February 1982:the Voyager 1 Saturn flyby

The second part of the far encounter phase started 14 million kilometres from Saturn. At this time the spacecraft was 1,500 million km from Earth and the one-way radio communication time was 85 minutes, nearly 3 hours for a round trip. This phase would last until 26 hours before closest approach when the near encounter sequences stored in the onboard computer would take control of the spacecraft as it passed through Saturn's satellite system.

By now the angular size of Saturn had grown so large that 2x2 mosaics were no longer adequate to image the entire planet and the cameras were commanded to switch to 3 x 5 high resolution colour mosaics of the planet and its rings. Voyager 1 was approaching from above the ring plane and was planned to cross it just after its approach to the satellite Titan,18 hours away from Saturn encounter.

As the picture resolution of the rings improved, another major surprise was in store for the scientists at Pasadena, because it was becoming obvious that the structure of the rings was far more complex than had been originally supposed. A two image mosaic of the ring system taken on 6 November, at a range of 8 million km, revealed nearly 100 individual concentric features in the rings. Saturn’s rings looked remarkably like a cosmic record 275,000 km across. After two centuries of explaining how Saturn’s rings were controlled by the well-understood gravitational influences of the major satellites, the astronomical community had only a few hours to prepare an explanation of why things hadn’t quite turned out as expected, and Saturn was still five days away.

On 6 November the final course correction prior to Saturn encounter was made. This manoeuvre did not succeed in exactly aligning Voyager 1 with its planned trajectory but project officials said that Voyager’s course was within mission requirements. Also on 6 November, a series of photographs was taken as part of a pre-planned survey of the space around the rings in the search for new satellites, and the discovery of moon number 15 was announced the following day. Detected at a range of 8 million km,the satellite 15 orbits every 14 hours and 20 minutes, a mere 800 km from the outer edge of the A ring. It was thought that this moon, only about 100 km across,played a crucial role in defining the outer edge of the main ring system.

The Rain in Spain

Although the approach to Saturn was going extremely well,a minor problem was appearing back on Earth where heavy rain at the Deep Space Network tracking station near Madrid was threatening to interrupt X-band communications with Voyager 1. The X-band (5.2-10.9 GHz) transmitters on the spacecraft are able to return data to Earth at a staggering 115.2 kilobits per second, but heavy rains near the Earth station where the signal is received can severely disrupt reception.

On the night of 7-8 November a series of thunderstorms developed near the Madrid station and about 5 hours of data was lost. Mission scientists seemed not to be unduly concerned by this since most of the lost information would be repeated later and in more detail as Voyager closed in on Saturn. The storms returned later in the week during the critical Earth-Titan occultation phase, but since the spacecraft was sending unmodulated signals as part of the occultation experiments there was no significant loss of data.

Nearing the target

Two days from closest approach there was another surprise when Voyager 1 discovered two eccentric rings, unlike the usual circular ones. One of these lay within the innermost C ring, the other in Cassini's division. Dr. Brad Smith, head of the imaging team, said these rings were the feature he least expected to see. The following day he revised this opinion as Voyager returned pictures of the faint outer F ring (discovered by Pioneer 11), showing it to have three components, two of which appeared to cross like the strands of a rope. 'Obviously pure gravitational forces were not the only ones operating here,' Dr. Smith said, ‘In the strange world of Saturn’s rings, the bizarre has become commonplace.’

While attention focused on the rings, the detail visible on the planet had increased to objects as small as 175 km and small-scale convective clouds were being clearly seen. Simultaneously, the other experiments continued to probe the electromagnetic and particle fields around Saturn looking out for the bow shock as the solar wind interacted with the planet’s magnetic field. Voyager 1 crossed Saturn’s bow shock on 11 November, a little over 1.5 million km from the planet and just prior to the start of the near encounter phase. Behind the bow shock was the turbulent boundary between the shock and Saturn’s own magnetic field. This area is known as the magnetosheath and in an hour Voyager 1 recorded no less than five crossings into and out of the magnetosheath as the exact boundary ebbed and flowed. Then Voyager turned its electronic senses towards its first major target within the Saturn system, the moon Titan.

The Sirens of Titan

Titan,Saturn’s largest satellite, had drawn the attention of astronomers for decades because it is the only satellite in the Solar System known to have an atmosphere.On 11 November Voyager 1 closed in on Titan and the pictures received in Pasadena showed it to resemble a fuzzy yellow tennis ball with no observable features. Like Venus, the surface was covered in an impenetrable layer of haze. Fortunately, Voyager’s senses were not limited to visible wavelengths and its infra-red,ultraviolet and radio occultation experiments were able to reveal several surprising new facts about this mysterious satellite.

Voyager 1 flew behind Titan on 11 November 1980 just 60 seconds from the planned time and less than 10 minutes after its closest approach. This Earth occultation allowed Voyager’s radio experiments to probe the atmosphere and measure the satellite’s diameter. Almost immediately Titan was dethroned as the Solar System's largest satellite, as radio measurements revealed its diameter to be about 5120 km,over 100 km less than Ganymede.

After passing by Tethys, Voyager 1’s trajectory began to curve upwards again, back towards the ring plane so that as it made its closest approach to Saturn on 12 November it was on the ascending portion of its trajectory and heading for a close encounter with Mimas, the innermost classical satellite.

Voyager 1 passed within 108,000 km of Mimas and obtained high resolution, photographs of the face which looks away from Saturn. This surface, like the opposite hemisphere, was heavily cratered and at the antipode of the massive crater observed on the approach, lines were detected, possibly related to the original impact.

Minutes after passing Mimas,Voyager 1 made its closest approach to Enceladus. Enceladus was not a prime objective of this first probe and the miss distance was large, a little over 200,000 km. At this range the best surface resolution possible for Voyager's cameras was about 12 km and surprisingly, at least down to this scale, the surface appeared smooth with no evidence for cratering. Enceladus was one of the prime targets for Voyager 2. eight months behind its twin.

For 90 minutes during the close approaches to Mimas and Enceladus Voyager 1 was hidden from the Earth by the bulk of Saturn, allowing the radio experiments to probe the detailed structure of the planet’s atmosphere. Shortly after emerging from behind Saturn,Voyager 1 (as seen from Earth) began to pass behind the ring system and further radio and optical measurements were made to probe the density and composition of the rings.

Dione was the next moon to be encountered and a series of beautifully detailed photographs was returned to Earth showing a highly reflective, heavily cratered surface with several sinuous valleys and what appeared to be a large mare-like basin. Bright streaks were observed on the surface,probably ice ejected from relatively recent impacts. Dione is about 1110 km in diameter and its density is higher than the other inner moons, suggesting a higher proportion of rock.

Rhea has a diameter of about 1500 km and Voyager 1 passed within 75,000 km of it, 6 hours after closest approach to Saturn. Like Dione. the surface of Rhea is highly reflective with bright streaks overlying parts of the surface. Rhea is the most heavily cratered of all Saturn’s satellites thus showing the extreme age of its surface, probably reflecting a cosmic bombardment about 4,000 million years ago. The largest crater observed by Voyager 1 has a diameter of about 300 km. Many of the craters show central peaks caused by the rebound of the floor after the explosion which formed the crater.

The Rings of Saturn

During its approach to the planet. Voyager 1 showed that Saturn’s rings were far more complicated than had been previously believed. As the spacecraft dipped below the rings,viewing them from the unlit side and then climbing up and away from Saturn, many more pictures were received which shed important new light on their structure.

Classically, the main portions of the rings are known by letters, and reading from the centre of the planet outwards they are D, C, B, A, F and E. The irregular alphabetical sequence comes from the order in which they were discovered. Some of the significant Voyager findings about the rings are given here, in order of increasing distance from the planet.

The very faint D ring has been observed from Earth but its existence has long been disputed. Voyager 1 has confirmed that it does exist and shown that it stretches right down to the cloud tops.

Analysis of radio signals passing through the C ring gives an average size of the particles in the C ring as 2 metres.

Cassini’s division, which separates the B and A rings, was once believed to be empty, but Pioneer 11 showed that it actually contained some particulate matter and Voyager pictures revealed over 20 ringlets within it.

The A ring is the outermost of the well known rings and its outer edge appears to be controlled by the presence of tiny satellite 15. The F ring was discovered by Pioneer 11 and Voyager 1 has shown that this ring consists of three interwoven ringlets, shepherded on either side by satellites 13 and 14. Within the three ringlets are clumps of material, short segments of increased density about 100 km long, their origin a mystery at present.

Light scattering experiments showed that the particles in the D,E and F rings are small, mostly about 0.0002 mm in diameter. The composition of all of the rings is believed to be a mixture of ices and ice covered silicates, similar to the composition of interstellar dust.

Saturn

The planet itself is a less colourful world than Jupiter,probably because it is 25 to 30 K colder, the temperature near the cloud tops being a chilly 97 K. This coldness means that the clouds on Saturn form lower in the atmosphere and appear muted by high level haze. Wind speeds, however, are up to four times faster than on Jupiter, reaching 1600 km/hour near the equator. Like Jupiter, Saturn has aurorae, oval atmospheric spots and lightning-like discharges, although these discharges are believed to occur in the rings, not the atmosphere.

Mission Accomplished

As Voyager 1 climbed away from Saturn in early November its sensors continued to study Saturn. Wide angle pictures taken during 16-18 November were assembled into another time lapse movie of the planet’s rotation. Looking back at Saturn, Voyager produced some beautiful photographs of the crescent planet and its rings, as well as conducting infrared,ultraviolet and radio astronomy observations. Imaging finally ceased just before Christmas 1980.

On 23 November the plasma instrument aboard the spacecraft stopped transmitting usable data, but was left on in the hope that ground controllers could reactivate it. A similar fault had disabled the instrument for three months earlier in the year. By 5 December Voyager 1 was 29 million km from Saturn and travelling at 21.6 km/s. Even though observations of Saturn were drawing to a close. Voyager would continue to study interplanetary space as it travelled towards the edge of the solar system.