed 200,000 km from Enceladus and had shown that at a resolution of 12 km it was unusually smooth. As Voyager 2 closed in. on Enceladus it was revealed as an exciting new world. In general terms, it resembled Ganymede, although with a diameter of about 500 km it was only a tenth of size of the Jovian satellite. Some regions of Enceladus showed impact craters up to 55 km in diameter while other areas, those imaged by Voyager 1 are smooth and uncratered. Like Ganymede and Europa,Enceladus is also criss-crossed by linear grooves several hundred kilometers long. These probably represent faults resulting from crustal deformations. The large uncratered areas on Enceladus must be young, probably less than 100 million years old, suggesting recent geological activity.

Voyager 2 was still working well 15 minutes after Enceladus closest approach as it disappeared behind Saturn to begin the 90 minute Earth occultation phase (Sun occultation occurred 6 minutes later). While out of sight from Earth the spacecraft crossed the ring plane about 745 km outside the C ring. Once out of contact with Earth, Voyager’s scientific observations were the responsibility of the spacecraft’s CCU which would use commands stored in its memory to control the complex array of instruments. Mission controllers waited expectantly for the spacecraft to reappear.

When contact was re-established it was obvious that all was not well with Voyager 2. Instead of Saturn, photographs showed only empty space. The 220 lb scan platform carrying Voyager’s cameras, spectrometers and photopolarimetcr had stuck and was pointing uselessly into space. The platform was jammed at 260 degrees in azimuth and 20 degree elevation and would not respond to commands to slew in azimuth. Immediately ground controllers commanded a slew in platform elevation to keep the sensitive instruments pointing away from the Sun. This was followed by an instruction to the scan platform not to respond to azimuth commands from the spacecraft. Several azimuth scans had already been sent by the CCU to the platform but the platform had failed to respond. Later in the day ground controllers were able to move the platform about 10 degrees in azimuth but further attempts to free it were delayed to allow time for a full evaluation.

As Voyager began to leave Saturn ground controllers strove to bring the scan platform back into action. Photographs were obtainable but only at a cost - slewing the spacecraft was using precious attitude control gas needed for the journey to Uranus. On 28 August, three days after closest approach, the platform was successfully moved by ground command to point the instruments at Saturn once more. Early tests showed that the platform could be moved, although response was sometimes hesitant and slow. Response improved steadily over the next few days and the platform was returned to the.onboard computer sequences for the imaging of Phoebe, on 4 September. Voyager -2 flew no closer to Phoebe than 2 million kilometres but even at that range was able to return useful results. Phoebe was shown to be about 200 km in diameter and very dark. Earth-based measurements had assumed a more reflective surface and deduced a diameter half that measured by Voyager. The rotation period was determined as about 9 to 10 hours, the only one of Saturn’s moons which does not always show the same face to the planet. Phoebe orbits Saturn in a retrograde direction, and in the ecliptic plane rather than the plane of the equator.