June-September 1981:Voyager 2 at Jupiter

As Voyager 1 receded from Jupiter, the scientific instruments aboard the spacecraft continued to look on the giant planet. On 9 April the spacecraft’s attitude control jets were fired to adjust its course towards an encounter with Saturn, still nearly 800 million km away and 19 months in the future. During the flypast of Jupiter the spacecraft had been accelerated and swung onto course for Saturn by Jupiter's immense gravitational field, the planet's orbital energy increasing the spacecraft's velocity to about 84,500 km per hour. As the distance from Jupiter increased,Voyager 1 was commanded to settle into a relatively quiet cruise mode,sampling the interplanetary medium and carrying out regular calibrations and tests. Meanwhile, at Pasadena in California, mission controllers prepared for the arrival at Jupiter of Voyager 2.

Voyager 2 nears Jupiter

On 24 April 1979, 76 days before the closest approach, and following a five day run of the events planned for the close encounter phase. Voyager 2 began Jupiter observations. The spacecraft was approaching Jupiter on a different sunline to its sister ship and would fly past the Galilean satellites on the inbound part of its trajectory. Since the satellites always keep the same hemisphere towards Jupiter this would help to improve the overall impression of the Jovian system by viewing the previously unobserved satellites.

By early May,Voyager 2 was in its observatory phase and imaging Jupiter at two hour intervals. From these photographs another time lapse film of the circulation in the planet’s atmosphere could be prepared. Comparison with a similar sequence taken by Voyager 1 would illustrate any large scale changes since March. As May turned into June it became clear that significant changes in the atmosphere had indeed occurred during the four months since the Voyager 1 flyby. Important changes were observed in the region of the Great Red Spot with movement of one of the previously observed white ovals relative to the Red Spot. The white oval was drifting east at a rate of 0.35 degrees per day while the Red Spot was drifting west at about 0.26 degrees per day.

On 5 June, 34 days from closest approach. Voyager 2 was in good shape with only one or two minor worries for mission controllers. Of these, the most significant was the heating of one section of the spacecraft bus when the craft was manoeuvred off the Sun line or when the power consumption changed. This heating caused frequency drifts in the one remaining radio receiver, limiting the ability of ground controllers to command the spacecraft. Events likely to cause the temperature variations were identified and plans for commanding the spacecraft during these periods were revised.

After a two month long heat soak, designed to retard the degradation of bonding material, the infrared interferometer spectrometer (IRIS) was successfully re-activated on 21 June. The IRIS was immediately put to use measuring temperature differences on the Jovian satellites as they disappeared into and re-appeared from Jupiter's shadow. The fields and particles experiments continued to sample interplanetary space as they had done during the long cruise between planets and were at last beginning to detect.the presence of Jupiter.

A final pre-encounter trajectory correction was made on 27 June finalizing the path through the Jovian system. Although the operations during the approach so far had been similar to those of Voyager 1, with imaging mosaics and daily scans with the ultra-violet and infrared instruments, the details of the closest approach would be quite different.

Since the trajectory of Voyager 2 was designed to include a possible flight to Uranus (arriving in 1986) mission controllers wished to keep the spacecraft clear of the most intense parts of Jupiter's radiation field. Voyager 2 would flyby further from the planet than Voyager 1 and deeper into the southern hemisphere. This flight path would allow much higher resolution of the surface of Europa but would preclude a close flyby of Io.

Ten days from closest approach, Voyager 2 was 9.5 million km from Jupiter and travelling with a heliocentric velocity of 9.9 km/sec (22,000 mph). Jupiter's gravity would treble this velocity during the final approach to the planet and slingshot the spacecraft towards Saturn. One-way radio communication time was 51 minutes. At this moment Voyager 1 was 113 million miles from Jupiter, travelling with a heliocentric velocity of 23 km/sec and with a one-way communication time of 54 minutes.

Voyager 2 encounter

As Voyager 2 fell in towards Jupiter, the innermost satellites came under further intensive study. The spacecraft passed within 213,000 km of Callisto, twice the distance of the Voyager 1 flyby, but nonetheless returned many useful pictures of this heavily cratered world. Twelve hours later came the closest approach to Ganymede. Observations of Ganymede showed bright young craters, light and dark terrain stripes resembling the outer rings of a large impact basin. Many bright and therefore young craters were observed, as were large areas of the grooved terrain first observed by Voyager 1.

Four and a half hours before closest approach to Jupiter Voyager 1 flew past Europa, the brightest of the planet's moons. Voyager 2 flew much closer to Europa than its predecessor and revealed a surface that was very smooth, with hardly any evidence of cratering. Europa appeared to be completely covered with a thin layer of ice, possibly overlying water or softer ice. Bright and dark linear features were interpreted as fractures in the crust through which ice was welling up to the surface. This continual resurfacing process would obliterate any impact craters almost immediately after they were formed, leaving an unmarked surface.

After Voyager 2 flew behind Jupiter it continued to look back on the satellite system, carrying out a volcano watch on the innermost Gallilean moon Io. Voyager 1 had discovered active volcanoes on Io and over a ten hour period on 9 July Voyager 2 took about 200 photographs. These would be used to produce a time lapse film of activity on the satellite. During this sequence the sunlit crescent of Io grew steadily thinner as the spacecraft flew deeper into Io's shadow until only a sliver of light, punctuated by the plumes of the volcanoes, was visible. Of the eight active plumes observed by the first Voyager, seven were visible to Voyager 2 and of these six were still active.

The day after closest approach a series of long exposure photographs of the recently discovered Jovian ring was taken. In these, the rings appeared much brighter than expected,probably due to forward scattering of sunlight by small particles within the ring. Within the main bright ring a fainter ring was observed. This inner ring appeared to stretch all the way down to the upper layers of the planet's atmosphere. Several more images of the rings were obtained the following day as the spacecraft looked back onto the night side of Jupiter searching for auroral activity and lightning. From these the dimensions of the ring were estimated at about 65,000 km wide and less than 1 km in thickness.

In addition to the intensive study of the satellites and the ring system Voyager 2 continued to record vast quantities of information about the electromagnetic and particle environment around the giant planet and returned additional thousands of images of the atmosphere. On 19 July, and again on the 23rd, trajectory corrections were carried out to point the Voyager towards its rendezvous with Saturn in mid-1981.

Pioneer 11 and Saturn

While the two Voyagers were having a well earned rest between planets, another milestone in planetary exploration was approaching. Some seven years after leaving Earth,Pioneer 11 was closing in on Saturn. The 258 kg Pioneer and its mission have been described in detail in previous issues of Spaceflight and so only a summary is given here. Pioneer 10 was the first spacecraft to investigate Jupiter, and the essentially identical Pioneer 11 was launched almost a year later. Spin stabilised and with relatively unsophisticated instruments,Pioneer 10 blazed a trail through the Jovian system which allowed mission planners to take a daring gamble with its sister ship. Pioneer 11 flew closer to Jupiter, passing through the planet’s intense radiation field more quickly, and was swung back, almost upon itself, onto a trajectory for Saturn. The coast across the Solar System lasted five years and by the time Pioneer 11 reached Saturn it had exceeded its design life by 40%. NASA was hoping that like previous Pioneers the spacecraft would continue to operate well beyond its nominal lifetime.

This optimism was justified, for as Pioneer 11 approached Saturn in August 1979 it was indeed in good condition, a tribute to the engineers who had built it and to the controllers at Mountain View, California who had nursed it throughout its monumental journey. On 1 September 1979, while NASA held its breath, Pioneer 11 swooped down past the outer edges of Saturn’s rings, past the cloud tops and up, out again towards deep space with hardly a jolt to its systems. During the flyby vital navigational data had been gained and it had been shown that damage to Voyager 1 from radiation or ring particles was unlikely, strengthening hopes that Voyager 2 would ultimately fly on from Saturn to Uranus.

With the excitement of the Pioneer 11 flyby over, mission operations slipped back into a routine as the two Voyagers cruised across the abyss between the Solar System’s largest planets. Analysis of results from the Jupiter flyby continued and a further two Jovian moons were discovered, the second while searching for confirmation of the first. Instrumental in the discoveries were G. Edward Danielson of the Voyager imaging team, optical navigation engineer Steven Synnott and David Jewitt,a CalTech graduate student.

Voyager approaches Saturn

On 16 October this routine was abruptly disturbed when contact with Voyager 1 was temporarily lost. After a 22 hour cruise science manouevre, which is performed in radio blackout since the main antenna moves off Earth line, radio signals did not arrive back on Earth when expected. This was because after the series of turns allowing the scientific instruments to scan the whole sky, the spacecraft's star tracker had locked onto Alpha Centauri instead of Canopus and the high gain antenna beam was no longer pointing at Earth. Once the situation was understood, the powerful 80 kW power carrier at Tidbinbilla in Australia was used to command, through a sidelobe of the spacecraft antenna, a switch over to the low gain antenna with its larger beamwidth. This done,passing commands to the Voyager was easier and the spacecraft was instructed to roll 56.8 degrees, resulting in Earth-pointing the antenna and placing the star tracker within 1 degree of Canopus. On ce the signal from Voyager was received at Earth. Voyager I was commanded to re-acquire Canopus and return to its high gain antenna. The emergency was over.

1980 dawned another year, and with it another award for the Voyager team. In 1979 the American Veterans of Foreign Wars had presented their National Space Award Gold Medal to the Voyager project and on 24 March 1980 President Carter presented the National Space Club's Goddard Memorial Trophy, America's most prestigious space award, to the project. The President said "The teams that made this flight possible deserve the highest accolades".

Voyager 1 had already begun to take photographs of Saturn for calibration and navigation purposes even as the president spoke, although at ranges of over 300 million km little detail was visible, even after computer enhancement. By May both spacecraft were on target and in basically good health, although some internal inconsistencies between the two computer command subsystem processors on Voyager 1 were causing a little anxiety. Four command anomalies had been noted on Voyager 1 since the encounter with Jupiter and a team was set up to investigate possible fault protection measures. Voyager 1 would move off Earth line several times before arriving at Saturn and it was vital that the high level of internal command activity during these events should not affect the mission. As a precaution against the failure of the remaining receiver onboard Voyager 2, the backup mission load (BML) stored in the spacecraft's computer was updated. The new BML would activate if contact with Earth was lost and would extend the data gathering capability beyond Saturn to Uranus.

During July and August, as the distance to Saturn decreased, mission controllers underwent a seven week test and training period. A variety of simulated emergencies, including a fire in the mission control area during commanding were sprung upon controllers without warning. This ensured that the long planetary coast had not dulled the skills of the scientists and engineers at Pasadena. Tests and training climaxed on 18-19 August with the near encounter test, a simulation of the activities during the closest approach to Saturn. During the test,a near-duplicate of the encounter computer sequence was sent to Voyager 1 and activated. Over the portion of the test simulating occultation of the Sun by Saturn, alternate pointing commands were supplied.to the scan platform, lest the cameras be damaged by pointing directly at the Sun.

On 22 August, 82 days before closest approach. Voyager 1 began its observatory phase studying the planet from a range of over 100 million km. At this point the spacecraft was 1.4x10 km from Earth and travelling at 20.4 km/sec. One way light time was 80 minutes. Apart from minor problems with the star tracker and the pointing accuracy of the scan platform,Voyager 1 was in good health for its encounter.

Saturn Encounter

The Saturn encounter was divided into five major phases beginning with the observatory phase. During a nine week period a long time base history of the Saturn system was compiled. At long range the battery of scientific instruments on the Voyager were brought to bear on the mysteries of Saturn. Seven times a day the ultraviolet spectrometer swept across the planetary system, searching for clouds or tori of ions in the orbits of the planet's moons and investigating their chemical constituents. The atmosphere, thermal structure and dynamics of Saturn were investigated by the infra-red spectrometer and radiometer aboard. Twice daily radio-astronomy scans, returned to Earth at 115.2 kilobits per second, defined the radio rate of rotation of Saturn to be 10 hours and 34.4 minutes. This compared well with the value measured from Earth for polar and temperate regions, but was longer than the previous result (10 hours and 14 minutes) for the equator. This indicated the presence of a high velocity equatorial jetstream suggested by Pioneer 11. Fields and particles experiments aboard Voyager 1 were also active,studying interplanetary space in the vicinity of the giant planet.

The most obviously spectacular results, however, were coming from the TV cameras on the spacecraft. Photographs were taken at approximately 2 hour intervals, one fifth of the rotation period, every day. From these, time lapse movies would be made which would appear as zooms towards five specific longitudes on the planet. In addition, during a 42 hour period on 12-14 September the planet was imaged continuously every 4.8 minutes. These images were taken through a set of 3 different colour filters every 8 degrees of rotation and from them a colour film of four rotations of the planet was prepared. The intensive imaging showed less detail in the atmosphere of Saturn than had similar footage of Jupiter for a variety of reasons. Saturn was revealed as a less active planet than Jupiter and much of the detail within the atmosphere was masked by high altitude clouds. In addition, the range at which these images were recorded was more than twice the distance for comparable photographs of Jupiter. The reasons for this were twofold, firstly the photographs were intended to capture any motion within the planet’s rings ancl, secondly, Voyager 1 was rapidly approaching its annual solar conjunction, which would interfere with data transmissions. Solar conjunction occurs when the spacecraft, as seen from Earth, appears to pass behind the Sun. When the angle from the Sun to Voyager and back to Earth is less than 5 degrees the intense activity within the solar corona seriously interrupts communications with the spacecraft. This period lasted for about 2 weeks during the middle of September for Voyager 1. Although this interfered with the scientific and engineering data flow from the spacecraft it did give the radio science team an opportunity to study the solar corona and make certain measurements crucial to Einstein's theory of relativity.

As Voyager 2 rushed towards Saturn, the detail in the photographs of the planet grew steadily better. On 17 September 1980 the distance to the planet had shrunk to 75 million km and picture resolution had increased to about 1600 km, three times better than the best picture ever taken from Earth. Most of the major satellites were clearly visible and from these photographs would come more detailed calculations of their paths around Saturn. The search for new moons had also begun; long exposure photographs through the clear filter were trying to capture a variety of suspected satellites. From 50 million km, previously unobserved features were seen in Saturn’s rings and, as was to be the pattern throughout the encounter, scientists were caught by surprise. Clearly visible in the inner B ring were dark spoke-like features which rotated around the planet. Some of these features were observed to persist for three or more hours despite the differing orbital velocities between inner and outer edges of the ring. These spokes should have been rapidly erased as the inner edge of the ring raced ahead of the outer portions. Although individual markings within the ring disappeared in a few hours, new features seemed to be continually regenerated by an unknown mechanism.

On 24 October Voyager 1 entered its far encounter phase and over the next day took its last non-mosaic images of the planet. For two months Saturn had been growing in the field of the camera and now a single frame could no longer capture the whole planet. During the far encounter phase, regular 2x2 three colour mosaics were made, supplemented by additional images of the rings. Infrared and ultraviolet data were also collected, the ultraviolet spectrometer attempting to gain information about the composition of Saturn,its rings and inner satellites.

Over a ten hour period on 25 October the rings were imaged every 4 minutes to provide a time lapse motion picture of the radial features forming and dissipating. These photographs revealed two new satellites, satellite 13 orbiting about 2500 km beyond the outer F ring and satellite 14 between the F and A rings. Both satellites were small, only 2-300 km in diameter.

On 2 November, Voyager 1 entered its far encounter 11 phase with only ten days remaining to closest approach, and detail of amazing complexity rapidly becoming evident in the rings. The army of scientists, engineers and journalists at Pasadena waited amid rising excitment for close up photographs of Saturn and its mysterious moons.